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ATLAS Level-1 Muon Barrel Trigger robustness study at X5 test facility
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 518 (2004) 529–531
ATLAS Level-1 Muon Barrel Trigger robustness study at X5 test facility A. Di Mattiaa, L. Luminaria, A. Nisatia, F.C. Pastorea,*, R. Varia, S. Venezianoa, G. Aiellib, P. Camarrib, R. Cardarellib, A. Di Ciacciob, A. Di Simoneb, B. Libertib, R. Santonicob a
b
INFN, Rome, Italy Dipartimento di Fisica, Universita di Roma ‘‘Tor Vergata’’ INFN, Sezione di Roma 2, Via della Ricerca Scientifica 1, Roma 00133, Italy
Abstract The present paper describes the Level-1 Barrel Muon Trigger performance as expected with the current configuration of the RPC detectors, as designed for the Barrel Muon Spectrometer of ATLAS. Results of a beam test performed at the X5-GIF facility at CERN are presented in order to show the trigger efficiency with different conditions of RPC detection efficiency and several background rates. Small RPC chambers with part of the final trigger electronics are used, while the trigger coincidence logic is applied off-line using a detailed simulation model. r 2003 Published by Elsevier B.V. PACS: 29.30.Aj; 29.30.Ep; 29.40.Cs Keywords: RPC detectors; First level trigger; Muon; Robustness; ATLAS; Background rates
1. Introduction The ATLAS Muon System [1] has been designed in order to face the very large production rate of background particles expected at the LHC energy and luminosity, which can contribute to the muon trigger rate by accidental coincidences of hits. The Level-1 trigger decision must reduce the trigger rate by a factor of Oð104 Þ from the initial bunchcrossing rate of 40 MHz and identify the regions of interest for the Level-2 trigger. The Level-1 muon trigger system in the Barrel region relies on layers of Resistive Plate Chambers *Corresponding author. Tel.: +39-6-49914242; fax: +39-649914320. E-mail address: [email protected] (F.C. Pastore). 0168-9002/$ - see front matter r 2003 Published by Elsevier B.V. doi:10.1016/j.nima.2003.11.077
(RPCs) to identify muons with transverse momentum above B5 GeV/c. The complete trigger logic and readout hardware is implemented in a system of Coincidence Matrix ASICs (CMAs) [2]. Trigger logic is split into two trigger decisions, the Low pt and the High pt coincidences, using the informations from 3 RPC doublets as shown in Fig 1. Trigger decision requires a time coincidence on both bending and non-bending projections and hit patterns compatible with a muon coming from the interaction region. Different majority coincidences can be set in each of the two subsystems: the baseline trigger configuration uses a 3-out-of-4 majority in the Low pt trigger and matches the latter result with a 1-out-of-2 majority in the High pt system. Rates [3] expected in the baseline Low pt trigger configuration, both for nominal
ARTICLE IN PRESS A. Di Mattia et al. / Nuclear Instruments and Methods in Physics Research A 518 (2004) 529–531
530
Fig. 1. The ATLAS Level-1 Muon Barrel Trigger.
5
Trigger Rate, Hz
10
and an uncorrelated background source reproducing the expected rate in ATLAS (B100 Hz/cm2). The RPC signals are picked up by a system of 32 strips, positioned orthogonally on both sides of each gas gap. The chambers are operated in avalanche mode, with the operating current varying between 10–150 mA when source is switched on. The readout is performed by six TDC modules triggered by the coincidence of three scintillators. The detector performance has been studied with different operating conditions and the efficiency measured with a tracking algorithm using clustered hits on the RPC layers. Time resolution, B1.6 ns, is not degraded by fluxes greater than 100 Hz/cm2.
3. Trigger efficiency study
4
10
Prompt muons nominal bckg
3
10
bckg x 5
1
Luminosity, nb-1
10
Fig. 2. The accidental trigger rate, for nominal background and for 5 times the flux, compared with the prompt muons as a function of luminosity, for Low pt 6 GeV/c threshold in the baseline configuration.
Using a detailed model of the CMA, the ATLAS baseline trigger efficiency has been evaluated with different detector conditions and proved not to be influenced by additional background rates. By changing the values of the programmable parameters of the CMA, other two trigger configurations have been studied: the loose, a more efficient 2-out-of-4 majority configuration, and the robust loose, which requires the additional confirmation from the outer layer, reducing background from softer charged particles. Starting from the trigger measurements, we
background and for a conservative scheme which includes a safety factor on hit rates ( 5 for Low pt and 10 for High pt), are shown in Fig. 2 compared with the prompt-muon rate as a function of luminosity.
Trigger Efficiency
1
0.8
0.6
Experimental 0.4
Expected from RPC Eff Binomial
0.2
2. Experimental set-up
0 0.7
0.75
0.8
0.85
0.9
0.95
1
Effective RPC gap efficiency
Background and trigger efficiency studies have been carried out [3] at the X5-GIF test facility, irradiating three reduced sized (50 50 cm2) ATLAS RPC doublets with a high-energy muon beam
Fig. 3. The baseline Low pt trigger efficiency versus the effective detector efficiency, extrapolated from the expected binomial curve, with the Monte Carlo estimation expected from measured RPCs efficiency superimposed.
ARTICLE IN PRESS A. Di Mattia et al. / Nuclear Instruments and Methods in Physics Research A 518 (2004) 529–531
531
Table 1 Expected accidental trigger rates and trigger efficiencies for different configurations Trigger configuration
L (cm2 s1)
Nominal (kHz)
Conserv. (kHz)
e (RPC 95%)
e (RPC 80%)
Baseline Low pt Baseline High pt Loose Low pt Loose High pt Rob. loose Low pt
1033 1034 1033 1034 1033
0.7 0.01 6.5 0.02 0.8
7.6 3.3 36.2 13.4 9.4
0.99 0.98 1.0 1.0 B0.95
0.82 0.79 0.92 0.88 B0.85
Efficiencies in the robust configuration are not accurate since the contributions from the muons in the Barrel-Endcap transition zone must be verified.
extrapolated an effective RPCs efficiency from the ideal binomial curves, allowing a cross-check of the trigger algorithm for different conditions. Using a simple Monte Carlo simulation, we calculated the expected trigger efficiency combining the hit probabilities measured in each gas gap. The good agreement, as shown in Fig. 3, was tested in all the aforementioned configurations.
4. Summary and conclusions In Table 1 all the trigger configurations studied in this work are presented, with their expected accidental-coincidence trigger rates (in both conservative and nominal schemes) and the corre-
sponding efficiencies, evaluated with two different detector operating conditions. The muon trigger system appears robust against accidental coincidences, even for a background level 5–10 times higher than that predicted by detailed simulation programs. It also looks robust against unexpected aging of the RPCs chambers and offers several handles to face multiple operating scenarios of the ATLAS experiment. References [1] ATLAS TDR, CERN/LHCC/98-24, 1998. [2] V. Bocci, et al., Proceedings of the Eighth Workshop on Electronics for LHC, Colmar, 2002. [3] ATLAS Internal Note ATL-DAQ-2002-015.
Nuclear Instruments and Methods in Physics Research A 518 (2004) 529–531
ATLAS Level-1 Muon Barrel Trigger robustness study at X5 test facility A. Di Mattiaa, L. Luminaria, A. Nisatia, F.C. Pastorea,*, R. Varia, S. Venezianoa, G. Aiellib, P. Camarrib, R. Cardarellib, A. Di Ciacciob, A. Di Simoneb, B. Libertib, R. Santonicob a
b
INFN, Rome, Italy Dipartimento di Fisica, Universita di Roma ‘‘Tor Vergata’’ INFN, Sezione di Roma 2, Via della Ricerca Scientifica 1, Roma 00133, Italy
Abstract The present paper describes the Level-1 Barrel Muon Trigger performance as expected with the current configuration of the RPC detectors, as designed for the Barrel Muon Spectrometer of ATLAS. Results of a beam test performed at the X5-GIF facility at CERN are presented in order to show the trigger efficiency with different conditions of RPC detection efficiency and several background rates. Small RPC chambers with part of the final trigger electronics are used, while the trigger coincidence logic is applied off-line using a detailed simulation model. r 2003 Published by Elsevier B.V. PACS: 29.30.Aj; 29.30.Ep; 29.40.Cs Keywords: RPC detectors; First level trigger; Muon; Robustness; ATLAS; Background rates
1. Introduction The ATLAS Muon System [1] has been designed in order to face the very large production rate of background particles expected at the LHC energy and luminosity, which can contribute to the muon trigger rate by accidental coincidences of hits. The Level-1 trigger decision must reduce the trigger rate by a factor of Oð104 Þ from the initial bunchcrossing rate of 40 MHz and identify the regions of interest for the Level-2 trigger. The Level-1 muon trigger system in the Barrel region relies on layers of Resistive Plate Chambers *Corresponding author. Tel.: +39-6-49914242; fax: +39-649914320. E-mail address: [email protected] (F.C. Pastore). 0168-9002/$ - see front matter r 2003 Published by Elsevier B.V. doi:10.1016/j.nima.2003.11.077
(RPCs) to identify muons with transverse momentum above B5 GeV/c. The complete trigger logic and readout hardware is implemented in a system of Coincidence Matrix ASICs (CMAs) [2]. Trigger logic is split into two trigger decisions, the Low pt and the High pt coincidences, using the informations from 3 RPC doublets as shown in Fig 1. Trigger decision requires a time coincidence on both bending and non-bending projections and hit patterns compatible with a muon coming from the interaction region. Different majority coincidences can be set in each of the two subsystems: the baseline trigger configuration uses a 3-out-of-4 majority in the Low pt trigger and matches the latter result with a 1-out-of-2 majority in the High pt system. Rates [3] expected in the baseline Low pt trigger configuration, both for nominal
ARTICLE IN PRESS A. Di Mattia et al. / Nuclear Instruments and Methods in Physics Research A 518 (2004) 529–531
530
Fig. 1. The ATLAS Level-1 Muon Barrel Trigger.
5
Trigger Rate, Hz
10
and an uncorrelated background source reproducing the expected rate in ATLAS (B100 Hz/cm2). The RPC signals are picked up by a system of 32 strips, positioned orthogonally on both sides of each gas gap. The chambers are operated in avalanche mode, with the operating current varying between 10–150 mA when source is switched on. The readout is performed by six TDC modules triggered by the coincidence of three scintillators. The detector performance has been studied with different operating conditions and the efficiency measured with a tracking algorithm using clustered hits on the RPC layers. Time resolution, B1.6 ns, is not degraded by fluxes greater than 100 Hz/cm2.
3. Trigger efficiency study
4
10
Prompt muons nominal bckg
3
10
bckg x 5
1
Luminosity, nb-1
10
Fig. 2. The accidental trigger rate, for nominal background and for 5 times the flux, compared with the prompt muons as a function of luminosity, for Low pt 6 GeV/c threshold in the baseline configuration.
Using a detailed model of the CMA, the ATLAS baseline trigger efficiency has been evaluated with different detector conditions and proved not to be influenced by additional background rates. By changing the values of the programmable parameters of the CMA, other two trigger configurations have been studied: the loose, a more efficient 2-out-of-4 majority configuration, and the robust loose, which requires the additional confirmation from the outer layer, reducing background from softer charged particles. Starting from the trigger measurements, we
background and for a conservative scheme which includes a safety factor on hit rates ( 5 for Low pt and 10 for High pt), are shown in Fig. 2 compared with the prompt-muon rate as a function of luminosity.
Trigger Efficiency
1
0.8
0.6
Experimental 0.4
Expected from RPC Eff Binomial
0.2
2. Experimental set-up
0 0.7
0.75
0.8
0.85
0.9
0.95
1
Effective RPC gap efficiency
Background and trigger efficiency studies have been carried out [3] at the X5-GIF test facility, irradiating three reduced sized (50 50 cm2) ATLAS RPC doublets with a high-energy muon beam
Fig. 3. The baseline Low pt trigger efficiency versus the effective detector efficiency, extrapolated from the expected binomial curve, with the Monte Carlo estimation expected from measured RPCs efficiency superimposed.
ARTICLE IN PRESS A. Di Mattia et al. / Nuclear Instruments and Methods in Physics Research A 518 (2004) 529–531
531
Table 1 Expected accidental trigger rates and trigger efficiencies for different configurations Trigger configuration
L (cm2 s1)
Nominal (kHz)
Conserv. (kHz)
e (RPC 95%)
e (RPC 80%)
Baseline Low pt Baseline High pt Loose Low pt Loose High pt Rob. loose Low pt
1033 1034 1033 1034 1033
0.7 0.01 6.5 0.02 0.8
7.6 3.3 36.2 13.4 9.4
0.99 0.98 1.0 1.0 B0.95
0.82 0.79 0.92 0.88 B0.85
Efficiencies in the robust configuration are not accurate since the contributions from the muons in the Barrel-Endcap transition zone must be verified.
extrapolated an effective RPCs efficiency from the ideal binomial curves, allowing a cross-check of the trigger algorithm for different conditions. Using a simple Monte Carlo simulation, we calculated the expected trigger efficiency combining the hit probabilities measured in each gas gap. The good agreement, as shown in Fig. 3, was tested in all the aforementioned configurations.
4. Summary and conclusions In Table 1 all the trigger configurations studied in this work are presented, with their expected accidental-coincidence trigger rates (in both conservative and nominal schemes) and the corre-
sponding efficiencies, evaluated with two different detector operating conditions. The muon trigger system appears robust against accidental coincidences, even for a background level 5–10 times higher than that predicted by detailed simulation programs. It also looks robust against unexpected aging of the RPCs chambers and offers several handles to face multiple operating scenarios of the ATLAS experiment. References [1] ATLAS TDR, CERN/LHCC/98-24, 1998. [2] V. Bocci, et al., Proceedings of the Eighth Workshop on Electronics for LHC, Colmar, 2002. [3] ATLAS Internal Note ATL-DAQ-2002-015.