ELSEVIER Nuclear Physics B (Proc. Suppl.) 75A (1999) 373-376

I l a m m m n ~ u

PROCEEDINGS SUPPLEMENTS

Reconstruction of showers at TeV energy by the CLUE Experiment and its application to recent data

B. Bartoli ~

D. Bastieri, M. Cresti, M. Mariotti, L. Peruzzo, R. Sacco, A. Saggion, G. Sartori, C. Sbarra b

C. Bigongiari, E. Cocca, D. Lucchesi, G. Marsella, A. Menzione, R. Paoletti, G. Parlavecchio, A. Scribano, A. Stamerra, N. Turini, F. Zetti ¢

F. Liello d

"Dipar t imento di Fisica dell 'Universit~ e Sezione INFN, Napoli, I taly

bDipartinlento di Fisica dell 'Universit~ e Sezione INFN, Padova, I taly

~Dipartimento di Fisica dell 'Universit£ e Sezione INFN, Pisa, I taly

dDipa.rtimento di Fisica dell 'Universit~ e Sezione INFN, Trieste, I taly

The CLUE UV C~erenkov telescope array has started to take data with 8 telescopes in January 1998. The UV (:erenkov images obtained by the CLUE experiment are very different with respect to the visible case, and a new method for reconstructing the shower direction has bees worked out. The shower reconstruction is shown and an application to recent data is given.

1. T h e C L U E e x p e r i m e n t

The CLUE experiment is operating in La Pah-na, a.t 2200m. a.s.1, with 8 units in the same site of the t tEGRA collaboration. Each tele- scope[l] is equipped with a 8 ° field of view UV detector [1] in the focal plane of 1.8 m F:I alu- minum coated glass mirror. The detector used is a MWP(:[1,2] sensitive to the UV photons thanks to the vapours of TMAE in the gas mixture. The chamber is equipped with a 576 pads system and is able to detect, single photoelectrons. Each pho- t.oelectron is resolved with 0.1 ° of accuracy.

The trigger and the DAQ architecture are de- signed tbr an arbi trary number of units. Each detector, from a t.rigger point of view, is divided in 36 "superpads" of trigger, wich are indeed the discriminated logic signals of the analog sum[3,4] of 4 × 4 neighboring pads: all 36 signals are pushed in a local fast p rogrammable look-up table named SMART[5]. The output of the SMART (normally a. ma~iority of 2) is called pre-trigger and is used lo- ca.lly to perform sampling and hold of the signals. The pre-trigger signal is sent. also to the central

DAQ where the global trigger is performed. All the telescopes are linked via Ethernet and fast signal cable to a central station where the da ta are saved by the central DAQ.

2. S h o w e r r e c o n s t r u c t i o n s t r a t e g y

Due to the absorption of UV photons in the at- mosphere, CLUE samples just the last kilometer of the shower cascade. The UV (~erenkov images look very different from what one can see in the visible spectrum: UV images are wider and com- posed by few scattered photons well separated in the focal plane. In the UV case, no evident mor- phological shapes are present, and so an imaging approach like in the visible is difficult.

The shower direction reconstruction we use is based on a max imum likelihood method with the likelihood function derived from Montecarlo. For a given photoelectron position detected in the chamber (i.e. direction) we assign a bi- dimensional density function that gives the prob- ability density to find the pr imary direction at a ~iven direction (c~, fl). q-'h~s probabili ty function

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374 B. Bartoli et aL /Nuclear Physics B (Proc. Suppl.) 75.4 (1999) 373-376

is deduced from Montecarlo and is shown in fig 1. By multiplying this probabili ty distribution cen-

:'~l , ' 7/]<l

I

!,/r,df ";),~ ¢ 44 'l ~2,, t 1

/,1 t'~°¢'2o9 ] p(, , 2 7 p , ~

DEG ,4 Itgi fill dLvtt'lblttiOtl (It Cluxters ~i th re.,pect It) ~'hower a.~l~

¢ ~

'I'AKELYIIOOD FOR

Figure 1. Probabil i ty density to find photons in a given direction for a 3 TeV shower coming from (0,0)

tered on the position of each photoelectron found in the detector, we obtain an overall probabil- ity density defined in the direction plane for each event. The max imum of this probabili ty function is our best est imate of the pr imary direction.

In fig. 2 the deviation of the a reconstructed angle with this method from the original angle of the sinmlated shower is shown as a function of the number of photoelectrons detected in the cham- ber. According to our simulation, our resolution function is well represented by a superposition of 2 gaussians, the narrower with o" ~ 0.2 0 and the other with cr --~ 1 °. The relative population of the 2 distribution is ,::hanging with the number of detected photoelectron.

3. R u n n i n g c o n d i t i o n a n d d a t a s a m p l e s

The data were collected starting roughly half an hour after sunset and ending half an hour be- fore sunrise. The trigger used was a major i ty of 3 out of 8 pre-trigger signals coming from the units. With a typical pre-trigger rate of 200Hz, we get a spurious 3-fold coincidence rate of l ev/hour,

resolution function vs phe

4O0 Sago

2OO ~00

250

o -2 ~ 0 I 2 o

alfa_ 1 3-10 phot. alfa_l 10-20 phot.

tvoo 1oo0

500 5o0

o o -2 - t 0 1 2 -2 - I 0 ~ 2

aYa_ 1 20-30 OhOt. alfa 1 30-40 phot.

~ v ~ ' 5 ° ° 5 o o ,, %2

o -2 -i o I 2 al~_ 1 40.50 phot.

Figure 2. Reconstructed shower a angle for ver- tical showers as a function of the number of pho- toelectron detected.

while the acquisition rate with the same trigger was 1-2 kev/hour. We have to remark that the chamber "dark" count is very low, whereas the trigger efficiency on single PHE's is very good [2]. A calibration run is made to evaluate gains and pedestals before each normal acquisition run. The pedestal count can also be obtained directly from a normal acquisition run, since the chance of having a pixel with a signal is less than 20%. In such a way the pedestals can be calculated and pursued during the run itself. The purpose of this first campaign with 8 telescopes was to test the apparatus in a g a m m a ray source study. The data taken since Winter '98 are grouped in four da ta samples. The off-source da ta were taken ev- ery night following the "ghost" of the source, i.e. doing a back-tracking of the source path. In to- tal we have 39 integrated hours for Crab, 34 for Mkn421 and 81 for Mkn501. Besides the gamma- ray da ta we have started to get Moon data to study the Moon shadow[6]. The Moon sample is still statistically poor and the da ta analysis is not yet done.

4. Data analysis

The analysis starts with the calculation of the first pedestal and follows with the subtraction of

B. Bartoli et aL /Nuclear Physics B (Proc. Suppl.) 75,4 (1999) 373-376 375

the pedestal eventually corrected for drift during the run. After the pedestal subtraction the sig- nals in the chamber are separated in clusters of which total charge, area and position are mea- sured.

A cluster of signals is defined as a group of neighboring pads with signals above a certain threshod (tipieally 4 sigma of pedestal width), the cluster area is the product of the m a x i m u m and min inmm width obtained by the covariance ma- trix. The cluster analysis is performed because even a single photoelectron signal involves more than one adjacent pad and the cluster center of gravity gives us the best estimation of its direc- tion. The chamber is sensitive also to charged particles and in this case the amount of charge is very high while the area is fairly low. Some pads, because of noise or preamp defects, appear as a spurious cluster: we define them "hot pads". The distribution "cluster area vs cluster charge" in each run helps us to flag charged particles and hot pads; in fact we can see from fig. 3 how easily we can cut. to well separate clusters coming from (Terenkov light with respect, to charged particles and hot. pad clusters. Once all light clusters have

..4 Rim I U 2 0 % . . . . .

i 10 Light Cluster,~ ( 'ut Cl!arge. Clusters

" , ,,.-C:,:, ) : : i? . . . .

.... :-~">i . . . . " " : : " " i Hot Pads

10 "1 t , ~ !_4 : _ t i t 11 _ i I Li J.i

~0 3 10 4 10 5 10

ctuster Charge (ADC counL~)

Figure 3. The cluster analysis in the Area vs Charge plane is used to flag cluster related with (',erenkov light

been identified we calculate the best estimate of t h e shower direction using the m a x i m u m likeli- hoo(l method described in Section 2.

4 . 1 . P r e l i m i n a r y r e s u l t s We analized three samples of da ta on sources

Crab, MkM21 and Mkn501 as described ill sec-

tion 3.1. The analysis is quite simple. For each shower we measure the angle 0 with respect to the source direction. The da ta are then stored in a M-bin histogram with variable bin size, where each bin covers the same amount of solid angle: the first bin width, 0.70 , is of the order of our est imated angular resolution.

We are looking for all excess of event in the first 3 bins of 0 plots comparing the distributions "on" and "offsouree". Each " off source" plot (the hatched one in figures 4,5) is normalized to the relative "on-source" using the density of events at more than 20 from the source direction. The plots are shown in fig 4,5.

MKN421 and all-Background

r.:: 1~2z, LU

20~

12!

7'.

5~

mkn cut3 0 (deg)

Figure 4. 0 plot for Mkn421 da ta sample and background (hatched istogram). The data where collected in Apri l /May '98, 34h.

If we look at fig. 6, we can observe that all the background shows the same theta distribu- tion, that reflects in some way the angular ac- ceptance of the apparatus. We can also say that Mkn501 in May 1998 is compatible with back- ground. The da ta are summarized in table 1. A rough simulation of our trigger tells us that our energy threshold shoud be around 3 Tev. Since we do not have yet a good simulation of our real trigger, and of course of our effective area, we can not evaluate correctly the expected flux from

376 B Bartoli et al. /Nuclear Physics B (Proc. Suppl.) 75.4 (1999) 373-376

Table 1 Results of this preliminary analysis

Mkn421 Mkn501 Crab Nebula Total excess 570 ± 85 0 4- 100 280 -t- 100 Total time 34h 81h 39h Rate 16 ± 2 . 5 e v t / h < 1 . 2 e v t / h 7 ± 2 . 5 e v t / h

Data taking A p r / M a y - 98 A p r / M a y - 98 F e b - 98

i _ _

Crab and Background

CLUE Pre l iminorv I

1500

1250

fO00

75O

5O0

25O

O 0

. . . . . . . . o (~eg) C r a b c u t 3

Figure 5. 0 plot for Crab data sample and back-' ground (hatched istogram). The data where col- lected in Feb. '98, 39h.

those sources. We can say that, ill Apt /May 98, CL[ E found Mkn421 a factor (2.3 + 1.1) more active than the Crab.

5. C o n c l u s i o n

Working with an array of 8-telescopes, in spring '98, CLUE was able to see an excess of events from Mkn421 and a less significant signal from ('.tab. No excess has been found for Mkn501. The result are preliminary and we still need a correct evaluation of our effective area. The algorithm used for shower reconstruction turned out to be successful. We expect to rise the sensitivity of our apparatus in the near future, also with the devel- opment of refined shower reconstruction methods.

background shape

cu~ 0 (deg)

Figure 6. 0 plot for all background data and Mkn501 data sample (gray istogram).

R E F E R E N C E S

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A (1998) 409. 6. M. Urban et al., Nuclear Physics B (Proc.

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