trigger system.pdf

143

3 Trigger system

3.1 Introduction

3.1.1 Trigger principle

The trigger has to select events of interest, containing a dimuon from, for example, the decay of aresonance J/ or amongst all possible background sources. The main source of background is clearlythe low pt component of muons from pion and kaon decays, as discussed in detail in Chapter 8 of thisdocument.

A cut on the transverse momentum of the tracks is for this reason performed by the trigger to reducethe background. A dimuon trigger is issued if at least two tracks, with opposite signs essentially, abovepre-defined pt cuts, are detected in an event.

The trigger decision has to be available very fast (1 s) because the signal is used as an early strobeby the muon tracking chambers.

The trigger information is also used to track identification during off-line analysis.

A special setup based on the RPC detector is built for triggering purpose and the results of a largeR&D effort, quite specific to ALICE requirements, are reported in this document.

The trigger algorithm and its hardware implementation in fast electronics is described as well as thedevelopment and the tests of first prototypes.

The choice of the trigger setup and algorithm has been accompanied by a large set of simulations:the main conclusions are reported and discussed.

3.1.2 Overview of the trigger systemThe trigger system used to perform the selection discussed in the previous section is represented inFig. 3.1. The muon trigger is generated using the information from two trigger stations (MT1 and MT2)located about 16 m from the interaction point and 1 m apart. These stations are located behind a 1.2 mthick iron muon filter (72int) which stops low-energy background particles.

Each trigger station consists of two planes of single-gap RPCs, each plane providing an X and Y read-out. Y measures the bending deviation (vertical) due to the dipole magnetic field, and X the position inthe non-bending direction. By convention, the X strips are horizontal and measure the deviation along Y(and vice versa for the Y strips).

This trigger detection system is divided in projective sub-detectors and the informations coming fromthe front-end electronics (X and Y strip pattern of the four planes) are sent to the local electronics, thepurpose of which is to perform a selection (via a cut on the transverse momentum pt on single tracks.The transverse momentum pt is computed using the deviation between MT1 and MT2 measured by theX strips (see details below) while the track is required to point back to the interaction point using theinformation given by the Y strips.

Two different pt thresholds are foreseen, a low threshold in order to select the muons coming fromthe decay of the J/ and a high threshold for the ones.

This local information is then gathered on a regional level (1 per crate) which delivers a signal forsingle muons as well as for muon pairs. In the same way, all these regional pre-triggers are then mixedto deliver the global dimuon trigger signal.

144 3 Trigger system

Figure 3.1: Overview of the trigger system.

B

Z

Y

MT1 MT2

d

pYZ

pt

ZF Z1 Z2

YF

Y1Y2

Figure 3.2: The muon arm trigger principle, based on the estimation of the transverse momentum of the tracks:the larger the distance between YF and the pt straight line, the lower the pt of the track.

The trigger algorithm is therefore a 3-step process.From the DAQ point of view, the patterns are transmitted in the same way to a DAQ interface card

which further sends the information to the DAQ ALICE system via a RORC.The number of signals, cards and crates needed is discussed in the following sections.The timing considerations for the data transfer and the trigger signal building are also discussed in

the following sections. Let us just remark now that this dimuon trigger signal has to be locally availablebefore 700 ns in order to be included in the ALICE level 0 trigger.

The principle of the pt cut with the trigger relies on the use of an estimated deviation angle d andis represented in Fig. 3.2. The track information (X1,Y1,Z1,Y2,Z2), where the index 1(2) refers to thetrigger station MT1(2), is enough to calculate a raw value of the transverse momentum pt. If only smallbending angles are considered in the muon spectrometer and the approximation of the bending plane(located at the middle of the dipole ZF ) and vertex point are used, the momentum pYZ in the bendingdirection is then given by

pY Z

qBLd

where

q is the charge of the particleB is the magnetic fieldL is the length of the dipole

3.1 Introduction 145

The deviation angle d is obtained by

d 1

ZF

Y1Z2Y2Z1Z2Z1

The particle crosses the bending plane at

XF X1 ZFZ1YF Y2 Y2Y1Z2Z1 Z2ZF

The transverse momentum is finally:

pt p

X2F Y 2FZF

pYZ

X2F Y 2FZF

Two different pt cuts, corresponding to low and high thresholds, will be pre-loaded in look-up tableslocated in the trigger electronics. Some improvement on pt could be achieved by the use of a fullsimulation of the setup, in particular by introducing the dipole actual field map.

3.1.3 Choice and requirements for the trigger detector

The considerations which have driven the choice of the trigger detector and the requirements to be ful-filled by this detector are discussed in this section.

A granularity around 1 cm [1] is mandatory to select high pt muons from J/ and decays and toreject the background from muons coming from pion and kaon decays. Such a resolution can be achievedby a scintillator hodoscope made of strips about 1 cm wide, but this option presents the disadvantage ofbeing costly. Gaseous detectors, such as Iarocci tubes or Resistive Plate Chambers (RPCs), are muchcheaper and offer the desired granularity.

Another advantage is afforded by gaseous detectors. Simulations based on GEANT and FLUKAcodes show that, beside muons coming from pion and kaon decays, neutrons, gammas and electrons(mainly produced by showers inside the absorbers and the beam shield) are another relevant source ofbackground. Hereafter, this background is referred to as soft background.

The trigger detector should have a low sensitivity to neutrons and gammas to keep the number ofhits to a reasonable level. This feature is fulfilled by gaseous detectors; for example the typical RPCsensitivities are of the order of 102and 103 respectively for gammas and neutrons.

RPCs have been preferred to Iarocci tubes because of their better timing properties: in the first onethe electric field is strong and constant inside the gas gap, while in the latter the field is weak far fromthe anode wire, which means larger fluctuations on the necessary time to create the signal. It is thenimpossible to get the signals from Iarocci tubes in a gate narrower than 70 ns without a significant loss ofefficiency [2], while for RPCs a 20 ns gate can be used, as will be shown in Section 3.2.3. This relevantproperty can be exploited to reject most soft background hits. Actually, while the time dispersion of themuons emitted in an event is about 5 ns, the soft-background is spread out in time on a s scale. Onlyabout 50% of the charged particles, 10% of the gammas and 1% of the neutrons reach the trigger detectorwithin 20 ns after the fastest muons. Therefore the number of background hits can be significantlyreduced by gating the signals of the trigger detector in a 20 ns wide window.

To illustrate the timing properties of the RPCs, a typical streamer is shown in Fig. 3.3. The rise timeis typically 2 ns and the pulse height 100200 mV/50 .

Furthermore, two other requirements have to be fulfilled by the RPCs: the rate capability and thecluster size.

Simulations show that the RPCs are expected to be fired (see Table 3.17) at rates of 3 Hz/cm2,40 Hz/cm2and 10 Hz/cm2 respectively for PbPb at luminosity = 1027 cm2s1, for CaCa at =


Figure 3.3: Typical example of a RPC streamer.

1029 cm2s1 and for pp at = 1031 cm2s1. These rates are the biggest and are located in the RPCregion close to the beam shield. They significantly decrease when getting away from the beam shield.Therefore, including a safety factor, a rate capability of 100 Hz/cm2 is needed for the RPCs in the ALICEmuon arm.

The cluster size (number of adjacent channels fired) should be as close to one as possible. A smallcluster size minimizes the occupancy of the read-out strips and gives a higher selectivity of the trigger,as will be discussed in detail in Section 3.5.3.

3.1.4 Requirements for the trigger electronics

The trigger electronics has three main components:

The Front-End Electronics (FEE) The local trigger electronics

The regional and global trigger electronics

3.1.4.1 The Front-End-Electronics (FEE)The detector response is collected on aluminium strips 14 cm wide and less than 70 cm long. Eachstrip is connected at one end to a resistive termination and to an FEE channel at the other end. The FEEmust be able to handle very fast signals, with a rise time of about 2 ns (see Fig. 3.3), of both positive andnegative polarity. Basically it consists of a leading edge discriminator stage followed by a shaper. Therequired timing performances do not justify more sophisticated discrimination techniques like constantfraction or zero crossing. No amplification of the signal is needed in the streamer mode of operation ofthe RPC. The output signal must have a width of about 20 ns:

this is the minimal width which ensures the correct signal capture by the trigger electronics, in-cluding all possible time jitter sources (detector response, strip length, etc.)

for pp collisions, the time interval between two bunch-crossing is 25 ns. To select the good bunchcrossing, the signal width must be narrower.

in heavy ion collisions where the bunch-crossing interval is larger (125 ns), a short signal is clearlya means of limiting the impact of the background as already discussed in Section 3.1.3.

For any detector information above the FEE discriminator threshold, a signal in ECL differential modeis sent to the local trigger electronics through cables about 15 m long.

3.2 R&D, tests and prototypes 147

3.1.4.2 The trigger electronics (local to global)

The local trigger electronics receives digital signals from the FEE: any sequence of those signals is calleda bit-pattern. Two main functions are implemented in the local electronics:

the backup of the input bit-patterns which are placed in a pipelined memory read out on occurrenceof an ALICE L1 trigger.

the local trigger algorithm. The local L0 trigger - on single tracks - above low and high pt thresh-olds is delivered on output.

The regional and then global electronics collect the information of the various local boards. A trigger isissued if at least two candidates, for a dimuon trigger, pass the low (high) pt threshold.

The dimuon trigger is involved in the level 0 trigger of ALICE [3] . L0 triggers act as an early strobe(before 1.2 s) for some ALICE detectors, like the muon tracking chambers for example. It is thenmandatory to have the dimuon trigger already at L0 because strobing the muon tracking chambers at thefull interaction rate (106 in pp) would create an enormous dead-time. The participation of the muontrigger in a higher ALICE trigger level like L2 has been envisaged but has not been retained (see Table3.26 and comments).The requirements for the L0 muon trigger are:

To work in pipelined mode at a frequency of 40 MHz, like all L0 ALICE trigger detectors.

To give a decision every 25 ns with a fixed latency. The dimuon trigger has to be delivered to thegeneral ALICE L0 trigger before 700 ns which leaves time to build the ALICE L0 decision andto transport it to the detectors. A validation of the L0 dimuon by an interaction trigger (given forinstance by the Forward Multiplicity Detector or any other interaction detector) is foreseen at thebuild-up time of the general ALICE L0.

To limit the dimuon rate to a maximum of 1 kHz.

To deliver a signal at low rate ( 100 Hz) for rare events like high pt muon pairs. These eventswill be given priority in the DAQ stream afterwards.

3.2 R&D, tests and prototypes3.2.1 General considerationsThe first ideas about Resistive Plates Chambers (RPCs) were developed by R. Santonico at the beginningof the eighties [4], and since then they have been successfully employed for the detection of cosmicrays [5], as well as in accelerator experiments [6]. Owing to their low cost and robustness, RPCs are infact well suited for covering large surfaces and offer time resolutions and granularities comparable to oreven better than those achieved with plastic scintillators.

In their early versions, RPCs were operated in streamer mode. This mode provides large signalswhich can be discriminated without amplification but imposes severe constraints on the rate capabilityof the detector: to our knowledge the best performance reported in the literature is about 100 Hz/cm2[7], [8], [9]. This limitation comes from the large charge released in the streamer which causes a localreduction of the voltage between the electrodes. Since these electrodes are made of high resistivity mate-rials (typically of 10111 cm), tens or hundreds of milliseconds are necessary to resume the operatingvoltage. Therefore, during this time the detector is blind in the area where the streamer developed.

To overcome this problem, RPCs operated in avalanche mode have been widely studied in the lastfew years [10], [11], [12], [13], [14]. The current dissipated in a single discharge is reduced in this


running mode especially when it is coupled with the use of a low amplification gas mixture. This meansa smaller local decrease of the voltage and consequently a better rate capability of the detector which isstill efficient for particle fluxes of several kHz/cm2 [15]. Obviously, in this mode, the lower gain has tobe compensated for by high-gain fast amplifiers integrated in the front-end electronics.

Indeed, the rate capabilities quoted above have been measured in beam tests; in these conditionsonly a small fraction of the detector surface is exposed to the particles (local illumination) for a timelimited by the duration of the burst. A non-negligible reduction of the rate capability occurs when thewhole RPC is exposed to an uniform flux of particles constant in time [16], which is indeed the workingcondition at the LHC. Then, for safe operation, the rate capability shown by the RPCs in beam tests mustbe considerably higher than the rates expected at the LHC.

RPCs operated in avalanche mode have been chosen to provide the muon trigger for ATLAS andCMS, because the rates are expected to be as high as a few hundred Hz/cm2 in these experiments. Thesituation is different for the ALICE muon arm, where the expected rates are 40 Hz/cm2 at most. Thisvalue does not exclude a priori the possibility of working in streamer mode, although some improvementof the rate capability would be needed.

Moreover, as already discussed, the rate capability is not the only relevant parameter: a time resolu-tion sufficiently good to strobe the RPC signal within a 20 ns wide gate is required together with a clustersize as close to one as possible.

These considerations led us to carry out a comparative study of both avalanche and streamer modeto understand which one turns out to be a suitable choice for the ALICE muon arm.

A special effort has been devoted to improving the rate capability of the streamer mode. For thispurpose, two parameters have been studied. The first is the gas mixture, which has been chosen in sucha way as to minimize the charge released in the streamer. The second one is the electrode resistivity: achamber with electrodes made of low resistivity Bakelite ( 35 109cm) to reduce the recovery timehas been built.

In Section 3.2.2 cosmic ray tests for the optimization of the gas mixture in the streamer mode arediscussed, while the experimental set-up and the results of beam tests are presented in Section 3.2.3.

3.2.2 Tests with cosmic rays

Tests with cosmic rays have been carried out to optimize the gas mixture in the streamer mode. In thesetests we used a single-gap RPC of 50 50 cm2, the structure of which is shown in Fig. 3.4. Theelectrodes are made of 2 mm thick foils of phenolic Bakelite with a resistivity of about 4 1011cm.The electrodes are separated by a 2 mm wide gas gap. The surface of the Bakelite foils on the gas sideis painted with linseed oil, while the external surface is painted with graphite. One of the two graphitelayers is connected to the high voltage, the other to the ground. Read-out strips 3 cm wide, terminatedby a 50 resistor at one end and connected (by coaxial cables) to the electronics at the other end, areused to pick up the signal.

The experimental setup is sketched in Fig. 3.5. The trigger consists of a coincidence of three scintilla-tors (SC1, SC2 and SC3) which matches the geometrical acceptance of one strip on the RPC (the so-calledcentral strip). This geometrical configuration gives both the global and the neighbour efficiency. The firstone is determined by the logical OR of 5 strips centred around the central one; the latter by removingfrom the OR the central strip. Obviously, a small value of the neighbour efficiency is related to a smallcluster size. Together with the efficiencies, which have been measured with a discrimination thresholdset to 35 mV, the time resolution, the signal amplitude and its shape have been studied.

Among the different mixtures that have been investigated, the results for four mixtures are reportedhere; the compositions of these are listed in Table 3.1 and the results obtained are summarized in Table3.2. Here are reported the neighbour efficiency, the signal charge and its amplitude for a value of theRPC HV which is about 400 V above the knee of the efficiency curve.

The study was started by testing a mixture (number 1 in Tables 3.1 and 3.2) made of argon, iso-


High Voltage (+H.V.)pick-up x-strips

Spacerspick-up y-strips Insulating film

Gas 2 mm

2 mm

2 mm

GND

Resistive electrodplates

Graphite paintedelectrodes

Figure 3.4: Structure of the RPC used for the tests with cosmic rays and for the tests with beam.

Figure 3.5: Experimental setup of the cosmic-ray tests for the optimization of the gas mixture in streamer mode.


Table 3.1: Composition of the gas mixturesMixture number Mixture composition

1 Ar/i-C4H10/C2H2F4= 70/20/102 Ar/i-C4H10/C2H2F4= 10/7/833 Ar/i-C4H10/C2H2F4/SF6= 49/7/40/44 Ar/i-C4H10 = 80/20 + 4% SF6

Table 3.2: Summary of the results for the gas mixturesMixture number HV (V) Neigh. eff. Charge (pC) Ampl. (mV)

1 6700 46% 330160 3901702 10000 15% 10643 183533 9500 11% 4825 113444 7300 13% 7040 14451

butane and tetrafluoroethane in the ratios Ar/i-C4H10/C2H2F4 = 70/20/10. For this mixture, which israther similar to the ones proposed in Ref. [17], the neighbour efficiency is higher than 40% (the smallerthe value, the better it is) and the mean amplitude of the signal turns out to be about 400 mV/50 (seeTable 3.2). This last value is close to those reported for RPCs operated with more usual gas mixturesmade of argon, iso-butane and a small proportion of CF3Br [18], [19], [20].

To reduce the signal amplitude, the first attempt to increase the percentage of tetrafluoroethane andto decrease the percentage of argon (mixture number 2). As displayed in Table 3.2, the mean amplitudefor the mixture Ar/i-C4H10/C2H2F4 in the ratios 10/7/83 is smaller than 200 mV/50 and also theneighbour efficiency is much improved. This last gas mixture was originally proposed for operation inavalanche mode [11], but recent studies have shown that a small percentage of SF6 is extremely effectivein providing streamer quenching [12]. Since the goal is to reduce the charge released in the streameras well as the cluster size (that might be to some extent related to the transverse size of the streamer)it was decided to investigate the effects of SF6 for operation in streamer mode. Therefore quaternarygas mixtures made of Ar/i-C4H10/C2H2F4/SF6 in different proportions were tested; the best results wereobtained with the ratios 49/7/40/4. As shown in Table 3.2, this mixture (number 3) gives a mean signalamplitude of about 100 mV/50 and a neighbour efficiency of about 10%. This is the mixture whichwas chosen for the beam tests reported in the next section.

For the sake of completeness, and to illustrate the quenching properties of SF6, the results obtainedwith a mixture (number 4) made of Ar/iso-butane in the ratio 80/20 and with the addition of 4% of SF6are also shown. This mixture has the same proportion of SF6 as the one chosen, but no tetrafluoroethaneand much more argon. In spite of that, the signal amplitude is only slightly larger than the one of thechosen mixture but the percentage of double pulses is much higher.

To conclude the discussion of the data collected in these tests we note that, although the signal ampli-tudes and the neighbour efficiencies are different, the global efficiencies as well as the time resolutions(better than 2 ns) are practically the same for all mixtures.

3.2.3 Tests with beam

3.2.3.1 Experimental conditions

The RPC used in the tests reported here has the same size (50 50 cm2) and mechanical structure asdescribed in Section 3.2.2 (see Fig. 3.4). The only difference is that the electrodes of the RPC testedwith the beam are made of phenolic Bakelite of much lower resistivity: 35 109cm. In May 1998,the low resistivity RPC was exposed to a 120 GeV/c pion beam at the H6 experimental area in the SPS


beam

RPC

S1

HOH - HOVscintillators

IMPH

IMPV

Pb block

Figure 3.6: Schematic side view of the experimental setup for the beam tests.

North Hall. The experimental setup is shown in Fig. 3.6. The S1 counter (20 20 cm2 in area) is placed12 m upstream of the RPC, while the horizontal and vertical scintillator hodoscopes HOH and HOV arebehind the chamber. They are made of four scintillator counters each, to cover a surface of 40 40cm2, slightly smaller than the one of the RPC. The trigger is defined by the coincidence S1HOHiHOVi,where HOHi and HOVi (i = 1,...,4) respectively indicates the signals of the i-th counter of the horizontaland vertical hodoscopes. The two scintillators IMPH and IMPV, placed just behind the RPC, do not enterin the trigger logic; their signals are used to select and count off-line the particles hitting the chamber inthe small central region of 2 2 cm2 defined by the geometrical overlap of these two counters.

The RPC is equipped both with horizontal and vertical strips of 50 impedance. For the tests instreamer mode, both 1 cm and 2 cm wide strips have been used, while in avalanche mode the chamberhas been tested with strips of 2 cm. One end of each strip is terminated with a 50 resistor, while thesignal is picked up at the other end and sent to the counting room via 50 m long coaxial cables. Forthe RPC operating in streamer mode, the signals of the strips are discriminated by a Constant FractionDiscriminator (CFD) with a threshold of 25 mV. One output of each CFD is sent to a TDC, the other to aPattern Unit. The start to the TDCs is given by the trigger (the timing is defined by S1) and the width ofthe Pattern Unit gates is 200 ns. For the tests in avalanche mode, the process of the strip signals is rathersimilar. The only relevant difference, for the avalanche mode, is that the signals are amplified by a factor300 and the threshold of the CFDs is set to 200 mV. The amplifiers were mounted on the chamber itself.

As previously stated, one of the aims of this test is to study the performance of the chamber as afunction of the incident flux. Since in beam tests such as the one presented here it is not possible toachieve a uniform illumination of detectors as large as the RPC prototype, the good parameter is thelocal flux in the region corresponding to the beam spot. The local flux is measured in the 2 2 cm2central region of the chamber by counting the number of TRIGGER*IMPH*IMPV coincidences and canbe adjusted by changing the aperture of the collimators placed along the beam line. A defocused beamis used to obtain a rather uniform illumination inside the chamber region defined by the coincidence ofIMPH and IMPV. Indeed, a constant illumination is only approximately achieved; actually the r.m.s. ofthe strip hit distributions turns out to be of about 1.3 cm in the horizontal coordinate (vertical strips) andbetween 13 cm in the vertical coordinate (horizontal strips), decreasing with the beam intensity. As anexample, in Fig. 3.7 (a) and (b), are shown respectively the hit distributions of the vertical and horizontal


strips (both 2 cm wide) for a local flux of 870 Hz/cm2 in the central region of the chamber.

3.2.3.2 Efficiency and rate capability

The low-resistivity RPC has been fluxed with the gas mixture Ar/i-C4H10/C2H2F4/SF6 = 49/7/40/4, cho-sen for operation in streamer mode, as reported in Section 3.2.2. At a high voltage of 9 kV, correspond-ing to the beginning of the efficiency plateau (see below) the dark current has reached a stable valueof about 30 A, while the single rate is of the order of 0.050.07 Hz/cm2. It is interesting to remarkthat both these numbers are very close to the ones reported in the literature for RPCs of considerablyhigher resistivity [18]. During the beam test, the efficiency of the RPC, its time resolution and the clustersize have been studied first in streamer mode and then in avalanche mode. For the latter, we used a gasmixture made of C2H2F4/i-C4H10/SF6 in the ratios 95/3/2. This is very similar to the mixtures proposedin Ref. [12], that allow operation in avalanche mode with very small streamer contamination.

The efficiencies presented here are local, that is they are computed by taking into account only thesignals from few strips (respectively 3 and 5 for 2 cm and 1 cm wide strips) covering the 2 2 cm2central region of the RPC, where the incident flux is maximum.

The efficiencies as a function of the HV are shown in Fig. 3.8 for operation both in streamer andavalanche mode. The efficiency curves visible in this figure have been obtained for rather low localfluxes, respectively of 180 Hz/cm2 and 100 Hz/cm2. As can be seen, the efficiency plateau starts at8.8 kV for the streamer mode and at 9.9 kV for the avalanche one.

The evolution of the efficiency as a function of the local flux is summarized in Fig. 3.9. In Fig. 3.9(a)is shown the efficiency for the streamer mode at two different values of the high voltage. This is stillbetter than 95% for fluxes of about 500 Hz/cm2 and 1 kHz/cm2 at 9 kV and 9.4 kV respectively. Toemphasize the straightforward dependence of the rate capability on the resistivity of the electrodes, wehave also plotted in Fig. 3.9(a) the efficiencies of two RPCs made with Bakelite of higher resistivity,respectively of about 3.51011cm and 61010cm. The former was tested at the SPS in 1997 [21],the latter in the test reported here. The efficiency obtained when the low resistivity RPC is operated inavalanche mode is visible in Fig. 3.9(b). As can be seen, also in this case, better rate capabilities areachieved by increasing the high voltage; at 10.3 kV the efficiency is still better than 95% at 10 kHz/cm2.

The efficiencies presented above are averaged over the duration of the burst (2.5 s). Actually, aprogressive reduction of the efficiency during the burst has been reported in the literature by differentauthors [22], [8]. This effect, which has been observed for electrodes of resistivity larger than the onewe use by at least one order of magnitude, is caused by the surface of the resistive plate that becomescharged, implying a reduction of the electric field applied to the gas gap.

To investigate the relevance of this effect for our low-resistivity RPC operated in streamer mode,the burst duration was divided in four intervals of about 0.6 s and the efficiency was measured for eachinterval. The results of this study are summarized in Fig. 3.10, where the efficiency as a function oftime during the burst is shown for different incident fluxes. As can be seen, although the efficiencydecreases with the incident flux, it remains constant during the burst and the RPC achieves its asymptoticefficiency already at the beginning of the first quarter of the burst. Therefore, any further degradationof the efficiency is not expected when the detector is exposed to a constant particle flux, as it will be inALICE.

The efficiencies presented up to now are measured with defocused beam. As has been previouslypointed out, in this experimental condition the flux of incident particles is not constant over the 2 2cm2 region selected by the IMPH and IMPV counters. This means that the flux at the centre of thisregion is higher than the mean value determined by counting the coincidences TRIGGER*IMPH*IMPVand implies that the rate capability of the detector is slightly underestimated.

To conclude this section, we note that the rate capability of RPCs with high-resistivity ( 10111012 cm) electrodes is improved when about 1% water vapour is added to the gas mixture [23], [24].Although during this test the low-resistivity RPC has been fluxed with dry gas, it is clear that the addition


0

10

20

30

40

-10 -5 0 5 10

strip position (cm)

cou

nts

/100

0

0

20

40

-10 -5 0 5 10

strip position (cm)

cou

nts

/100

0

Figure 3.7: Coordinate distributions of fired strips. In (a) and (b) are shown respectively the distributions forvertical and horizontal 2 cm wide strips obtained with defocused beam.


0

0.25

0.5

0.75

1

8 9 10 11

streamer

avalanche

H.V. (kV)

effic

ienc

y

Figure 3.8: Efficiency vs. high voltage for the streamer mode at 180 Hz/cm 2 (full circles) and for the avalanchemode at 100 Hz/cm2 (open circles). We note explicitly that the gas mixtures for streamer and avalanche modes aredifferent.

of moisture to the gas might represent a method to further improve its rate capability, if needed.

3.2.3.3 Time resolution

The time resolution of the RPC has been studied for avalanche and streamer modes as a function of thelocal flux. For both modes, the RPC was equipped with constant fraction discriminators and the TDCstart was given by the trigger signal provided by the scintillator counters.

In the streamer mode, the TDC distributions at 9 kV are shown in Fig. 3.11(a) for local fluxes of45 Hz/cm2 and 380 Hz/cm2, while in figure 3.11(b) the distributions at 9.4 kV measured at 105 Hz/cm2and 650 Hz/cm2 are plotted. At 9.4 kV the distribution is nearly Gaussian for the lower incident flux;when the flux increases, the width of the Gaussian peak as well as its position are basically unchanged,but a tail appears on the right side. For the same incident flux, the fraction of events in the tail is largerat 9 kV than at 9.4 kV.

These results are qualitatively similar to those reported in the literature ( [7], [9]) for RPCs of higherresistivity and with conventional gas mixtures. However, the effects of the incident flux on the shape andposition of the TDC distributions are less pronounced here.

The time performances of the detector are summarized in Fig. 3.12(a) and 3.12(b), where, for HV=9.4 kV, the r.m.s. of the TDC distribution and the fraction of events contained in a 20 ns window arerespectively plotted as a function of the incident flux. Figure 3.12(a) shows that the r.m.s. is about 1ns below 100 Hz/cm2 and then increases up to about 4 ns for incident fluxes of the order of 1 kHz/cm2.However, as can be seen in Fig. 3.12(b), even for the highest rates, more than 98% of the events arecontained in a 20 ns window which is a relevant point to operate the chambers in ALICE, as alreadydiscussed (see also Ref. [25]).

For the avalanche mode, the TDC distribution measured at 10 kV for an incident flux of 1.15 kHz/cm2is shown in Fig. 3.13. The distribution is nearly Gaussian, with a r.m.s. of 0.9 ns. The time resolutionshows a weak dependence upon the incident flux for the avalanche mode: it increases to 1.1 ns forincident flux up to 15 kHz/cm2.


0.6

0.7

0.8

0.9

1

10 -1 1 10 10 2 10 3

H.V.= 9000 V H.V.= 9400 V

H.V.= 9000 V H.V.= 9000 V

H.V.= 9200 V

particle flux (Hz/cm2)

Effic

ienc

y

0.6

0.8

1

10 2 10 3 10 4H.V.= 9900 V

H.V.= 10300 V

H.V.= 10000 V

particle flux Hz/cm2

Effic

ienc

y

Figure 3.9: Efficiency as a function of the local flux: (a) efficiency of the low resistivity RPC in streamer mode forHV values of 9 kV (full circles) and 9.4 kV (open circles) together with the efficiencies of two RPCs of resistivity 6 1010 cm (open triangles) and 35 1011 cm (full triangles); (b) efficiency of the low resistivity RPCin avalanche mode, at 9.9 kV (full circles), 10.0 kV (open circles) and 10.3 kV (stars).


0.6

0.8

1

0 1 2 3 4 5

section of burst

effic

ienc

y

380 Hz/cm2

630 Hz/cm2

870 Hz/cm2

Figure 3.10: Streamer mode efficiency as a function of time during the burst for incident fluxes of 380 Hz/cm 2(full circles), 630 Hz/cm2 (open circles) and 870 Hz/cm2 (triangles).

3.2.3.4 Cluster size

The cluster size distributions for the streamer mode at 9 kV are plotted in Fig. 3.14(a) and 3.14(b). Thedistribution shown in the first figure is obtained by equipping the RPC with 2 cm wide strips, while theone plotted in the second figure is for 1 cm wide strips. In both cases, the events in which only onestrip is fired are the most probable and the fraction of events with more than three adjacent strips fired issmaller than 103. However, the mean cluster size for a 1 cm strip is 1.46, a value that is significantlylarger than the one for 2 cm wide strips which is 1.12.

The mean cluster size is plotted in Fig. 3.15 as a function of the applied high voltage for both thestrip widths. The cluster size increases slightly with the voltage; for instance at 9.4 kV it turns out to be1.7 and 1.25 respectively for the 1 cm and 2 cm strip.

To conclude the discussion on the cluster size in the streamer mode, it is interesting to note thatthe cluster size values reported here for this low-resistivity chamber are very close to the one measuredfor the RPC with high resistivity electrodes ( 35 1011 cm) which had been tested in 1997 at theSPS [21]. This suggests that, for the streamer mode, the dependence of the cluster size upon the bulkresistivity of the Bakelite is weak.

For practical reasons, it has not been possible to equip the RPC with 1 cm wide strips when it wasoperated in avalanche mode. Therefore only the results for 2 cm wide strips are presented in Fig. 3.16,where the cluster size distribution in avalanche mode at 10 kV is plotted. Such a distribution is remark-ably different with respect to the one in the streamer mode. The mean value is higher (1.41) and there isa much larger fraction of events (about 1%) with more than three adjacent strips fired. These results arerather similar to those reported in the literature by different authors [15], [26].

3.2.3.5 Conclusions and remarks

In Table 3.3 the performances of the low resistivity (35 109 cm) RPC operated in streamer and inavalanche mode are compared.

During the beam test the chamber operating in streamer mode has shown a stable behaviour and itis still efficient when exposed to a local particle flux of about 1 kHz/cm2. The time resolution is higherthan the one obtained in avalanche mode but it still well fulfils the ALICE trigger requirements as can


0

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100 150 200 250 300 350

TDC ch. (1ch.=0.1ns)

even

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0

1000

2000

100 150 200 250 300 350


even

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Figure 3.11: Time distributions for incident fluxes of 45 Hz/cm2 and 380 Hz/cm2 at 9 kV (a) and for fluxes of105 Hz/cm2 and 650 Hz/cm2 at 9.4 kV (b).


0

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4

5

10 2 10 3


TDC

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ns)

90

92.5

95

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% ev

ents

Figure 3.12: Root mean square of the time distribution (a) and fraction of events contained within a 20 ns widewindow (b) as a function of the incident flux. The data are for the streamer mode at HV = 9.4 kV. The position ofthe time window has been optimized at low rates and has been kept constant at higher rates.


0

1000

2000

3000

350 400 450 500 550 600


even

ts

Figure 3.13: Time distribution for avalanche mode at HV = 10 kV and for an incident flux of 1.15 kHz/cm 2.

Table 3.3: Summary of test beam results for the low resistivity RPC operated in streamer and in avalanche mode(with different gas mixtures)

Streamer Avalanche9 kV 9.4 kV 10 kV

Rate capability (Hz/cm2) 500 1000 7000Time resolution r.m.s. (ns) 25 14 0.91.1% of events in a 20 ns gate > 98% > 98% 100%Cluster size (strip) for 2 cm strips 1.12 1.25 1.41Cluster size (strip) for 1 cm strips 1.5 1.7% of events with more than 3 strips fired (2 cm strips) < 0.1% < 0.1% 1%

be seen from the high fraction of events included in a 20 ns time gate. The cluster size is lower whenthe chamber is operating in streamer mode, and moreover, when the chamber is operating in avalanchemode, the fraction of events with more than three adjacent fired strips is not negligible.

3.2.4 Tests at the GIF

As mentioned in Section 3.2.1, the RPC rate capabilities measured in tests with beam (i.e. under localillumination, limited in time by the burst duration) are much better than those found when the wholedetector is exposed to a particle flux constant in time. The latter conditions are close to the ones expectedin ALICE, and can be achieved at the CERN Gamma Irradiation Facility (GIF). Therefore, after the SPStests, the rate capability of the low-resistivity RPC has been studied at the GIF.

The experimental setup of the GIF test is shown in Fig. 3.17. The RPC is exposed to 660 keV -raysemitted by a 137Cs source. The distance between the source and the detector is about 5 m, correspondingto a RPC counting rate of about 300 Hz/cm2, constant over the RPC surface. Cosmic rays crossingthe chamber are selected by a telescope, made of two pairs of 40 10 cm2 plastic scintillators. Thetrigger is given by the coincidence of the four counters signals. To reduce the scintillator hit rates dueto -rays, both scintillator pairs are shielded by a 5 cm thick lead absorber. Moreover, a 0.5 cm thickcopper absorber is placed between the two scintillators of each pair, to stop the electrons emitted in the


10

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10 3

10 4

5 10 15 20

IDEntriesMeanRMS

5551 42255 1.115 0.3222

Vertical cluster size

even

ts

1

10

10 2

10 3

10 4

5 10 15 20

IDEntriesMeanRMS

5552 15248 1.459 0.5196

Vertical cluster size

even

ts

Figure 3.14: Cluster size distribution in streamer mode for 2 cm (a) and 1 cm (b) strips at HV = 9kV.


0

1

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3

8 8.5 9 9.5 10

2cm strips1cm strips

H.V. (kV)

Clus

ter s

ize

(strip

)

Figure 3.15: Mean cluster size for 1 cm (open circles) and 2 cm (full circles) strips as a function of the highvoltage.

1

10

10 2

10 3

10 4

5 10 15 20

IDEntriesMeanRMS

5551 45360 1.406 0.7677

Cluster size

even

ts

Figure 3.16: Cluster size distribution for avalanche mode at 10 kV for 2 cm wide strips.


Gammasource (Cs-137)

Pb blocksScintillators

Cu shield

RPC

Cosmic ray

Figure 3.17: Experimental setup of the GIF tests.

interaction of a -ray in the first scintillator before they reach the second one.During this test the RPC was operated in streamer mode, with the front-end electronics mounted on

the chamber itself (see Section 3.2.6).The RPC efficiency for cosmic rays in the presence of a hit rate of 330 Hz/cm2 (determined by

counting the logical OR of the strips) is shown in Fig. 3.18 for a RPC HV of 9400 V. It is measuredduring 20 subsequent runs, covering an overall period of about 3 days. As can be seen, the behaviourof the RPC is stable and the efficiency is about 90%. Actually, this value is only a lower limit, because afraction of the triggers (about 5%) comes from random coincidences of the four scintillators due to thehigh single rates in the GIF environment.

In Fig. 3.19 is shown a typical T.O.F. distribution of the logical OR of vertical read-out strips: theHV is set at 9400 V and the counting rate was about 350 Hz/cm2. The TDC spectrum is similar to thoseobserved in SPS tests; a Gaussian peak ( 2.3 ns) is clearly visible as well as a tail, due to late signals.It can be noticed that, with good approximation, the whole TDC distribution is comprehended within a20 ns window.

In conclusion, this test shows that low-resistivity RPCs can be successfully operated in streamermode in conditions much more severe than those expected in ALICE.

3.2.5 Tests in ALICE-like conditions

In ALICE, energetic hadrons ( 100 GeV) emitted during the ion collision intercept the beam shield anddevelop hadronic showers. The charged particles leaking out from this thick absorber are responsible formost of the background on the trigger.

An experiment at the CERN/SPS was carried out in 1998 where a hadron beam of 120 GeV/c wasdumped in a thick lead absorber (100 cm long in the beam direction and 60 60 cm2 of section) whichsimulates the ALICE beam shielding. The RPC described in Section 3.2.3, operated in streamer mode, islocated on the side of the Pb absorber as shown in Fig. 3.20, orthogonally to the beam, at 60 cm from thebeam entry face. The lateral Pb thickness between the beam axis and the RPC is varied between 20 cm


Figure 3.18: Efficiency measured at the GIF for a uniform counting rate of 330 Hz/cm 2 at 9.4 kV (streamermode).

Figure 3.19: Time distribution (100 ps/ch) for a uniform counting rate of 350 Hz/cm 2 at 9.4 kV (streamer mode).


Figure 3.20: Setup of absorber test.

and 40 cm.The goal of the experiment is twofold:

cross-check the simulation absolute yields of hits (the lateral punch-through probability of hadron-ic showers is not very well known) with the actual detector;

operate the RPC in uncomfortable conditions: the simulations give for example that the incidentangle of the particles reaching the detector is peaked at 50.

The general behaviour of the RPC during the test was quite satisfactory. The cluster size increases asexpected up to 1.4 (to be compared to 1.12 for direct beam in the RPC) with 2 cm wide strips.

The simulations of the background radial distributions, normalized per cm2 and per incident hadron,are made with FLUKA - with a simplified geometry - for three lateral Pb thickness of 20, 30 and 40 cm.Data and simulation are compared in Fig. 3.21. The shape of the experimental and simulated distributionsare in good agreement. However, FLUKA simulations underestimate the hit yields by a factor 1.4 to 1.6.

The simulations find that the photons and neutrons interact mainly in the Bakelite respectively byCompton effect and production of recoil protons. The interactions in the gas become important only forthe lowest energy neutral particles and do not contribute significantly to the hit yields whatever the gasmixture composition. The mean sensitivity, averaged on the particular energy distribution obtained inthis test, of the detector to the gammas and neutrons is found to be of the order of 5 103 and 5 104respectively.

Table 3.4 displays the measured and simulated hit rates, measured at 20 cm away from the absorberside, as well as the simulated flux of particles crossing the RPC. About one-half of the hits are comingfrom neutrons and the contribution of the gammas is less than 10%: these ratios are a little bit differentin ALICE because the material of the beam shield is different (see section absorber).

Nonetheless the conclusion from this test is that a correction factor of 1.5 must be applied on theFLUKA simulations of the hit rates on the dimuon trigger in ALICE (see Section 3.5.1).

3.2.6 Tests of the front-end electronics

A prototype of the FEE has been built and successfully tested on RPCs in the laboratory and during theGIF runs (see Section 3.2.4), in the extreme conditions in the GIF area.


0

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40 45 50 55 60 65 70R (cm)

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(/cm2/

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60 65 70 75 80 85 90R (cm)

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(/cm2/

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Figure 3.21: Experimental and simulated (FLUKA) radial distributions of soft-background hits.

Table 3.4: Comparison of data and FLUKA simulation of the testLateral Pb Measured hit rate FLUKA hit rate FLUKA simulated fluxthickness charged gamma neutron

(cm) (104cm2) (104cm2) (104cm2) (104cm2) (102cm2)20 1.65 1.20 0.72 8.0 4.430 0.48 0.36 0.18 3.9 2.540 0.20 0.14 0.08 1.3 1.5


Figure 3.22: Main elements of a FEE channel prototype.

Table 3.5: Main characteristics of the FEE prototypeTechnology Discriminator Minimum width Monostable t p (max) Power

at thresh. level consumptionECL MAX9687 1 ns latch of 1 ns 200 mW

or AD96687 comparator per ch.

This prototype was developed for the streamer mode and is rather simple. The main elements arerepresented in Fig. 3.22. The outputs are grouped by octets.

The main features of the prototype are given in Table 3.5. It is built around a commercial comparatorin ECL technology (MAX9687 from Maxim or AD96687 from Analog Devices - 2 channels). TheECL technology is fast enough to handle RPC signals but the well-known drawback is its large powerconsumption (200 mW/ch for this prototype).

The difference of propagation time t p between channels does not exceed 1 ns. The output stage isable to drive differential ECL signals in long cables.

In the GIF test, very stable performance during the experiment was noticed. Full efficiency of thedetector and a time resolution better than 3 ns were reached without any correction on the start signaltiming delivered by a 50 cm long scintillator. Some cross-talk was observed in case of large pulses(1 V), affecting the cluster size quality: clamping diodes (D1) were added later on to limit the inputpulse height, and the cross-talk has become negligible.

Even though the prototype described in this section has given satisfactory results, new studies on amicro-electronics chip have been started. The reasons for this and the description of the chip are givenin Section 3.4.1.

3.2.7 Tests of the trigger cards prototypes

A prototype of the local trigger board (VME 6U format) has been built and successfully tested. A schemeis shown in Fig. 3.23 (see also colour Fig. III.2).

The prototype houses essentially two programmable ALTERA (FLEX 6012) chips which performthe L0-X (bending plane) and L0-Y (non-bending plane) local logic. The logical functions implementedin the ALTERA (see Fig. 3.23) are a little bit different from the one described in Section 3.4.2 but theprinciple and the technical solution are the same so that the extrapolation to the second generation of

3.3 Description of the trigger detector 167

!"

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!

"

#

$

#$

#%$

%&

'

(

(

!"#

$(

'#

Figure 3.23: Trigger electronics prototype.

prototype should be straightforward. All output information needed for the pt cut is available and can beread-out but the look-up table (see Section 3.4.2) is not yet implemented.

The prototype is designed to receive inputs from the RPC front-end electronics: 160 logical differ-ential signals (a bit-pattern configuration) from X and Y strips are processed simultaneously at the clockfrequency of 40 MHz.

For test purposes, it is however more interesting to have the possibility to choose the input config-urations. Hence a dedicated bit-pattern generator has been built (VME 9U board). Selected bit-patternconfigurations are loaded in the memories (12 memories of 32K words of 16 bits) of the generator andthen transmitted to the trigger card at the clock frequency. The expected outputs are software computedand compared to the measured ones. The LabView VME interface is used for loading the bit-pattern con-figurations in the generator as well as for the read-out of the trigger card response, through a CAEN/V533VME module. The system allows up to 106 bit-pattern configurations to be tested per day.

3.3 Description of the trigger detector

3.3.1 General descriptionThe trigger detector of the dimuon arm is made of two trigger stations MT1 and MT2 respectively placedat 16 m and 17 m from the interaction point just behind the muon iron filter.

Each station consists of two identical trigger planes, positioned at 15 cm one from the other, which aremade of 18 single-gap Resistive Plate Chambers read out on both sides with strips in the two orthogonaldirections.

The active surface of the planes in the first station is 6.12 5.44 m2. All the dimensions of thesecond station are scaled by a factor equal to the ratio between the station positions along the beam axis,so the active surface of the planes in this station is 6.50 5.78 m2. A hole of diameter 96 cm is leftfree in the centre of the trigger to accommodate the beam pipe and shielding. With this configuration the


Figure 3.24: Structure of the trigger detector.

Table 3.6: Dimensions of the RPC modules (see Fig. 3.27)Chamber External dimensions Active surface

MT1, chamber 5 2230 720 mm2 2210 680 mm2MT1, chamber 14 and 69 2740 720 mm2 2720 680 mm2

MT2, chamber 5 2376 765 mm2 2356 725 mm2MT2, chamber 14 and 69 2920 765 mm2 2900 725 mm2

trigger detector defines a geometrical acceptance ranging from 2 to 9.6( 2 to 10.8) in the non-bending(bending) plane. It can be seen from the analysis presented in the chapter Physics performances thatthe tracking and trigger systems should both optimize the acceptance at around 2 in the bending plane.

The structure of the trigger detector is shown in Fig. 3.24. Each plane is divided into two halves inthe vertical direction containing nine chambers. The mechanical structure of each half-plane allows aslight geometrical overlap of the inactive zones on the top and bottom edges of the RPCs avoiding deadzones between two adjacent chambers. In the vertical direction there is a dead zone of about 4 cm widewhere the two half-planes are connected.

The sizes of the RPCs in the two stations are shown in Table 3.6. There are 4 small chambers and32 large chambers for each station, with a total of 72 RPC modules. To accommodate the beam shield,chambers 4 and 6 of each half-plane are slightly different than the other large chambers (see Fig. 3.24).

The RPCs are operated in streamer mode with a gas mixture of Ar/C2H2F4/C4H10/SF6 in the pro-portion of 49/40/7/4 as described in the previous paragraph. The gas and the high voltage are distributedto all chambers independently.

The read-out strip planes are divided into different zones in which the strips have the same widthand length; the strip widths range from 1 to 4 cm, from the inner to the outer part of the trigger planes.There are 21K read-out channels for the 4 RPC planes. The signal are shaped and discriminated by the


Figure 3.25: Cross section of a RPC module.

front-end electronics located just where the signals are picked up.The mechanical support of the two stations is tied to the muon filter and is divided into two parts

which can be separated and moved with each respective half of the muon filter in the perpendiculardirection to the beam axis. This open configuration allows the dipole magnet to be inserted, with asystem of rails, in between the two halves of the trigger and muon filter.

It is also possible to move each half-plane on rails in the beam direction to separate the planes insidea station for maintenance operations on the RPC modules and on the front-end electronics. This movecan be performed either when the muon filter and the trigger stations are opened, that is in the garageposition, or when they are closed in the data acquisition configuration.

3.3.2 Structure of the RPC modules

The cross-section of a RPC module is shown in Fig. 3.25. Each RPC element is made of a high-voltagemid-plane and two read-out planes. A couple of U-shaped steel girders give the element the necessarymechanical strength. The weight of each RPC is about 30 kg and its thickness is 40 mm.

3.3.2.1 High-voltage planes

The high-voltage plane consists of a couple of resistive electrodes made of 2 mm thick Bakelite foils andof a 2 mm thick gas gap. In order to keep the thickness of the gas gap constant over the whole detector,an array of cylindrical spacers (10 mm in diameter, 100 mm distant from each other) is inserted in thegap, as shown in Fig. 3.25 and 3.26.


Figure 3.26: Plan view of the internal part of the RPC high-voltage plane (not to scale). The gas path and asample of cylindrical spacers are shown.

According to the results of the tests reported in Sections 3.2.3 and 3.2.4, the RPC electrodes will bemade of Bakelite with resistivity of few 109 cm. To smooth out the inner surface of the Bakelite foils(i.e. the one in contact with the gas), this is painted with linseed oil. The outer surfaces of the Bakelitefoils are painted with graphite; one of the graphite layers is connected to the high voltage, the other toground. The resistivity of the graphite layers is such as to ensure, on the one hand, a uniform distributionof the high voltage to the whole electrode and, on the other hand, the transparency needed to pick up thesignal by means of the read-out strips. The electrical insulation of the high-voltage plane is achieved bymeans of an outer film of Mylar (PET).

In order to have a gas flow as homogeneous as possible, it has been decided to have a double inletand outlet of the gas for each RPC. For reasons of accessibility, all the four gas connectors are placed onthe RPC side opposite to the vertical plane passing through the beam axis. As a flux conveyor, a coupleof longitudinal spacer bars is used, as illustrated in Fig. 3.26, where the internal part of the high-voltageplane is shown. We note explicitly that, for the sake of a clear presentation, the drawing is not to scaleand only a sample of cylindrical spacers is shown.

3.3.2.2 Read-out planes

One great advantage of the RPC is that any cell of the order of1 cm2 of the total area can be consideredas an independent detector. Hence the read-out planes can be organized as needed.

The dimuon trigger setup has basically to measure the deviation of the muons, with the informationof MT1 and MT2 only. The strip width is then a compromise solution between many parameters likethe magnetic deviation of the particles, the distance between the trigger stations, the maximum deviationrange (8 strips, see Section 3.4.2 concerning the trigger electronics), etc. For example the strips arenarrower at small angle because the muon momenta are higher (the deviations are smaller): with the


Table 3.7: Read-out planes main specificationsNumber of read-out strips (total) 20 992

Strip pitch (mm) 1045Strip length (mm) 170720

Table 3.8: Read-out planes - number of strips.Strip X (horizontal) Strip Y (vertical)

Strip pitch (mm) MT1/MT2 10.6/11.3 21.2/22.6 42.4/45.2 21.2/22.6 42.4/45.2Number of strips 3840 8448 2688 3008 3008

Total 14 976 6016

chosen configuration, the deviation is almost constant over the whole area of the trigger, in terms ofnumber of strips, for a given pt (e.g. 46 strips for pt = 1 GeV/c). The minimum strip width is somehowlimited by the cluster size capability of the RPC detector but also because thinner strips would makeprohibitive the number of channels of the trigger.

The segmentation of the read-out planes has also been chosen to fulfil the following requirements:

occupancy in central PbPb collisions

connections to the trigger electronics which need a special arrangement of the strips on a givenRPC, modulo 16 for example in the bending direction (see Section 3.4.2).

The trigger has a total of 21K read-out channels made of aluminium strips. The two planes of one triggerstation, MT1 or MT2, have exactly the same readout planes. From one station to the other, the readoutplanes (and hence the strip length and width) are projective in X and Y relative to the interaction point(the projection ratio 1700/1600, from MT1 to MT2, is the ratio of the positions of the stations along thebeam axis).

The values of the strip pitch are given in Table 3.8. They increase from the centre to the edges ofthe trigger planes. The gap between two strips is about 2-3 mm. Each trigger station has only three striplengths: 170(180), 340(361) and 680(722) mm for MT1 (MT2).

A detailed description of the trigger segmentation in X (left) and Y (right) is shown in Fig. 3.27 forone quarter of MT1.

The two readout planes, in X and Y, are mounted independently (see Fig. 3.28) from the RPC high-voltage plane. A final readout plane includes:

The strips units, with the required segmentation. Each module is 3 mm thick. The strips are gluedon some foam on the side close to the RPC. The other side is an aluminized surface which isconnected to the ground.

The stiffener planes. They are made of foam of 1112 mm thick covered on each side with a thinaluminium layer.

The FEE connectors for the signal pickup.

The detector unit, HV and read-out plane, is then assembled with stainless steel U-profiles. The positionaccuracy of the strips on the High Voltage plane is expected to be better than 1 mm. This is ensured byspecial mechanical references positioned on the RPC and on the readout plane.

Because of the electrical segmentation, the strips are read out not only at the sides of the chambersbut also at different point on their surfaces. The signal pickup should then be done through the various


Figure 3.27: Trigger segmentation: upper quarter of MT1.

Figure 3.28: Assembly of a detector unit.


Figure 3.29: Signal pickup on the strips.

layers of the readout planes, with obviously an electrical connection to each individual strip insulatedfrom the back side of the strip unit. It should preserve the readout plane rigidity. The way to do that isshown in Fig. 3.29 where, from left to right, is shown the assembly procedure of the various elements,with the connection of the FEE printed circuit. These operations have to be simple because there areabout 42K connections of this kind (21K FEE channels + 21K resistors at the other end of the strip). Foran easy connection of the FEE boards, a tolerance of 0.1 mm should be imposed between two connectors.

The layout of the FEE boards on the detector unit is illustrated in Fig. 3.30 for the detector unit N5(see Fig. 3.27) in X (top view) and Y (bottom view). The FEE boards, outputs are organized by octets.The smaller boxes represent the boards supporting the resistive terminations while the larger boxes arethe FEE boards.

A test line is implemented on the same side as the resistive terminations. A fast signal, simulatinga RPC pulse, is sent and induced on the strips. The test signal distribution must be achieved with amaximum time difference of 34 ns between different strips. Such a test allows one to check the fullline, from the strip end to the DAQ, including time jitters in the FEE for instance. A remote controlsystem for timing compensation of FEE octets with a range of a few ns is presently under study (seeSection 3.4.1).

3.3.3 Half-plane structure

As stated in Section 3.3.1, where a general description of the trigger detector is given, the mechanicalstructure of this detector consists of eight independent half-planes. The nine RPC modules of each half-plane and the relative read-out, high and low voltage cables are attached to the same mechanical frame,the structure of which is shown in Fig. 3.31(a). It consists of six vertical rods with cave square section(30 30 3 mm) and ten horizontal rods with L-shaped section and equal flanges (50 50 4 mm).

The RPC and cable layout on the half-plane mechanical frame is shown in Fig. 3.31 and 3.32. Forreasons of positioning and alignment, each RPC element is independently movable with respect to themechanical frame. The vertical position of each RPC can be adjusted by acting on the two bracketswhich sustain the RPC and which can be moved up and down within a range of 10 mm. The horizontalalignment can be tuned directly by sliding the RPC on the brackets. For each detector, a position accuracyof the order of 1 mm is expected to be achieved.

Each RPC module and the corresponding horizontal rod of the half-plane frame lie at the oppositesides of the vertical rods (see Fig. 3.32). To minimize the overall thickness of the half-plane (about 120mm), the horizontal rods are used as a support for cables. There is one sheaf of cables for each RPCmodule; each sheaf runs on the corresponding horizontal rod towards the outer side of the frame, andthen vertically, as shown in Fig. 3.31, to reach the trigger electronics and the power supplies that areplaced on a platform on top of the half-planes (see Section 3.3.4). As explained in Section 3.3.2, each


Figure 3.30: Position of the FEE boards on the detector planes.

Table 3.9: Number of x and y read-out cables for each RPC and total number of read-out cables per half-planeChamber Number of x-cables Number of y-cables

Chamber 1 and 9 14 8Chamber 2 and 8 26 8Chamber 3 and 7 26 13Chamber 4 and 6 36 13

Chamber 5 30 10Total 234 94

RPC is equipped with x and y read-out planes. The x-plane is the one with the larger number of channelsand hence with the larger number of read-out cables. Each RPC module is mounted on the frame withthe x-plane close to the vertical rods of the frame. In this way, the x-plane cables are directly collectedonto the horizontal rod, while the y-plane cables are bent below the RPC to reach the horizontal rod. Thenumber of read-out cables for each RPC and the total number of read-out cables for a whole half-planeare given in Table 3.9 (the number of cables has been computed by assuming eight channels per cable).

3.3.4 Overall mechanical structure

The eight half-planes are independently suspended from an upper frame. There are two identical upperframes; each of them is fixed to the corresponding half of the iron wall (muon filter), as shown in Fig. 3.33(only the right upper frame is represented in this figure).

As already mentioned in Section 3.3.1, the half-planes have to be displaced independently in the beamdirection (z axis) to allow the access to the RPC and to the related front-end electronics for maintenance.As shown in Fig. 3.34, this movement is achieved by means of eight electrical motors (one per half-plane) whose pignons are engaged in a rack. The movement of the each half-plane is guided by a couple


cablecolumn

(a) (b)

beamaxis

Figure 3.31: Half-plane structure: bare mechanical frame (a) and mechanical frame equipped with RPCs andcable columns (b).

Figure 3.32: Side view of the half-plane structure.


Figure 3.33: Overall mechanical structure.

Table 3.10: Gas mixture for the trigger detectors% volume

Argon 49R134a 40

Isobutane 7SF6 4

Moisture 1%

of skids sliding on guides. The rack and the guides are fixed to a couple of IPE beams, which are in turnfixed to the iron wall.

Each half-plane is connected to the upper frame by means of a couple of cylindrical hinges. Thisallows one to tilt the half-planes from the vertical position (for displacement along the z axis and duringmaintenance) to the one orthogonal to the beams for data taking (we recall that the beams are inclinedby an angle of 0.8 deg. with respect to the horizontal plane).

In order to minimize the length of the read-out cables, the racks housing the trigger electronics areplaced on a platform on top of the upper frame, as shown in Fig. 3.35. The movement of the half-planesalong the z direction is allowed by eight cable chains (each chain houses the cables from an individualhalf-plane).

3.3.5 Gas distribution system

3.3.5.1 Introduction

The trigger system of the dimuon arm comprises four planes of Resistive Plate Chambers (RPC) assem-bled into two stations. These stations are located directly behind the muon filter. Each plane is split intotwo halves, and each half is built up from nine individual chambers. The preferred gas mixture for thetrigger system at the present time is shown in Table 3.10.


Figure 3.34: Details of the half-plane movement system.

Figure 3.35: Side view of the overall mechanical structure.


Table 3.11: Design parameters of the trigger gas systemTotal volume 0.286 m3

Number of chambers 72Chamber volume 0.00397 m3

Volume exchange/day 8Total flow rate 95 l/hr

Working pressure 1 mbar

The proposed design consists of a single-pass gas system, which distributes gas to individual cham-bers. The chambers will have eight volume changes per day of gas. The working pressure of the chamberswill be 1 mbar. The design parameters of the system can be seen in Table 3.11.

Primary mixing and pressure regulation will be carried out in the surface gas building (SGX). Thefinal distribution and flow control to the individual chambers will take place on the shielding plug areain the access shaft (PX24), which is accessible during LHC machine operation. Since the proposedgas mixture is flammable, the return gas from the chambers will be exhausted into the dedicated gasextraction system located in the shielding plug area. An overview of the system can be seen in Fig. 3.36.

3.3.5.2 Pressure regulation and mixing

A standard LHC gas-mixing unit is proposed (see Fig. 3.37). The pressure of the incoming gases willbe regulated by the pressure control valves, before they pass into the mixer stage. The flow of thecomponent gases will be metered by mass flow controllers, which have an absolute precision of 0.3% inconstant conditions, and are monitored by a process control computer. The mixing unit can work eitherin a constant ratio mode, or, alternatively may be derived from comparison of the running mixture to areference gas mixture. Normal flow rates are typically about 30% of full-scale flow. The medium-termstability in constant flow conditions is better than 0.1%, absolute stability will depend on the absoluteprecision of the analysing instrument. The RPC chambers performance could be improved if neededwith the addition of moisture to the gas. After primary mixing a certain percentage of gas will be deviatedthrough a bubbler containing water at a fixed temperature, this could give up to 1% moisture content tothe gas.

3.3.5.3 Gas purity and analysis

On-line analysis will monitor the incoming gases for adherence to specification. Automatic shut-downand alarm will result if any of the primary gases falls outside of the prescribed limits. The gas mixtureis also analysed for any deviation from the required mixture, by comparison with a pre-mixed referencegas.

3.3.5.4 Distribution system

Pressure regulation and flow distribution to each plane will take place on the shielding plug area. Gasentering the distribution manifold from the mixer will pass through a common pressure regulator beforebeing distributed to each plane. A safety relief device is incorporated into the distribution manifold toprevent over-pressure of the chambers, this device will either be a simple relief valve or a rupture discdepending on the pressure limits.

Each distribution channel will incorporate a throttle valve to regulate the flow of gas to each of thefour planes (see Fig. 3.38). If a suitable low-cost differential flow-meter can be obtained, each of the fourdistribution channels will be monitored allowing a direct comparison between inlet and outlet.


control room

IP

UX25MAGNET

PX24

PLUG

SG2

Gas Extraction System

Gas Distribution Unit

Gas to DistributionLength ~90m

Gas Flow & ReturnLength ~130m

Trigger Chambers

Gas Supplies Mixing Unit

Figure 3.36: Overview of the trigger gas system.

Figure 3.37: Gas mixer unit located in the surface building.


Chamber Gas FromSurface Building

PLUG (PX 24)

CAVERN (UX 25)

BufferVessel

EXHAUST

Low Pressure Collector

Chamber 1

Chamber 2

Chamber 3

Chamber 4

Chamber 5

Chamber 6

Chamber 7

Chamber 8

Chamber 9

PLANE 1 RightPLANE 1 Left

Chamber

Gas FlowGas Return

Plane 2

Plane 3

Plane 4

Plane 2

Plane 3

Plane 4

Argon Purge Supply

To Plane 1 Right

Ch 9Ch 1 Ch 1 Ch 9

Local Extraction

Controller

Differential Flow

Bubbler

Pressure Protection

From Plane 1 Left

From Ar Purge

NO

Figure 3.38: Flow distribution for plane 1 of the trigger chambers.

Further distribution manifolds will be incorporated into the system, and will be installed in closeproximity to the detectors. These manifolds supply gas to each of the nine individual chambers on eachhalf-plane. Each channel has a short flexible metallic pipe with self-sealing quick-connector on bothinlet and outlet lines. This permits individual channels to be disconnected from the circulation systemfor flushing with inert gas and exhausting to a separate vent line.

The four output pipes return the gas from the detectors to the plug area. Back pressure regulatorsin series with a membrane pump, control the pressure to 1 to 2 mbar below atmospheric pressure atthe inlet to the pump. Allowing for the pressure drop in the return pipes, the working pressure in thechambers can be maintained near to 1 mbar at nominal flow rates.

Since the gas mixture proposed is flammable, exhaust gas from the pump outlet will be exhaustedinto the dedicated extraction system on the plug, this extraction system is designed specifically to handleflammable gases.


Table 3.12: Main piping parameters of the trigger gas systemNo. Size

SGX-Plug 1 22/20mmPlug-Manifolds 4 16/14mm Flow

4 18/16mm returnManifolds-Chambers 72 6/4mm Flow

72 10/8mm Return

3.3.5.5 Distribution pipework

All pipes, valves and fittings within the system will be made of stainless steel. Existing gas pipes at point2 will be re-used as far as possible. Table 3.12 shows the main piping parameters. At the shielding plugend they will be modified to link up to the new distribution rack. In the Experimental Area (UX25) theywill be extended onto the Muon Arm structure and up to the chambers.

3.3.6 High and low voltage

A high-voltage channel is foreseen for individual RPCs, thus the total number of high-voltage channelsis 72. The RPCs will be operated with a high voltage of 99.6 kV (see Section 3.2.3) and the currentabsorbed by each RPC is expected to be of the order of 100 A. Therefore commercial power supplieswith Vmax = 1215 kV and Imax = 1 mA per channel are well suited for our purposes. The HV system isremotely controlled and the voltage and the current of each channel are continuously monitored.

The main low-voltage needs are for the front-end electronics (FEE) and for the trigger electronics.Industrial solutions already exist for the power supply units as well as for their slow control.

The front-end electronics supplies can be put close to the detector. The estimated level of radiationis quite low at this place: less than 0.1 Gray and 1010neutrons/cm2 integrated over 10 years. The totalpower consumption is less than 3 kW under 3.5 V.

The VME crates housing the trigger electronics have their own power supplies, cooling and slowcontrol system. They are located on top of the trigger.

3.3.7 Monitoring of the RPC efficiency

A system for measuring and monitoring the efficiency of each individual RPC module during data takingis foreseen. Two options are being considered. The first one consists of eighteen telescopes, nine foreach half (left and right) of the trigger detector, as shown in Fig. 3.39. Each telescope is made of threeplastic scintillators or plastic Cherenkov counters, to select muons coming from the interaction point.For this purpose, each telescope is tilted by a different angle, depending on its position [see Fig. 3.39(a)].The telescopes are mounted on the same vertical rod that can be moved horizontally [see Fig. 3.39(b)]in such a way so as to explore, in turn, different regions of each RPC. The second option consists of twotelescopes, one for each half of the trigger detector, that can be moved both in the vertical and horizontaldirection. The former movement allows one to measure, one at a time, the efficiency of different RPCs,while the latter is the same as for the first option.

3.4 Description of the trigger electronics

3.4.1 Front-End Electronics (FEE)The main numbers concerning the FEE are summarized in Table 3.13. As previously discussed in Chapter3.2.6, after the tests of prototype no. 1 which was made with commercial components, it has been decided

3.4 Description of the trigger electronics 183

9

8

7

6

5

4

3

2

1

MT1 MT2

T9

T8

T7

T6

T5

T4

T3

T2

T1

MT1 MT2

T1-T9

(a) (b)

side view top view

Figure 3.39: Pictorial view of the system (first option) for monitoring the RPC efficiency for one-half of thetrigger detector: side view (a) and top view (b).

Table 3.13: Front-end electronics: main specificationsNumber of front-end boards (8 channels) 2624

Technology BiCMOS 0.8moutput pulse width 20 ns

Max. consumption per channel 100 mW


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Table 3.14: Local trigger cards: main specificationsNumber of local trigger cards 234Decision time (fixed latency) 300 ns

Working frequency 40 MHzNumber of crates - 9U VME format 16

to design a micro-electronics chip for the following reasons:

to lower the power consumption. A factor 2 reduction is expected as compared to prototype no. 1

to include more functionalities in the chip, with a final cost comparable to that of prototype no. 1

The integration question is not the main motivation in this case.

The design and the software tests of the chip with CADENCE are now almost completed. A first proto-type should be available at the end of 1999. Two additional functions, as compared to prototype no. 1,are presently implemented in the design:

a remote control delay, programmable up to 50 ns, common for a whole octet of FEE. It could beused to compensate for any possible source of time delay.

a one-shot system which prevents the chip from re-triggering during 100 ns.

A sketch of the circuit is presented in Fig. 3.40.Simulated examples of input/output are shown in Fig. 3.41. The amplitude of the input signal varies

from 40 mV (threshold) to 200 mV, with a constant rise time of 2 ns and constant width of 5 ns FWHM.The input signal is repeated every 15 ns. The width of the output signal is constant and the time slewingeffect can be observed. The output signal is also delayed according to a pre-defined value. The effect ofthe one-shot is clearly illustrated.

3.4.2 Local trigger cards

The main numbers and parameters concerning the local trigger cards are summarized in Table 3.14.The local trigger card receives digital signals from the FEE called a bit-pattern sequence. It houses

a pipelined memory where an exact copy of this information, the local pattern, is buffered and read outon occurrence of a L1 ALICE trigger (see Section 3.4.4).


Figure 3.41: Examples of input/output (simulations, see text).

The main function of the local trigger card is to deliver the local L0 decision after going throughthe trigger algorithm. The trigger algorithm describes all the operations done on the input bit-patternsequence up to the local L0 decision which include:

the local logic

the pt cut

In the following the bit-pattern sequence received by each local card and the trigger algorithm are de-scribed.

3.4.2.1 Description of a local trigger card

The main Input/Output of a local trigger card are described in Fig. 3.42.The front side receives general signals such as the LHC clock (through the TTC interface) and the

ALICE L0 trigger. It also receives:

16 bits (2 front-end octets) from X strips of each detector plane. This information comes directlyfrom the FEE via cables.

16 bits (maximum) from Y strips of each detector planes. This information comes directly fromthe FEE in general, but it happens that different local boards use the same Y inputs. A copy of theY input pattern is then done in each local board.

The part of the rear side not occupied by the VME bus is also used. It receives X and Y inputs from (andgets ready outputs for) neighbouring boards. This kind of input/output concerns only the trigger stationMT2 and is obviously needed to account for the particle deviation (maximum 8 strips i.e. one FEEoctet) from MT1 to MT2.


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A total of 48 connectors, 16 contacts each, are supported by a local board: small connectors and alarge board format like VME 9U are necessary. All the connectors of a given board are not used but arekept for standardization.

The VME bus is used to transmit for instance the local bit-pattern and trigger decision to the regionalboards (see Section 3.4.3).

3.4.2.2 Description of the local trigger algorithm

The trigger algorithm, which will be described, has been intensely optimized by simulation. However,because some cuts may change or may be missing, it is very important to have a programmable elec-tronics which offers very flexible possibilities and allows re-programming of the trigger algorithm at anymoment.

1- The local logic

The trigger logic is programmable (FLEX ALTERA) and works at 40 MHz. A general scheme canbe seen in Fig. 3.43 for the local logic in the bending plane L0-X and in the orthogonal plane L0-Y.

In the bending plane L0-X, each trigger card collects 16+16 data points from MT1, plane 1 and 2,and 32+32 from MT2 (directly from the FEE and from copies, see Section 3.4.2.1). A few steps ofprocessing are done:

Declustering. If N is the number of adjacent strips hit, the declustering algorithm described on topof Fig. 3.44 is applied. For N = 1 and N = 2, the centre of the cluster is selected: experimental mea-


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surements show that a cluster with N = 2 corresponds to a particle crossing the detector betweentwo strips [27]. The algorithm is (2N - 5) for N 3. This kind of algorithm is very powerfulbecause the position resolution of the trigger is enhanced but an additional bit is needed to definethe middle of two real strips. Indeed, the remainder of the trigger logic has to carry double bitpatterns: 31+31 for MT1 and 63+63 for MT2.

Mini-road and DS reduction. A mini-road 1 strip wide (2 bits after declustering) is openedbetween the two planes of the same station. This is necessary to account for the particle deviationbetween these planes separated by 15 cm. This leads to the introduction of the notion of Singles(S) and Doubles (D), illustrated in the bottom of Fig. 3.44. The general cases are given in thefirst three columns and the parenthesis indicate the width of the mini-road. The output pattern, interm of Singles and Doubles, is given in lines 3 and 4. The last two columns are more complexexamples. The bit-pattern of the Singles and Doubles per trigger station is then substituted for theinitial bit-pattern.At this stage the so-called DS reduction is applied: if the bit-pattern of the Doubles is non-zero, and only in this case, the bit-pattern of the Singles is reset. Examples of DS reduction areshown in the last line of Fig. 3.44. This is obviously very efficient for reducing the effect of thesoft-background hits on the trigger rates without compromising on the signal detection efficiency.

Roads 8 strips with 3/4 majority. For any Single or Double on MT1, a road is opened with afixed width of 8 strips (actually 15 bits after declustering) which is the maximum range. Avalid road requires the 3/4 majority condition i.e. S-D, D-S or D-D respectively on MT1-MT2.The full road width is indicated in Fig. 3.45. The vertical bar indicates the zero-deviation bitof MT2 in correspondence with the considered bit of MT1: the road defined by these two bitspoints back to the interaction point in a straight line. Two open roads are shown in the example


+

+

+

+,

+-

Figure 3.44: Local logic in the bending plane L0-X.

! Figure 3.45: Example of roads in the bending plane L0-X.

of Fig. 3.45: the left one is not valid because the 3/4 condition is not fulfilled but the right one isvalidated by one or the other Double of MT2.

Minimum deviation. The computation of the deviation is the last stage of the L0-X logic. Eachcircuit actually calculates the minimum deviation which corresponds to the track with the higherpt (that has to be kept). Still taking the example in Fig. 3.45, the final answer of this circuit isa valid road for bit 2 (from MT1) with a deviation of 3 bits (from MT2). The sign of thedeviation is associated to the particle charge with four possibilities: ,, 0 (empty road) and (particular case of the zero-deviation). The bit number (1:31) from MT1 and the minimumdeviation (15:+15) from MT2 of the valid road are coded in two words of 5 bits each.

In the non-bending plane L0-Y, each trigger card collects a maximum of 16 data points from eachplane. The processing steps are the following:

declustering. Same as L0-X.

mini-road 1 bit (after declustering) and DS reduction (not mandatory). roads 1 strip (2 bits after declustering) with 3/4 majority. This road is introduced to account

for any possible deviation coming for example from multiple scattering, the magnetic field in non-bending plane or bad alignment of the strips, etc.


random choice in case of more than one valid road per circuit (no deviation criteria). On outputthe bit number (131 max.) from MT1 of the valid road is coded in a 5 bit word.

2 - The pt cutThe local logic described previously selects tracks with a loose cut on the corresponding transverse

momentum (see Fig. 3.54, simulation results). The second step of the local trigger algorithm concernsa more precise pt estimation. It is performed thanks to Look-Up Tables (LUT), which are static randomaccess memories (SRAM). The principle of this pt estimation is based on the fact that each detection cell(XCOORD, YCOORD, Minimum Deviation) corresponds to a single value of the transverse momentum.This value could be estimated by the formula given in Section 3.1.2. However, the flexibility of thechosen system (loaded LUT) allows the use of a full GEANT simulation of real muon tracks, includingin particular the actual field map of the dipole. The loading of the LUT will be performed, via the VMEbus, from a program implemented on a computer in charge of this process. Since this loading lasts onlya few seconds, the LUT content could easily be updated depending on running conditions.

At this level, two different thresholds are considered in order to select muons coming from the decayof the J/ or resonance families. The corresponding values are roughly the following (see Fig. 3.55,simulation results): pt 1 GeV/c as low threshold, for the J/

pt 2 GeV/c as high threshold, for the

For each threshold, the (X, Y, Deviation) configuration of the current output of the L0 logic addressesthe LUT and the local trigger decision is issued. The LUT has a size of 128 Kbits:

215 bits for all possible combinations of bits delivered by the local L0 logic (5+5+5) (2 2) bits for the low and high pt local trigger. The four possible outputs, for each threshold,

are: no trigger, trigger with or deviation, trigger with deviation. This information is usedby the regional logic.

3.4.3 Regional and global triggerAs seen in the Section 3.1.2 overview, the dimuon trigger signal is built via a 3-step process:

the local trigger

the regional trigger

the global trigger

The first step is performed by the local algorithm. It consists of the single-track pt cut presented in theprevious section. The last two steps concern the elaboration of the global dimuon trigger thanks to anintermediate level, the regional one.

3.4.3.1 Regional trigger cards

The goal of the so-called regional trigger is to provide an intermediate trigger signal in order to dealwith the 234 4 bits coming from the local cards. The corresponding regional card merges theinformation from the local cards of each crate. The 234 local cards are distributed amongst 16 crates,each crate holding at most 16 local cards plus 1 regional card. The information from the local cards istransmitted via the back plane bus (64 wires at the most).

At this regional level, an event with more than one good track per regional crate has to be considered.Actually, the only relevant combinations are the following:


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