2£l52>.2Il

Contactman: U. Goerlach OCR OutputSpokesman: G. London

University of Torino and INFN, Torino, ItalyL. Riccati, E. Scomparin, E. Vercellin

G. Dellacasa, P. Giubellino, F. Martelli, M. Masera, L. Ramello

University of Stockholm, SwedenS. Nilsson, B. Sellden

CEN DPHPE, Saclay, FranceJ. Bystricky, A. Gaidot, G. London, J.P. Pansart

University of Rome and INFN, Rome, ItalyS. Di Liberlo, F. Meddi

Lebedev Institute, Moscow, USSRl. Gavrilenko, S. Muraviev, V. Tikhomirov, A. Shmeleva

Moscow Physical Engineering Institute, USSRB. Dolgoshein, V. Tcherniatin, S. Smirnov, A. Sumarokov

University of Montreal, Montreal, CanadaP. Depommier, J. Gascon, L.A. Hamel, J.P. Martin, L. Lessard, P. Taras

McGill University, Montreal, CanadaA.L.S. Angelis, C. Leroy

CERN, Geneva, SwitzerlandH. Beker, C.W. Fabian, U. Goerlach, M.A. Mazzoni, G. Poulard

University of Bari and INFN, Bari, ItalyM. Gallio, M.T. Muciaccia, S. Simone

University and INFN TorinoUniversity and INFN Roma · Saclay CEN DPHPE - University of Stockholm

,_,. University of Montreal - Moscow Physical Engineering Institute - Moscow Lebedev InstituteUniversity and INFN Bari - CERN - McGill University Montreal

WITH AN OPTIMIZED HELIOS MUON SPECTROMETER

SULPHUR-NUCLEUS COLLISIONS

MEASUREMENT OF LOW MASS MUON PAIRS IN

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CERN/SPSC/88_43CERN LIBRARIES, GENEVA; {/QjggF25:

%>\ EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH(;l< ll,.

very forward rapidities. OCR Outputexplore the rapidity dependence of the signal from high energy density at nearly central up toacceptance will be possible. The wide range of rapidity from 3.5 to 6.0 will enable us tospectrometer, the measurement of the low mass dimuon continuum with better resolution andfor its ability to reject non—target dimuons. With the upgrade of the HELIOS muoncovering the muon rapidity acceptance. A strong magnetic Held at the target is being evaluatedoptimized distance to the target, and tagging of central collisions via multiplicity measurements

We propose a modest upgrade which combines a carefully designed light absorber at an

setup for the measure of muons (i.e. dilute high Z absorber).thermal behavior. However, this analysis is hampered by the less-—than—ideal experimentalmajor aim of the on-—going analysis is to investigate the possible signs of collective and

A low mass, low pq- dimuon signal has been observed in the HELIOS/2 experiment. The

signs of quark gluon plasma formation in nucleus — nucleus interactions.Dileptons are a unique and specific tool to detect collective behavior and to probe for

Abstract:

CERN

known sources, such as Dalitz decays of n and w mesons and hadronic bremsstrahlung, the production OCR Output

seen both in p. "';t ` and e+e" charmels, and at rf s nom 4.5 GeV to 63 GeV. After subtraction of

masses < 0.6 GeV/cl, i.e. below the known resonance region [9]. This low mass continuum has been

rahlung. In addition, a number of experiments have reported observation of a dilepton continuum with

The well—understood sources of low mass lepton pairs are meson decays and hadronic bremsst

1.2 Dilepton Continuum at Low Masses in p- p and p -A Interactions

pendence of thermal photons, real or virtual, on the multiplicity.

No other evidence of collective behavior has been shown as yet, in particular a non—linear de

tinguishing.

an indication of parton deconfmement. Other explanations have now been proposed which need dis

been observed in the rapidity region 35 y5 4, especially for low pT(J/*1* ) [14]. This was predicted as

A suppression of .}/*1/ production for central collisions, as compared to more peripheral ones, has

eH`ects in ultra —relativistic nucleus - nucleus interactions.

tendency in the region 0.55 y5 2 [13]. lf this tendency is confirmed, this would be proof of collective

There is a tendency towards thermalization, at least in the rapidity range 25 y5 3, with no such

could be induced.

energy densities reached are estimated to be at the level at which a transition to a Quark Gluon Plasma

states of high energy density can be created rather frequently in Oxygen and Sulphur collisions. The

The analysis of the energy flow and multiplicity distributions show that, in central collisions,

polation of known mechanisms in p —nucleus interactions.

The global characteristics of the events have been measured and described successfully by extra

and 1987 at the CERN SPS.

Much experimental information is now available from the first two runs of nuclear beams in 1986

1.1 The CERN Pilot Program (1986/1987)

l. Introduction

CERN

where E = mTcoshy, making a natural connection to the photon PT dependence. OCR Output

stituent scattering. The mT dependence comes simply hom the Boltzmann factor exp(—E/T)

contrast to Drell—— Yan production, characterized by a strong `/ s—dependence typical of con

Thermal production of { +! ` pairs depends only on the transverse mass at a Exed y, in

sions can be described from the superposition of nucleon-nucleus collisions [1 1].

associated larger volume of the plasma while only a linear increase is expected if nuclear colli

The rate of I `l` I ` is expected to increase with the square of the total multiplicity due to the

distinguishes itself from other sources in three important ways:

tions, in probing the interior of the hot quark-gluon plasma. Dilepton production from a plasma

ment since (together with y.’s) they are unique, due to the absence of significant &nal—state interac

Dileptons from qi annihilation are proposed as a signal for collective behavior and for deconfxne

1.3 Dimuons, a Powerful Tool to Probe the Quark- Gluorz Plasma

urement of dimuon production in the forward hemisphere as a function of multiplicity.

In the upcoming sulphur run, we wish to continue and to concentrate our eiforts on the meas

aim of the proposal described here.

actions (one of the aims of the HELIOS/2 program). An optimization of the latter experiment is the

i.e. p-nucleus interactions (one of the aims of the HELIOS/l program) and nucleus—nucleus inter

therefore should be also studied in interactions with a larger volume and greater degree of equilibrium,

time is long enough. A similar, but statistically marginal, result has been seen with soft photons [18]. It

nation if the origin of these pairs is the annihilation of produced qq pairs [4], i.e. the hadronization

pairs on the associated charged multiplicity in the central region, Fig. 1.2. This can be given an expla

A startling result has been observed at the ISR [9]: a quadratic dependence of low mass lepton

connected to the observation of soft photons at low pT [18].

see Fig.l.l. lf normalized to the pion cross section, it is independent of `/ s. The continuum may be

Drell—Yan process. The low mass continuum is produced centrally and shows a l/MZ dependence,

rate of these pairs is up to two orders of magnitude larger than what would be expected from the

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resolution etc. OCR Output

possible choice for the muons. This leads to problems in the reconstruction of the vertex, the mass

measuring the missing energy. However, &om the point of view of multiple scattering, it is the worst

comes from the interest of HELIOS 1 in measuring electrons in the same acceptance as muons, and in

(ULAC) and two uranium- scintillater calorimeters (Beam and Veto). The main reason for this choice

absorber in front of the muon spectrometer is composed of a uranium-liquid argon calorimeter

The present set-up of the muon spectrometer is optimized for the HELIOS/l experiment. The

1.4 Modest Upgrade with Maximum Effect 0n Data Qualizjy

to a proposed measurement of electron pairs in a narrow rapidity region ofthe highest particle density.

baryon density are probed. We believe that our present data and proposed upgrade are complementary

tance increases constantly. Due to the large rapidity acceptance, both high energy density and high

the very central rapidity with reasonable acceptance (see later). Towards forward rapidity the accep

this rapidity cutoff is at about 3.5 depending on MT. This means that our spectrometer almost reaches

herent momentum cut off by the energy loss in the absorber. For the llA absorber considered here,

A low mass dimuon experiment is limited in acceptance towards small rapidities due to the in

to the temperature.

important parameter in the QCD phase diagram for the formation of quark gluon plasma, orthogonal

we enter the regime of high baryon densities. The net baryon density reached in a system is the second

to 4.5 where the rapidity density is still about 0.5 of the maximum value. At more forward rapidities,

stopping. The width of the regime of high density is very broad reaching up to forward rapidities of 4

idity of the nuclear system. For the sulphur-tungsten system the central value is 2.6 for complete

States of high energy densities are indicated by high hadron rapidity densities close to central rap

be suppressed.

• Since there is no confinement in a quark- gluon plasma, one would expect the resonances to

CERN

preliminary result of the previous sulphur run in 1987 in the next chapter. Some of the results of exOCR Output

Before we describe the proposed experiment in more detail in chapter 3, we will briefly report our

suppression via the imposition of a strong magnetic Held between the target and the dump.

are studying two options concerning this background: a direct measurement with hadron beams and a

'I`he dump - generated muon background is important and not well understood quantitatively. We

higher multiplicities must then be handled in the spectrometer. See Appendix A.

A shorter dump would enable us to approach the central region with better mass resolution but

the shorter target absorber distance will considerably improve the combinatorial background.

trometer system at an acceptable level. This geometry will give us a much better mass resolution and

which we experimentally found to be of adequate thickness to keep the particle leakage into the spec

11A, 25 cm down stream of the target. The total interaction length is the same as our present dump

\Ve will concentrate on replacing the present calorimeters by an Alumina (Al2O3) absorber of

of acceptance in mass is necessary, and that we have provided for this design.

mechanisms contribute to this distribution. In order to begin to investigate this effect a very wide range

there is physics to be learned by studying the PT distribution at fixed MT [19], because different

before accurate measurements of the cross—section will allow to make a subtraction. ln addition,

background due to products which may be enhanced in nuclear collisions, and it will take some time

cesses, ir1 particular particle decay or simple combinatorial background. It is important to control the

and about 400 MeV, giving a mass distribution which is quite distinct from that given by other pro

ing the characteristic effect of the virtual photon pole in muon pairs with eH`ective mass between 2m,L .`

son is that it allows a positive identification of the electromagnetic nature of the muon pair by observ

This has two purposes: first, of course, is to allow the largest possible range ir1 MT. The second rea

Our design has been chosen in order to allow measurement of pairs down to the MT = 2m

However we would like to point out the potential for further substantial improvements beyond this.

changes for the 1990 sulphur run, we propose only a relatively modest upgrade on this time scale.

muons in nuc1eus—nucleus interactions. Considering the limited resources and time available to make

see later. ln order to improve the quality of the data, we are lead to make a dedicated study of di

Despite these dillticulties, we are able to extract a prompt low mass dirnuon signal from ou.r data;

amount of ET determined in the electromagnetic section of ULAC which covers an approximate OCR Output

ond—1evel trigger requirement is at least two tracks in the muon spectrometer and at least a certain

at least two coincidences between corresponding elements of the H3 and H2 hodoscopes. The sec

required, each with a threshold of approximately to 5 charged particles. The muon first — level trigger is

petals which covers an approximate pseudo —rapidity region of 1.45 1;.5 3.7. At least three petals are

The muon interaction-·trigger involves a tiny hexagonal- shaped scintillator hodoscope with six

2.2 Trigger for this AnaLvsis

scopes (H3/H2).

veto calorimeters), the second-level trigger chambers (PC3/5/6) and the iirst—level scintillator hodo

particular, the positions of the target, the inter·action—trigger hodoscope, the absorber (ULAC/beam/

In figure 2.1, we show the parts of the present HELIOS/2 which concern the muon physics, in

2.] Present HELIOS/2 Muon Setup

get, and on 5% Pt and Ag targets. There is also data with no ET requirement.

ULAC. There are also in the pipeline approximately equal samples with this trigger on a 10% Ag tar

coincidence with a certain amount of ETCm, the ET measured in the electromagnetic section of the

The data sa.rnple considered is the following: 8 on a 10% Pt target with a dimuon trigger in32

2. Preliminary results with HELIOS/2

A, a possible further improvement of the spectrometer is briefly mentioned.

scheme of installation and our requests to CERN for beam time and quality in chapter 5. In Appendix

ment ends with a summary of the responsibilities of the institutions forming this collaboration, a

tensive Monte Carlo simulation leading to the proposed design can be found in chapter 4. The docu

CERN

is hadron decay in the 50 cm space between the ULAC and beam calorimeters (5.3 to 5.8m), a conseOCR Output

ment production, minimized however by the ET requirement. Another possible source which we study

the region 4 < Z, < 6 meters and at small radial distances. This could be understood as projectile &ag—

tribution of radial distance at the exit of the absorber (Fig.2.4). A small excess (5- 10%) is apparent in

There is a good reproduction of the measured Z1 distribution, (Fig.2.3), and of the measured dis

has been quite successful in understanding our p — Be data at 450 GeV/c and last year’s O — W data.

Z, distribution similar to muons coming from decays late in the shower development. This approach

We assume that the latter sample, which corresponds mostly to liadronic punch -·through, gives a

which do not penetrate the Fe wall.

• the sample of measured tracks which do not have a hit in a corresponding H2 slab, i.e. tracks

a signal; see next section), and

cays in flight (in principle, we should add a certain fraction of direct muons since we have such

• decay muons from the hadrons produced at the target, simulated by IRIS [15], and their de

sources to which the same cuts were applied:

(i.e. in the beam direction). We have compared the distribution in shape to two properly normalized

For each track, one calculates the Z, called Z,, for the distance of closest approach to the Z- axis

hits in corresponding H3 and H2 scintillator elements.

magnet and connected to the upstream chambers, imposing a number of quality criteria and requiring

Each track is reconstructed in the muon spectrometer in both projections downstream of the

2.3 Selection Criteria for Tracks from the Target

100 GeV with an average of about 70 GeV.

triggers (with and without ET). The threshold for the reconstructed ET varied in the run from 40 to

In Fig.2.2, we see the reconstructed (i.e. correctly weighted) ET distribution and its relation to the

among the chamber planes (PC3/PC5/PC6).

pseudo-rapidity region of 1.665 ng 4.25. Each track is determined by a straight line constraint

CERN

duction integrated over the mass range below the p. OCR Output

(d¤(pp)/dy)/(d¤(·n)/dy) = 4.0 x IO"), where (da(pp)/dy) represents the cross-section for pair-·pro

[1]. Relative to pions, its level in minimum bias collisions, is found to be (y` = O):

assumed a level which corresponds to the ’anomalous’ low mass pair signal observed in pA—collisions

timated to be the combinatorial background from pion and kaon decay. For the pair sigial we have

We have estimated S/B using a Monte Carlo (Chap.lV). The most important contribution is es

muon Z2 distribution.

derstood qualitatively since the decay muon Z2 distribution is shifted to higher Z2 relative to the direct

increase more rapidly. (Note that the errors are correlated, point —to ·—point.) The increase can be un

One notes the trend: S/B increases slowly from no Z2 cut to a Z2 cut of 4.5 m, and then begins to

like — sign pairs as a measure of the latter source.

two—track events composed of muons Brom the target and muons from decays in flight. We use the

We have studied S/B as a function of a Z2 cut, see Fig.2.6, to determine the cut which keeps only

of1ike—sign pairs (=B), 2.`/ (N+ +N- -).

Our signal (= S) is defined as the sample of unlike- sign pairs, N+ .. (= S+ B) after subtraction

tially no overlap in rapidity between the ET and dimuon measurements.

ceptance is well under control: y?_ 4.1, M; 0.3 GeV/cl and p·I—; 0.4 GeV/c. Note that there is essen

A study of the acceptance has lead us to determine a kinematic region in which the dimuon ac

2.4 A Low Mass Dimuon Signal

< Z2 > approaches the target, as expected.

Fig.2.5b, we present the average values of Z2 for different mass bins. We note as the mass increases,

Z, of the two tracks, weighted by their errors. In Fig.2.5a, we present the distribution of M vs Z2, In

comparing the < Z2 > of data samples defined by diH`erent mass bins, where Z2 is the average of the

Another important indication that we select target—produced dimuons (all charges) comes from

ground and the latter, uncorrelated background.

quence of the ’diluteness’ of our absorber; see next chapter. The former source gives correlated back

CERN

multiplicity dependence of the signal. We are encouraged to use dimuons to undertake a systematic OCR Output

ber. Presently we are working on the exact absolute level of the background and the signal, and on the

uurn in nucleus—nucleus interactions despite the tremendous multiplicities and an unfavorable absor

To summarize our present analysis: we have succeeded in detecting a low mass dimuon contin

2.6 Summary ofthe Present Analysis

after correction for acceptances.

error bars, by about 1.7. In Fig.2.8, we show the same spectra in the kinematically—reduced region,

probably be better estimated by using tracks from diEerent events. This would significantly reduce the

binatorial background subtraction was made bin-·by-bin. The spectrum of this background can

In Fig.2.7, we show the mass and transverse mass spectra for a Z2 cut of 4.5m in which the com

2.5 Calculation 0f Mass Spectrum

polate from hadron—nuc1eus interactions in our kinematical region.

Carlo would indicate that any quadratic dependence on the multiplicity must be small when we extra

Assuming that the absolute levels are satisfactory, the agreement between the data and the Monte

correct reproduction of the rapidity profile of our events.

solute multiplicity level can be deduced under the assumption that a given event generator gives a

most no overlap in rapidity with the dimuons. From a simulation of the calorimeter response, the ab

magnetic transverse energy measured ir1 the ULAC in the rapidity interval of 1.65 <n < 4.25 with al

measured, in our case y(pp)3 4.1. The events of our data sample have been triggered on the electro

we need to know the charged multiplicity ir1 the same rapidity interval where the muon pairs are

calculation is not yet finished; it is not straight-forward. To evaluate the absolute background level,

The result for S/B is qu.ite satisfactory but we need to reproduce the absolute levels of B. This

into the ULAC. We shall use a Z2 cut= 4.5m for the rest of this discussion.

The data in Fig.2.6 start to deviate significantly from the MC at about 4.5m, approximately 0.5m

10 CERN

overall view irom the top shows the dimensions of the proposed 11A alumina dump including the OCR Output

Fig.3.1. shows a schematic drawing of the new setup including the optional target magnet. The

3.2 [I A AIumina(Al2O3) Dump

cluding the muon spectrometer acceptance.

place the ET measurement by a direct multiplicity measurement covering the region 2.0 <n < 5.5, in

making model—dependent extrapolations of multiplicities in the dimuon acceptance, we intend to re

purposes: 1) selecting central collisions and 2) indirectly measuring multiplicity. In order to avoid

As we have noted, there is little overlap in our ET and dimuon measurements. ET serves two

overall dimensions in terms of interaction length, namely llA .

the maximum was already reached. Therefore we considered basically only absorbers with the same

the maximum multiplicity which can be tolerated in PCO and PC l. In the previous Sulphur running

imposes serious constraints to the layout of a new beam dump for early 1990. The main limitation is

The necessity of keeping the present muon spectrometer through 1989 for HELIOS/1 running

rating the diHerent sources of tracks found in the muon spectrometer as seen in the previous chapter.

From a qualitative point of view, the tirst point is the most serious since one has didiculty sepa

of the experiment limited the maximum beam intensity to lO° Sulphur ions per burst.

tracks and (2) poor resolutions (dimuon mass, etc). In addition, in the past run, the available shielding

The dilute high Z absorber leads to problems of (l) reconstruction of the vertex for individual

3.1 Limitations 0f the Present Spectrometer

3. Description of the new lay — out of the Experiment

without the shortcomings of the present set-up.

search for quark gluon plasma formation in the next sulphur run with an optimized spectrometer

ll

3.3 Layout 0f Target Area OCR Output

the observed hit distributions in PCO and PCI.

absolute level of charged hits in the chambers, the simulation gave reasonable results in agreement with

computations was gauged by a simulation of the present HELIOS geometry. It turned out that, at the

very low particle energies, thus obtaining realistic numbers for the multiplicity. The validity of our

multiplicity. It was possible, in particular, to simulate the last part of the hadronic showers down to

chambers. Therefore a complete shower simulation was done for various geometries to study this

against lateral shower leakage out of the Tungsten—rod which will increase the multiplicity in the Hrst

value of 10 mrad still gives sufficient acceptance at rapidity 5. This acceptance has to be balanced

dimension will limit the forward rapidity acceptance and should therefore be as small as possible. The

opening angle of 10 mrad. The dimensions of the Tungsten- core need some attention. The transverse

be larger than 3 times the z——vertex resolution. The length of the Tungsten—rod is 20A and has an

interaction point of leading fragments from the target region. The required separation was assumed to

The absorber has a central hole in the forward part followed by a Tungsten—rod to separate the

mina dump.

background from decays is controllable, so we decided to chose the better mass resolution of an alu

whereas the spatial resolution is basically unaffected. Our results have shown that the combinatorial

tribution of showers in the absorber. The price which has to be paid is the poorer mass resolution,

The effective decay length within the dump would be shorter by a factor of 2, reducing the decay con

The overall length of the layout could be reduced by about 30% if the Al2O3 is replaced by Cu.

background within reasonable limits.

iiciently short absorption length (A =29 cm as compared to 32 cm for HELIOS) to keep the decay

(X0= 7 cm as compared to 5 1 cm for the HELIOS absorber) to reduce multiple scattering and a suf

Alumina is chosen as the absorber material. This is a compromise between a long radiation length

(approximately IA thick) because it is desireable to have the option of varying the length of the dump.

central Tungsten rod. The outer dimensions are not yet optimized. The dump will consist of slices

l2 CERN

3.4 Trigger and Beam Intensizjy OCR Output

the multiplicity distribution is smeared.

A new version of the petal counter is required. The space for Si rings is reduced and, more important,

other rapidities being bent into the acceptance of the spectrometer by the magnetic Held at the target.

is necessary to surround the dump with some additional shielding material, to avoid particles from

The target layout is more complicated in the case of the target magnet. Studies have shown that it

A compact magnet with a somewhat reduced Held was studied and in fact seems to give good results.

considered a shirnmed PS magnet delivering the Held of 2.8 Tm as shown in Fig 3.3. Also a more

LHC magnets have a very limited acceptance and require some additional cryogenics. Therefore we

detail. The choice of the magnet was done on practical reasons. All presently available prototypes of

The option of a magnet generating a very strong Held at the target [2] was investigated in great

3.3.2 Target Magnet Option

about 1- 1.5 mm, similar in dimensions to the 1987 beam.

the position of the most forward detector. This restricts the required beam diameter (base width) to

measure up to very forward rapidities, the inner diameter will be of the order of 2- 3mm depending on

maximum beam diameter allowed is defined by the inner diameters of the ring counters. As we want to

The detailed geometry of the target area deHnes also the requirements on the beam quality. The

petal counter provides a.n interaction trigger.

tiplicity in the rapidity coverage of the muon spectrometer up to rapidity 5.5. As before, the scintillator

of two pairs of silicon ring counters. Silicon ring (or pad) counters are needed to measure the mul

magnet. Visible are the target, the tentative shape of the target magnet pole and the different positions

A more detailed drawing of the target area is shown in Fig. 3.2 which includes the optional target

3.3.1 General

CERN

4.1.1 The Dimuon Source (see Appendix B for details) OCR Output

interactions.

GEANT framework, the Fritiof Monte Carlo code was used to generate central and peripheral 5 Pb32

To study details of the proposed geometries with a full simulation of hadronic showers within the

of the rapidity and PT distributions will be improved in order to compare to our 1987 data.

of 1.7 was considered to give a reasonable representation of the shape. These simple parameterizations

and the dimuons generated in the primary collision. A Gaussian, centered at rapidity 2.6 with a sigma

muons from decays of pions, kaons and etas. The same rapidity distribution was used for the hadrons

leading to muon production. We simulated the production of dimuons in the target and the dump, and

tion and because it is easier to determine the relative normalization between the diHerent processes

Simple parameterizations of experimental rapidity and pj- distributions were used for fast genera

4.1 Event Generators

also [6].

We present a summary of extensive Monte Carlo studies of the proposed configuration [7], see

4. Monte Carlo simulation of the performance

Sulphur ions/burst, limited only by deadtime, but requires additional shielding of the target zone.

Based on our experience in 1987, our experiment can be run with a high bea.rn rate of 5xlO

as before. A second level multiplicity trigger from the ring counters will be available in addition.

scintillator petal counter. The muon hodoscopes and chambers provide iirst and second level triggers

teraction will be deined by a coincidence between the beam counters and a minimal multiplicity in the

The dimuon trigger will follow the present system. Only new beam counters are required. An in~

14 CERN

ference is the cut of at the 1; mass, resulting in a slightly steeper mass spectrum. OCR Output

lepton spectrum has the same virtual photon pole as the expected low mass continuum. The only dif

The decay of the ·q is an inherent problem to all low mass dilepton experiments. The Dalitz di

4.1.3 The Eta Dalitz decay mode

consuming, so only a limited sample of events was generated. See Appendix C.

decays from secondaries produced in the hadronic shower. Such detailed simulations are very time

A full simulation was carried out for some geometries, including the remaining background due to

in this fast simulation, contains only the decays of primary particles.

Erst interaction. No secondary particles were kept. Therefore the combinatorial background referred to

In the computations described here, primary pions and kaons were tracked by GEANT up to the

for the kaons.

slope of 6.0 for the pions (consistent with our results in the HELIOS/2 external spectrometer) and 5.3

tribution described above. The p-I- spectra were assumed to be purely exponential with an average

The decay background of pions and kaons (8% of pions) is evaluated assuming the rapidity dis

4.1.2 Decays of Pions and Kaons

spect to mass) was assumed to be 4xlO ` 5 dn(rr + )/dy independent of rapidity.

For the absolute normalization of the muon source relative to pions, the rate (integrated with re

,.. erably increase our real acceptances.

that there is some experimental evidence for an isotropic distribution [3] and [10]. This would consid

have a cos@ dependent acceptance, results are sensitive to the use of this distribution. \Ve are aware

ficient in front of a cos‘@ term. For example at M=4mlL this coemcient is 0.6. As all spectrometers

muon decay in its rest frame was taken from [5]. For our mass range, there is a mass dependent coef

The pq- distribution was assumed to be purely exponential in MT. The angular distribution of the di

l/ml, modified at the threshold due to the reduced phase space following a prescription given in [5].

as a "test’ source for the simulations. The mass dependence of the continuum dirnuons is described by

To study the response of our experiment to muon pairs we used the known low mass continuum

Drell Yan pairs. We believe that the addition of such sources will not change our overall conclusions. OCR Output

the mass resolution. At high masses the yield of dimuons will be mainly due to charm production and

the p decay which will start to dominate the low mass region above M(pp)= .65 GeV, depending on

Note that some sources of dimuons have not yet been implemented. One of the major sources is

4.1.5 Other Sources of Muons and Dimuons

so a good rejection is essential.

vertex into the iirst absorption length of the \V—plug. However, the absolute normalization is dimcult,

The generation of dimuons by leading fragments can easily be simulated by placing the primary

ln`/ s to take into account the \/ s —dependence of the multiplicity in the secondary interaction.

system of the iirst hadronic interaction of the pions in the absorber. These dimuons are weighted with

this source of dimuons cannot be completely ignored. Muon pairs were generated in the center of mass

interaction. Although there is some kinematic suppression against this process, calculations show that

Hadrons emitted from the primary collision can produce prompt dimuons in their lirst hadronic

4.1.4 Dimuons generated by Leading Hadrons and Projectile Fragments

modified at low p-I- according to MT scaling.

For our study we assumed that etas were produced with the same PT distribution as the pions but g

by one experiment at high pT [20]. Their value of 0.771- 0.23 is consistent with the pp value of 0.55.

assumption of MT scaling. I.n nucleus —nucleus interactions, the n production has been measured only

urement was carried out at low p·I— and central rapidity [16], which gives a level compatible with the

errors due to the uncertainty in the knowledge of the cross section at low pq-. So far only one meas

Experiments relying on the absolute normalization of the n production are aB”ected by systematic

son. The mass resolution obtained with the proposed dump may allow the use of this procedure.

the low mass pairs. It is clear that one needs a good mass resolution to be able to do such a compari

In [8], this diHerence in the spectral shape is used to exclude these decays as one of the sources for

CERN16

ground. OCR Output

If a magnetic field is present, the Z, distribution is improved, Fig.4.1.b, but a soh cut fiom back

well as on R-1-.

similar although not identical. We therefore use only a soft cut on the normalized Z1 distribution, as

lf the error expected from multiple scattering is taken into account, these distributions become very

difference in the distributions from diHerent sources is just a reflection of the diH`erent PT distributions.

signal and background. However, some caution should be applied since Z1 is correlated with pr1~. The

and dimuons generated by secondaries in the absorber. A cut on Z1 would allow some separation of

A We compare muons from dimuons generated in the target with muons from combinatorial background

in [12] a systematic shit} due to multiple scattering is expected for the z coordinate, visible in the plot.

is calculated by tracing back through the dipolar tield.

to finite extension of the magnetic field is taken out by a constant displacement of the distribution. RT

plane was determined, called RT. ln the case of a magnetic field at the target, the systematic shift due

beam axis was calculated, called Z1 as before. Secondly, the intersection of the vector with the target

The vertex reconstruction was performed as follows. Firstly, the point of closest approach to the

4.2.1 Single Muon Vertex Distributions

the analysis as already noted in chapter 2.

interactions. Therefore the vertex distribution of both single muons and dimuons plays a crucial role in

Many muons originate from the dump by either decays of shower particles or directly by hadron

ing a perfect reconstruction of the muon in the spectrometer.

from the W—plug. For the subsequent analysis, the four—vector after the absorber was used, assum

trical limits of the spectrometer. A cut was made at the exit of the absorber to exclude tracks exiting

lt was required that each muon pass through all chamber planes up to H2, H3 within the geome

4.2 Vertex Resolution and Event Selection

CERN

milliradian displacement of the beam spot is required. Only the pretrigger would be changed in that the OCR Output

e.g. 20, 50 and 80 GeV/c, aimed at diiferent spots on the face of our absorber. No more than a few

One way is to measure directly this background by using hadron beams of appropriate momcnta,

4.3.1 Direct Measurement with Hadrons

with this problem, one globally and one event —by- event.

later. Even with the target magnet, soft vertex cuts only get rid of 50%. We have two ways of dealing

Dump-produced dimuon background is quite important but its normalization is uncertain, see

4.3 The Problem 0f Dump — produced Dimuons

be seen in figure 4.3.b.

now this quantity a well defined value to study the vertex distribution of low mass muon pairs as can

coordinate Zminllowever, boosting the pair artificially to a higher mass with a magnetic field gives

defined for low mass pairs. This can be seen in figure 4.3.a., which shows the distribution of the 2

approach relative to each other, independent of the beam axis. In general this quantity is very badly

tex definition is used. It employs only the two vectors of the muon pair, dehning the point of closest

The power of a target magnet for the vertex reconstruction is also visible if another common ver—

shown in Fig. 4.2.b. Here, the soft cut eliminates about 0.5 of the dump produced dimuons.

50% of the signal, primarily at low mass (Fig.2.5). The same distribution with the target magnet is

as before. This is to be compared to the hard cut necessary in t.he present analysis which eliminates

tion than for Z1. We can normalize this quantity by the expected error and again apply only a soft cut

Fig 4.2.a shows the Z2 distribution for the 1lA alumina dump. We observe a narrower distribu

of the two muons, called Z2 as before.

of the pair was defined as the weighted average of the two points of closest approach to the beam axis

Pairs of muons were then formed in all charge combinations: (+ —), (+ +), (——). The vertex

4.2.2 Vertex Distribution of Dimuons

18 CERN

HELIOS configuration with the ll A Alurnina absorber. There is more than a factor 2 improvement; OCR Output

Fig.4.5.abc. presents the mass resolution in diB`erent mass intervals, comparing the present

method [12] to reconstruct the mass of the muon pairs.

ergy loss and L = path length). The best estimator of the angles was obtained by the Branson plane

to the empirical formula dE= L ( 1.18 + 0.051og(P)) dE/dx, (where dE/dx = minimum ionizing en

For the selected tracks, the momenta were corrected for energy loss in the dump, calculated according

4.4 Mass Resolution

vertex distributions.

and dump—produced pairs achieved with the mass boost is much better than the one obtained by the

spect to the 20% signal level without a magnet, see later. It is clear that the separation between target

sidering. The dump background is reduced to 1% , a gain of about 20 for the target magnet with re

In Fig.4.4.b, we compare the pseudo —mass distributions for the three sources we have been con

&om pairs originating from inside the absorber which see a smaller part of the field integral.

gain a boost in mass so the whole mass spectrum is shifted. This will only partially happen to muons

do~mass is shown in Fig.4.4.a. The result is that, on average, low mass pairs generated at the target

without correcting for the magnetic field. The relationship between the generated mass and the pseu

do —mass’, which is the reconstructed mass calculated directly from the four~vectors after the dump,

separation of the target dimuons and dump produced dimuons through the introduction of the ’pseu

A second method comes from the presence of the target magnet which would allow a significant

4.3.2 The Pseudo Mass Distribution with Target Magnet

global correction.

This would give several points of absolute normalization for our Monte Carlo calculation, giving a

The aim would be to obtain at least 100 dimuons in each configuration after the standard cuts.

Perhaps a large veto counter would be required as well.

petal counter would be replaced by a small counter which would move on the face of the absorber.

19

phasize. OCR Output

small mass and small rapidity, especially for small p·1—. This is just the region we wish to em

• Replacement of the caloximeters by a new alumina dump increases slightly the acceptance at

target magnet.

Let us now compare the present situation with a new 11A alumina dump, with and without a

the limits of the acceptance at low and at very forward rapidity.

is increased, a nearly flat p]— independent acceptance is obtained. This behaviour is a bit distorted at

the shape of the acceptance function. At small mass, the acceptance rises strongly with pT. Ifthe mass

the constant length of the dump of 11A. ln general, at iixed rapidity, there is a substantial change in

kick of the spectrometer magnet. This cutoff is nearly the same for all 3 coniigurations and given by

lower limit on the muon momentum coming from the energy loss in the absorber and the momentum

All acceptances display a similar feature: the inherent cutoff towards small rapidities due to the

the spectral weight given by the generator.

played as a function of mass, transverse mass, and rapidity, integrating over the other variables with

A second representation is given in the Figs. 4.6b, 4.7b and 4.8b. Here the acceptances are dis

net), each histogram giving the acceptance as a function of pT for a given mass and rapidity.

graphically in the Fig. 4.6a (present absorber), 4.7a (no target magnet) and 4.8a (with a target mag·

was made to account for the finite mass, p-1- or rapidity resolution. The information is displayed

numbers are the ratios, in percent, of the reconstructed pairs divided by the generated pairs. No cut

3.5, 4, 4.5, 5). Subsequentially they were tracked through the apparatus and reconstructed. The quoted

values of mass (0.25, 0.4, 0.6, 0.8 and 1 GeV/c‘), PT (0.2, 0.6, 1.0, 1.4, 1.8, 2.2 GeV/c) and rapidity (3,

The acceptances are presented in two diH`erent ways. First, dimuons were generated at discrete

4.5 Acceptarzces

resolution of the muon spectrometer, important for example at the J /*1* mass.

over the p·I— and rapidity distributions of the generated muon pairs. One should also include the iinite

in particular at the p mass, the resolution goes from 170 MeV/cz to 70 MeV/cl. The plot integrates

CERN20

at small rapidities. OCR Output

to the minimal energy cut off in the absorber. This changes specifically the sigial to background ratio

observed signal to background ratio. There is an implicit PT cut in the apparatus at lower rapidity due

tion of the prompt dimuon source. Therefore a PT cut applied to the data will strongly change the

The PT distributions of the various background sources are in general softer than the PT distribu

least a factor of 2- 3.

the rr"' multiplicity as those at the target. We estimate the uncertainty for this contribution to be at

secondary interaction. The dimuons from these interactions have the same strength, proportional to

multiplicity of 4 in the h—nucleus interaction at `{/s= 20 GeV, weighting the events with `/s of the

the interactions in the absorber of primary produced hadrons. \Ve assumed an average forward ·n"

By far the most diiiicult and uncertain contribution is the one of dimuons which are induced by

central pion multiplicity of 55 at the maximum.

assumed to be proportional to the charged pions independent of rapidity, using a value of the positive

All normalizations are done relative to the pion multiplicity. The strength of the prompt signal is

new configuration.

following we will first describe the method of relative normalization and then present the results for the

interactions in the absorber. The different event generators have been described in section 4.1. ln the

combinatorial background from K, rr decays, n Dalitz decays and muon pairs generated by secondary

In this section, an evaluation is done comparing the genuine prompt low mass dimuon signal to

4.6 Signal and Background, a Comparison

gating this option to optimize the magiet Held settings.

recogmition after the magiet. It may also cause a somewhat higher trigger rate. We are investi

this increased acceptance is vcry benehcial, providing we can continue to control the pattern

and low rapidity. Since the momentum resolution is not a major contributor to resolutions,

Lowering the muon spectrometer magietic Eeld increases the acceptance at low mass, low pT

The target magnetic Held increases the acceptance for low mass and low pT at high rapidity.

21CERN

performance is achieved. The optimal value of the magnetic field still has to be found. OCR Output

We also tested the same dump with a somewhat smaller target magnetic field. Still a very good

shown in Figure 4.12.ab.

tion of this contribution, as mentioned before. The range in MT accessible at rapidities below 3.7 is

ies. We consider this as very attractive because of the dimculty in calculating the absolute norrnaliza

feature is the complete removal by the pseudo -mass cut of all low mass pairs produced by secondar

higher level of the combinatorial background due to the improved low MT acceptance. The striking

Fig 4.1 labc. shows the results with the same dump with the target magnet switched on. Note the

20 — 25% level depending on the kinematic region.

300 MeV/cl. Again the n Dalitz decays and the background of secondaries produced dimuons is at the

ground ratio is obtained at low masses. The MT distribution reaches to much lower values, starting at

resolution (Fig. 4.10abc.), as well as the softer cuts employed in the analysis. A good signal to back

The reconstructed mass spectrum of this coniiguration reflects the better acceptance and mass

4.6.2 The Alumina Dump (ll Interaction Lengths)

the signal, see Fig. 4.9.ab.

data analysis, Fig.2.6. The n Dalitz decays and the dump—produced dimuons are at a level of 20% of

An overall signal to background ratio (M < 0.8) of 0.56 is observed in good agreement with the

disguised by the bad resolution. Therefore care should be taken when comparing with the data.

been included in the Monte Carlo yet, whereas this must be present in the data, obviously strongly A

quite distorted by the imposed cuts and the bad mass resolution, see Fig.2.7. No p ··production has

to decay background ratio of about one—half but the shape of the mass spectrum (originally 1/ml) is

The strong Z2 cut at 1.55 m relative to target 2 on the dimuons helps to achieve a relative signal

4.6.1 The HELIOS 1987 Configuration

except for the HELIOS 87 comparison where we consider dimuons up to rapidity 6.

mass for the mass interval below 800 MeV/ci. Generally we refer to the rapidity range from 3 to 5,

In the figures of this section, reconstructed spectra are plotted as functions of mass and transverse

22 CERN

target magnet. The geometry of the dump will be compatible with the installation of a target magnet, OCR Output

The option of a direct measurement of the dump dimuon background is proposed in addition to a

access to the passive 20A tungsten core.

mina dump. The description of the dump is in chapter 3. It will be variable in length and allow good

The first item to be designed, ordered and prepared for installation will be a segmented 11A alu

5.1 Organization of Installation

stood experiment for the next ion runs.

The collaboration commits itself to the goal of upgrading and operating a working and under

5. Operation of the upgraded Muon Spectrometer

range between 3.5 (MT > 600 MeV/cd) and 6.0.

either case, it is possible to do an excellent measurement of low mass dimuon pairs in the rapidity

duinp—produced dimuon is proposed to gauge the Monto Carlo simulations in a quantitative way. In

complicated multiplicity measurement, target area, shielding and analysis. A direct measurement of the

be obtained with a target magnet at the cost of a slightly higher combinatorial background and a more

___ ceptance at forward rapidities and the separation between target and dump produced muon pairs can

and a shorter dump target distance. An additional improvement of the vertex reconstruction, the ac

present HELIOS muon spectrometer can already be improved considerably by a new low Z absorber

By the Monte Carlo simulations described in this chapter, it has been demonstrated that the

4.7 Summary of the Monte Carlo Simulations

rial background.

In Appendix C, one can find a comparison of the fast with the full simulation of the combinato

23

tion. OCR Output

crease is that we should have a few hundred J /*1/ which should enable us to control our mass resolu

We expect to increase our present statistics by about 20 overall. An important aspect of this in

ittance achieved in the 1987 runs will be necessary and sufficient for our experiment.

hur ions per burst and two periods also for additional proton running for comparison. The beam em

We request a minimum of two 17-·day periods in 1990 with an average intensity of 5xl0° Sulp

5.3 Beam Requests

and successful way.

for which a detailed list is available. We are convinced that we can carry out this program in a timely

preparation, operation and analysis of this experiment and have identified the physicists for these tasks,

The collaboration agrees to share the costs for the new items. We have analyzed the needs for the

5.2 Responsibilities

Further possible improvements discussed in Appendix A will continue to be investigated.

runs [17]. They will also provide a multiplicity trigger.

are already available and well understood from the operation of silicon ring counters in the 1986/1987

trigger which is able to operate in a magnetic Held, Most ofthe electronics required for these counters

ty from central to forward rapidities and, in the target magnet option, a scinti]lator—·based interaction

Further 'must' items for 1990 are a set of silicon ring counters to measure the charged multiplici

out the target Held.

to share the time of the ion run between periods with the magnetic held at the target and those with

hadron beam preceding the ion running. Depending on this final test, the decision would be made how

most likely a modified PS -·magr1et. We would envision to carry out a test of the target magnet with a

24 CERN

A.l.3 Calorimetrized Plug OCR Output

scopes has to be envisaged, though quite expensive (ca 400KSFr).

and were installed about IO years ago. In the long range also a complete replacement of the old hodo

H2 and H3 have aged considerably; 2/3 of the scintillators come from the old NA3 experiment

scope plane. We are prepared to implement it if found to be useful.

We are presently studying the reduction in trigger rate which may be achieved with this third hodo

with which roads pointing to the target area could be set —up with the triple coincidence: Hl· H3- H2.

...._ area. It could be very useful to have a new sma.ll hodoscope (H1), just downstream of the magnet,

The scintillator hodoscopes used in the muon pre—trigger do not provide pointing to the target

A.l.2 A New Scintillator Hodoscope

imposes stronger requirements on the performance of the detectors i.e. the granularity.

showers starting already before the position of the silicon detector may affect the measurement. This

space is available and shorter target—dctector distances have to be envisioned. Also side—leakage of

In the case of the "nose"—geometry, the multiplicity measurement is more dillicult. Much less

background ratio at more central rapidities where the acceptance is small, see Fig.A.l.ab.

the decay length for particles emerging from the target at smaller rapidities, improving the signal to

get geometry. A "nose" was added to the dump surrounding the target in a close way. This shortens

Besides the standard dump geometry with a flat front face, we also studied a more restrictive tar

A.l.l 'Nose'

A.! Options

OPTIONS AND PROJECTS

APPENDIX A

CERN

sorber is feasible. OCR Output

wards central rapidity. The option of a target magnet becomes increasingly interesting if a thinner ab

resolution of 35 MeV/cz, a better vertex reconstruction and a significantly increased acceptance to

much lower multiplicity environment [3]. The great attraction for this approach is the improved mass

from the thin dump in a realistic high multiplicity environment. This technique was already used in a

to operate in such an environment. The limiting feature is the ability to reconstruct all tracks emerging

trometer, so only true two dimensional devices (such as wire chambers with cathode pads) will be able

Such an absorber would of course lead to much higher multiplicities ir1 the chambers of the spec

carried out.

but keeping the Eeld of the analyzer magnet at its present high value. Further optimizations have to be

trum of dirnuons in the rapidity range 3.05Y5 3.7. This is for the case of a target magnet configuration

at nearly central rapidity. This is demonstrated in figure A.2, which shows the reconstructed MT spec

lower momentum cutoii`. Preliminary calculations indicate an acceptance in M-I- down to .350 GeV/cz

rough estimates [7] show that a 6A absorber together with a 2A Fe—filter could be feasible yielding a

It would be possible to reach nearly central rapidities with a much shorter absorber and filter. Some

A.2 Project: 6A Dump

studying a similar calorimeter.

We will follow closely the progress made by the foccussing spectrometer experiment which is

should be introduced by this modification.

to bring out the read—out lines for this calorimeter. Of course, no significant cracks in thegabsorber

mentary to a multiplicity measurement. The segmentation points of the absorber could be the places

detection of the forward energy could provide a measure of the participating beam nucleons, comple

We have considered the possibility of a modest calorimeterization of the Tungsten-rod. The

26 CERN

acceptance of dimuon pairs would be better than assumed here. OCR Output

malized quark gluon plasma where one might expect a isotropic angular distribution. In this case the

photon as expected from qq annihilation. This may be not the case for photons coming from a ther

ilat distibution close to it. The above pararneterization is valid for transverse polarization of the virtual

This parameterization gives a 1+ cos26 angular distribution for masses well above the threshold, but a

W(@) cc 1 + A/(I —A) cos‘@ where A= (M4m)/2M2—22

The angular distribution of the dimuon decay in its rest frame was taken from [5]:

dn/dpT ¤¤ pq- exp(— 4.6 MT)

The dimuon p·I— distribution was parameterized according to

(m = muon mass, M = dimuon mass)

mmm cc f(M)/Ml where r(M) = (1+ 2m/M) _/ (1 —4m/M)22 22

at the threshold due to the reduced phase space. In the generation we follow a prescription given in [5].

The mass dependence of the dimuons is described by a l/m·‘ dependence. This shape is modified

DETAILS OF THE MUON PAIR GENERATOR

APPENDIX B

CERN

by up to a factor of two. OCR Output

ground. The additional contribution from secondaries seems to increase the primary decay background

theless one can conclude that the fast simulations give a realistic description of the total decay back

due to the deviations in the rapidity and PT distributions. This has to be further investigated. Never

whereas the MT spectrum seems to be steeper in the full simulation. These diHerences are most likely

on the parameterizations. The mass spectra show a rough agreement between the two evaluations,

sample (crosses in the Figure) should be comparable to the level of combinatorial background based

masses and MT. Muons from primary pion and kaon decays were selected and the contribution of this

Alumina dump, some increase of the combinatorial background is observed, mainly towards lower

simulation is compared to the simulation which includes only decays of primary particles. For the 11A

of secondary particles produced in the dump. In Fig. C.1, the decay background derived &om this

We use a full shower simulation (GEANT— GEISHA) to estimate the contribution of the decays

INTERACTIONS

COMPARISON WITH A FULL SIMULA TION OF SECONDAR Y

APPENDIX C

28 CERN

[20] WA80, private communication OCR Output

[19] V. Emelyanov, Lebedev preprint 5.9. (1986); available as Helios Note 167

. Pfeiffer, thesis in preparation

[18] P.V. Chliapnikov et a1.,Phys. Lett. 141B (1984) 276,

[17] P.Giube1lino et al., Performance of..., CERN—EP/88- 89, submitted to NIM

[16] T. Akesson et al., Phys. Lett. B178 (1986), 447

J.P. Pansart, Nucl. Phys. A461 (1987) 521c[15]

[14] A. Bussiere et al., Z. Phys. C38 (1988) 117, and references the

[13] T.J. Humanic et al., Z. Phys. C38 (1988) 79, and references therein

G. London, HELIOS note 308- "` [12]

V. Cemy et al., Z. Phys. C31 (1986) 163

E. Shuryak, Phys. lett. B78 (1978) 150,

[11] R.C. Hwa and K. Kajamie, Phys. Rev. D32 (1985) 1109,

[10] G.Henry, unpublished thesis (1978)

[9] V.Hedberg, thesis (1987) unpublished

B. Haber etal., Phys. Rev. D22(1980), 2107[8]

U. Goerlach, HELIOS note 339[7]

J.Bystricky et al, HELIOS NOTE 319[6]

[5] D. Bloclcus et al., Nucl. Phys. B201(1982), 205-249

V. Cemy et al., Z. Phys. C31 (1986), 163 and references therein

.I.D. Bjorken and H. Weisberg, Phys. Rev. D13(l976), 1405,[4]

[3] M.Bink1ey et al., Phys. ReV. Lett. 37 (1976) 571 and 774

[2] B. VV111is, HELIOS note 273

[1] KJ. Anderson et al., Phys. Rev. Lett. 37 (1976), 799

Rcfcrcncm

29CERN

Fig. 4.8.ab. Acceptance for 11A alumina dump with target B —tie1d. OCR Output

Fig. 4.7.ab. Acceptance for 11A alumina dump.

Fig. 4.6.ab. Acceptance for present absorber

tion and the proposed 11A alumina dump.Fig. 4.5.ab. Mass resolution, integrated over mass, PT and rapidity for the HELIOS 1987 configura

Fig. 4.4.b Pseudo -mass: signal, decay and dump background sources

Fig. 4.4.a Generated mass at target vs pseudo —mass (see text for definition)

dump background sources a) target magnet od`, b) on.Fig. 4.3. a,b. Point of closest approach between the two vectors of the muon pair: signal, decay and

and dump background sources a) target magnet oif, b) on.Fig. 4.2. a,b. Weighted average of points of closest approach to beam axis for dimuons: signal, decay

ground sources a) target magnet oi`, b) on.Fig. 4.l.a,b. Point of closest approach to beam axis for single muons: signal, decay and dump back

Fig. 3.3. Drawing of the shimmed PS magnet used as a target magnet and the absorber.

Fig. 3.2. Target area with silicon ring detectors a.nd pole of target magnet.

target magnet option is shown.Fig. 3.1 Schematic layout of the ll A A12O3 dump. The central portion is made from tungsten. The

the restricted kinematic region defined in text (preliminary).Fig. 2.8 The acceptance corrected distributions in mass M and transverse mass MT of dimuons in

32s —w am sample (pmummaiy)Fig. 2.7 The uncorrected distributions in mass M and transverse mass MT of dimuons for the 1987

Fig. 2.6 S/B vs Z2 cut

Fig. 2.5 Z2 distribution as a function of dirnuon mass (a) scatterplot and (b) average values of Z2

Fig. 2.4 Distribution of the radial distance at the absorber exit

each track. éFig. 2.3 The distribution of Z (along the beam direction) of closest approach to the beam axis for

Fig. 2.2 dN/dETcm(ULAC) vs type of trigger. ETcm is about 60% of the total ET.

studies.

Fig. 2.1 Partial view of HELIOS 2, as it was used in the 1987 S—Nucleus run for muon-pair32

multiplicity [9].multiplicity (upper scale), showing the quadratic dependence of the low mass pair rate on the particleFig. 1.2 The (e*e`)/n——ratio observed in pp—collisions at (/s = 63 GeV as a function of particle

Fig. 1.1 Compilation of low mass dilepton production

OCR OutputOCR OutputOCR OutputFigure Captions

CERN

particles for the 11A alumina dump, no target magnet. OCR OutputFig. C.l.ab. Comparison between the full simulation of all decays and the decays of just primary

case of a 6A A12O3 absorber and a 2A Fe filter.Fig. A.2. Reconstructed low mass MT distribution close to central rapidity (3.0 < Y(pp) < 3.7) for the

torial background is observed. (3.0 < Y(pli) < 3.7)Fig. A.1.ab. The eH`ect of a "nose' in front of the dump at low rapidity: A better signal to combina

with and without target magnet.Fig. 4.l2.ab. The MT distribution of the low mass dirnuon signal at low rapidity (3.0 < Y(pp) < 3.7),

configuration.Fig. 4.1l.abc. The same as Fig. 4.10. but with a magnetic field at the target. Shimmed PS-magnet

0.2 < M(p;i) < 1.2 and 0.2 < M(pp) < 0.4 GeV/cthe relative contributions as a function of mass, and of MT for__the two mass regions:Fig. 4.10.abc. Prompt dirnuon signal and background sources for the 11A alumina dump. Shown are

relative contributions as a function of mass and of MTFig. 4.9.ab. Prompt dirnuon signal and background sources for the present absorber. Shown are the

CERN

Fg CA _ [ . 1 OCR Output

The acceptance corrected mass distribution.

Mass M (MeV/c‘) of lepton pair

S 10 20 S0 100 200 500 1000 2000 5000

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55db(¢¤“`,<:sbtr0l)/GY

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PNK A.`\,Q OCR Output

!M (GeVO.2 O.60.4 O.8

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MHD Sicmb

= 55b(vT“`,<;@rwtrb>GY)/

W /\ %\\2O5—DumbW

Fjs OCR Output

T W (GQVO.75 W.5

WO

CA

W

OCR Output_W++***2

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bw mcsbbbmw SUNG

55db(¢¤+,<:©bb*0\)/GY

2.86 A A\2O3——Dbmb1

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LM (GeVW.50.75

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OCR OutputOCR Output_ W

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