Performances of the RPC trigger system of the L3 Forward—Backward muon spectrometer

UCLEAR PHYSICS

PROCEEDINGS SUPPLEMENTS El,SEVI ER

Nuclear Physics B (Proc. Suppl.) 44 (1995) 417422

Performances of the R,PC Trigger System of the L3 Forward-Backward Muon Spectrolnet, er Presented by G. Carlino A. Aloisio a, M.G. Alviggi a, G. Carlino a, N.Cavallo ~, R. de Asmundis a, V.Innocente b, S. L a n z a n o ~t, L. Lista ~, P. Paolucci ~, S. Patricelli ", D. Piccolo ~, C. Sciacca ~, V. Soulimov ¢ ~INFN Sezione di Napoli and Dip. Scienze Fisiche Universit£ "Federico II" di Napoli, Mostra d ' O l t r e m a r e , 1-80124, Napoli b E u r o p e a n L a b o r a t o r y for Particle Physics, C E R N , Switzerland c o n leave of absence from Nuclear Physics Institute, Gatchina, Russia In view of LEP200 physics, the L3 detector has been upgraded installing the Forward-Backward muon spectrometer to increase the angular acceptance in muon detection. We describe the trigger system for this spectrometer which makes use of Resistive Plate Counters (RPC) covering an area of 300 m s. This system is the first large application in high energy physics of this kind of detectors and its operation will constitute an important test for their future use in the LHC experiments. The main features of the RPCs, the trigger architecture and the preliminary results from the 1994 LEP run are given.

1. I n t r o d u c t i o n ~

Muon detection in the L3 experiment at L E P [1] (fig.l) is achieved in the angular region 44 ° < 0 < 136 ° (to the beam axis) by the barrel spectrometer consisting of three layers of high precision drift chambers. The m o m e n t u m resolution in the solenoidal magnetic field of 0.51 T is Ap/p = 2.5% at 45 GeV [2]. The F o r w a r d - B a c k w a r d ( F / B ) spectrometer extends the m u o n detection in the angular regions 220 < 0 < 44 ° and 136 o < 0 < 158 ° and it will be essential for future analysis at the higher energy of LEP200. T h e F / B detector consists of three layers of precision drift chambers (FI, FM, FO) m o u n t e d on each m a g n e t door which is toroidally magnetized [3]. Half of the detector, in the two diagonally opposite half doors, has been installed for L E P running in 1994. Figure 2 shows the side view of the F / B detector and the definition of the Solenoidal (S region, 360 < O < 440 ) and Toroidal regions (T region, 220 < 8 < 36°). In the S region muons are reconstructed by the MI and MM chambers of the bar0920-5632/95/$09.50© 1995 Elsevier Science B.V. All rights reserved. SSDI 0920-5632(95)00563-3

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tel detector and the PI chamber. Trigger in this region is provided adding the information given by the FI chamber to the system already used in the barrel.

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in the same mechanical structure with independent H.V. and a single pickup plane. Signal readout is obtained through induced pulses collected on aluminum strips 3.1 cm wide. Each R P C unit is equipped with 32 readout strips. Hence in every octant there are 96 strips per plane oriented in the orthogonal direction with respect to the octant center line.

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In the T region muons are deflected by the 1.5 T toroidal magnetic field and the moment u m is analyzed by the FI, FM and FO chambers. Trigger is provided by a system of Resistive Plate Counters chosen for their fast time response, high efficiency and the possibility of large scale industrial production. Two layers of R P C s are assembled outside the magnet door between the FM and FO chambers. Every layer is made of three units of trapezoidal shape to fit the octagonal shape of L3. The middle unit is overlapped to the other two to avoid dead areas. 2. T h e R e s i s t i v e P l a t e C o u n t e r s An R P C is a particle detector utilizing a constant and uniform electric field produced by two parallel electrode planes made of a material with high bulk resistivity. The R P C s used in L3 are the double layers detectors developed by Santonico et al. [4] (fig. 3) with small modifications. They consist of two independent counters superimposed on each other

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The sensitive volume is a 2 m m thick gas gap between the electrodes operating in the limited streamer region under a uniform electric field of about 4 k V / m m . The gap is filled with a mixture of Argon (58%), Isobutane (38%) and Freon (4%). The electrodes are 2 m m bakelite plates, with a volume resistivity p ~ 1011f~ cm, painted on the external surface with a conductor coating of graphite where the high voltage is applied. PVC spacers (0.8 cm 2 area), located inside the gap on a grid of 10 cm side, guarantee the planarity of the electrodes. When a charged particle crosses the detector,

419

A. Aloisio et al./Nuclear Physics B (Proc. Suppl.) 44 (1995) 417-422

the primary electrons produced by the ionization process in the gas generate a discharge. The discharge is quenched by the following mechanisms:

track impact points on the R P C s we measured a resolution cr = 7.7 + 0.1 mm.

1. the UV photon absorption by Isobutane, preventing secondary discharges due to gas photoionization; Space Resolution 2. the capture by Freon of the outer electrons, reducing the size of the discharge;

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3. the switching off of the electric field, due to the high resistivity of the electrodes, in a limited area around the point where the discharge occurred. Typical times are the duration of the discharge of the order of 10 ns and the relaxation time of the electrodes pe ..~ 10 ms where e is the dielectric constant of the bakelite. Due to the large difference between these times, the R P C behaves, during the discharge, like a set of independent cells insulated by the high electrodes resistivity. 3. T h e

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The Resistive Plate Counters have been extensively tested, in 1992 and 1993, both in the laboratory of Naples University and at C E R N using cosmic rays [5]. In the 1994 LEP shut down the F / B detector has been installed in the L3 experiment and in the following LEP run we collected a sample of about 4000 dimuon events Z ~ /~+/~- reconstructed in the F / B spectrometer [6]. These events have been used to analyze the R P C performances. The RPCs provide a measurement both of the position of the muon tracks and of the time they crossed the detector. When a particle crosses the RPC, a signal is generally induced on more then one strip and the contiguous strips are joined in a cluster. The average multiplicity of the clusters is of 2 strips. The centre of the cluster gives the measurement of the coordinate of the particle track. We made use of the F / B spectrometer tracking system to determine the R P C space resolution. From the distribution (fig 4) of the relative positions of the cluster centres with respect to the

Figure 4. Space resolution of the R P C s .

The R P C detection effieiency is a function of the high voltage applied to the electrodes. At L3 running conditions (H.V.= 7.5 kV) we obtained the value erpc = 99.5 + 0.06% checking the presence of a cluster of strips for each reconstructed muon track. The resolution of the time measurement performed by the R P C depends on the applied high voltage as well. Tests with cosmic rays showed that the time resolution decreases as the high voltage increases and values of the order of 1 ns could be achieved. At our running conditions, the preliminary value of time resolution (figure 5) is ~ti,n, = 2.19 + 0.05 ns. All the R P C performances have been included in the L3 simulation program to have a good simulation of the trigger system behaviour.

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for muons with different m o m e n t a at the vertex. They populate "roads" whose width (the number of strips) is dependent on their m o m e n t u m . The trigger logic is therefore based on the search in each octant, a m o n g all the fired strips in the two R P C layers, of a coincidence inside a road which is usually populated by muons coming from the vertex.

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The R P C system provides the Leve]-I muon trigger for the Toroidal region of the F / B detector [5] [7]. This system searches for tracks coming from the interaction point to select muons from Z decays rejecting background events. The muon trajectory outside the magnet door can be approximated to a straight line with the slope determined by the fired strips in the two R P C layers. Due to the multiple scattering (mainly in the hadron calorimeter and in the 1 meter thick magnet door) and to the bending in the toroidal magnetic field, to each strip fired in the inner layer correspond a limited number of strips in the outer one. This number depends on the muon moment u m and it has been studied using a Monte Carlo simulation. A 96 x 96 coincidence matrix (the Trigger Matrix), with the inner and the outer strips on the axis, can be defined. Figure 6 shows a Trigger Matrix obtained in the Monte Carlo simulation

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Fig 7 shows the Monte Carlo trigger efficiency for two roads of different width as a function of the m o m e n t u m . The trigger generation is performed by two kind of VME modules: the Zero Supressor and Encoder (32 modules, 2 per octant) and the Track Finder and Trigger Generator (16 modules, 1 per octant) [8]. For every octant, two groups of 96 signals, ge-

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Aloisio et al./Nuclear Physics B (Proc. Suppl.) 44 (1995) 417-422

Monte Carlo Trigger Effic[ercy

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nerated by the strips of both the RPC layers, are sent to a couple of Zero S u p p r e s s o r and Encoder modules. These encode the number of the fired strips and provide the input for the Track Finder and Trigger Generator module. This module is responsible for the detection of the coincidences, between the fired strips in the two layers, inside a predetermined road of the Trigger Matrix. The road is programmed, depending on the running conditions, using the information from the Monte Carlo simulation, and is downloaded into the modules at the initialization time. If a coincidence is found a trigger signal is sent in a typical time of the order of few ps. For a road 11 strips wide, the Level-1 muon trigger rate of the R P C system, because of the high background contamination due to the beam halo, is about 10 Hz. To reduce this rate a coincidence between the RPC system and the FI drift chamber is required. The trigger rate goes down to 0.4 Hz.

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A. Aloisio et al./Nuclear Physics B (Proc. Suppl.) 44 (1995) 417-422

Figure 8a shows a typical time distribution when the trigger is given by the R P C system whereas figure 8b shows the same distribution for events which have not been triggered by the RPCs. The peaks correspond to the beam halo background. The first peak is in time with the incoming beam and the second one with the outgoing beam. "Physical" events are expected to be approximately in time with the delayed peak. The comparison of these distributions shows that the R P C trigger provides a complete rejection of the off time background due to cosmic rays. A further reduction of the background contamination, that will be necessary at LEP200 because of the increasing of the beam luminosity, can be obtained acquiring only the events in the temporal region where the physics peak is located. Figure 9 shows this peak for the sample of dimuon events we used to analyze the trigger behaviour. Corrections for different cable length are not yet applied in the previous plots. Fig 10 shows the Trigger Matrix defined by those dimuon events. The 45 GeV muons, as expected after the Monte Carlo simulation, populate a very narrow road. We determined the trigger efficiency measuring the number of events with a trigger provided by the R P C system among those with a muon reconstructed in the T region of the detector. The preliminary value of the trigger efficiency we obtained is ~trigger = 99.3 :t: 0.06%.

REFERENCES 1.

2. 3.

B.Adeva et al., Nucl. Inst. Meth. A 2 8 9 (1990) 35. B.Adeva e~ al., Nucl. Inst. Meth. A 3 2 3 (1992) 109. for a detailed description of the F / B detector and its performances see: U. Becker, "The New Forward-Backward Muon Detector of L3: features and implementation", these proceedings; L3 Forward-Backward Muon Detector, L3 internal note, Dec.3, 1991, unpublished.

Trigger Matrix

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5.

6.

7.

8.

R. Santonico e R. Cardarelli, Nucl. Instr. Meth. 187 (1981) 377; R. Cardarelli et al, Nucl. Instr. Meth. A 2 6 3 (1988) 20. S. Patricelli et al., Proceedings of the "International Workshop on the R P C in Particle Physics and A s t r o p h y s i c s ' , R o m e 1993, Scientifica Acta, v.8, n.2 (1993). R. Bock, G. Fernandez and P. de Jong, "Track Reconstruction in the L3 F / B muon detector", L3 internal note, unpublished. D. Piccolo et al., Proceedings of the "6th Pisa Meeting on Advanced Detectors", Isola d'Elba 1994, to be published on Nucl. Inst. Meth. A. Aloisio e~ al., Proceedings of CHEP-94 conference, San Francisco, 1994