- Email: [email protected]
Operations and performance of the CMS RPC muon system at LHC
Nuclear Instruments and Methods in Physics Research A 718 (2013) 412–413
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Operations and performance of the CMS RPC muon system at LHC Anna Cimmino,1 Universiteit Gent, Ghent, Belgium
a r t i c l e i n f o
a b s t r a c t
Available online 13 December 2012
The Compact Muon Solenoid (CMS) experiment is one of the two general-purpose detectors at the CERN Large Hadron Collider. CMS combines three different gaseous detector technologies to trigger and reconstruct muons: Drift Tubes in the barrel region (9Z9 o 1:2), Cathode Strip Chambers in the endcaps ð0:9 o 9Z9 o 2:4Þ , and Resistive Plate Chambers (RPCs) in both regions, as a dedicated muon trigger. We report on the operations and performance of the RPC system after 2 years of LHC running with increasing instantaneous luminosity. Special attention is given to the stability of the system and to the working point calibration procedures. & 2013 EURATOM. Published by Elsevier B.V. All rights reserved.
Keywords: Muon system RPC Gas detectors CMS
1. Introduction The Compact Muon Solenoid (CMS) [1] is a general-purpose particle physics detector built at the Large Hadron Collider (LHC) [2]. Its compact geometry is built around a 6 m diameter superconducting solenoid with a 3.8 T magnetic field. The iron return yoke of the magnet hosts the muon system, which combines three different gaseous detector technologies. In the barrel region, Drift Tube chambers (DTs) are arranged in coaxial layers to detect muons up to pseudorapidity 9Z9 o 1:2. The planar endcap regions are equipped with Cathode Strip Chambers (CSCs) ð0:9 o 9Z9 o2:4Þ to handle the higher rates and non-uniform magnetic field. Both detector regions are instrumented with Resistive Plate Chambers [3] (RPCs) ð9Z9 o1:6Þ, as a dedicated muon trigger. The RPC system is characterized by a time resolution t 3 ns and a spatial resolution of the order of 1 cm. Each of the 912 chambers has a double-gap (2 mm of width) design formed by two parallel bakelite electrodes (bulk resistivity 1010 O cm) and a common readout plane of copper strips. CMS RPCs are operated in avalanche mode, with a gas mixture composed of 95.2% C2H2F4, 4.5% C4H10 and 0.3% SF6, with a humidity of 40% at 21 1C [4]. This paper presents the results of performance studies of the RPC system related to 2011 operations. Focus is put on detector stability and working point calibration procedures.
2. High voltage scan A high voltage (HV) scan was carried out during early 2011 [5]. Collision data was recorded at 11 different HV settings, during a E-mail address: [email protected] & CERN for the benefit of the CMS Collaboration
1
series of dedicated runs, to define the optimal operating voltage for each chamber. Dependency of the applied HV on environmental pressure (P) and temperature (T) where taken into account [6], thus defining the effective HV : HVeff ¼ HV P=P 0 T 0 =T. P0 ¼965 mbar and T0 ¼293 K are the reference pressure and temperature respectively. RPC hits are predict by extrapolating, to the associated RPC strip plane, track segments reconstructed within the neighboring CSC or DT chamber. The extrapolated impact point is then matched to the closest cluster (set of adjacent strips fired by the passage of a ionizing particle). For each chambers partition, called roll, the measured efficiency as a function of HVeff , E, was fitted with a sigmoid function [7] and working points ðHVWP Þ were defined as: HVWP ¼ HVknee þ 100ð150Þ V for the barrel (endcap) region, where HVknee is the HVeff at E ¼ 95%. 3. Detector performance 3.1. Efficiency The agreement between efficiency measured in the subsequent runs (observed) and the predicted one measured using the fitting procedure confirmed the effectiveness of the technique. Average efficiency is found to be 94.9(93.8)% in the barrel (endcap) region. A maximum fluctuation of the efficiency of about 7 0:5% was observed, once the automatic online correction of the HV working point with the atmospheric pressure measured in the CMS cavern was applied. 3.2. Cluster size Pressure variations influence also the cluster size (CLS), defined as the number of continues strips fired at the passage of
0168-9002/ - see front matter & 2013 EURATOM. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2012.12.062
A. Cimmino / Nuclear Instruments and Methods in Physics Research A 718 (2013) 412–413
413
of fired strips in three or more planes within a 25 ns window and with hits matching predefined patterns. CMS Preliminary 2011
Without Pressure correction
The residuals (the distance in the RPC plane between the extrapolated impact point and the center of the RPC cluster) can be used as a measurement of the spatial resolution of the RPC system. The standard deviations of Gaussian fits to the distributions of the residuals are then taken as spatial resolutions. Results depend on strip width, CLS, and detector alignment. Measured resolutions goes from 0.81 to 1.32 cm in the barrel and from 0.86 to 1.28 cm in the endcap.
With Pressure correction
April 2011
3.4. Spacial resolution
October 2011
Fig. 1. CLS in the endcaps as a function of the run number (April and October 2011). The two regions, before and after the automatic pressure correction, are shown in the plot.
CMS Preliminary 2011
Outermost station
4. Background studies Single rate data were analyzed to study the radiation background level. The average background rate, measured in the RPC system at luminosity 3 1033 cm2 s1 , was 1:3 Hz=cm2 . The dependence between the background rate and luminosity was found to be linear as shown in Fig. 2. Linear extrapolation to 1034 cm 2 s 1 gives an average background of 6 Hz=cm2 and a maximum rate of 35240 Hz=cm2 that is still well below the limit of 100 Hz=cm2 used in the trigger design [9].
5. Conclusion Innermost station
Fig. 2. RPC background rate as a function of the instantaneous luminosity, for four radial stations of barrel region. Outermost station suffers from neutron background, while the innermost mainly affected by particles coming from the vertex.
Detector efficiency, cluster size, noise, spatial resolution, background rate have been studied. RPCs performance was well understood and tuned using regular and dedicated collision runs. The results show that the RPCs form a stable and reliable subdetector, effectively contributing to event triggering and reconstruction for the CMS physics programme. Data collected were also compared to Monte Carlo simulations. The agreement between the two confirms the excellent behavior of the RPC system. The HV scan procedure will be repeated at the beginning of each data taking year, to monitor in time the performance of the chambers and spot any aging effect. References
a ionizing particle. The stability of the system may be seen in Fig. 1. An overall average of 1.8 (RMS ¼0.3) was found, compatible with the trigger requirements of CLS o 2:0. A detailed description of the RPC trigger may be found in Ref. [8]. 3.3. Noise By measuring the hit rates during no-collision periods, the intrinsic noise per chamber was found to be well below 0.5 Hz cm2. These rates values are negligible for accidental muon triggering, since the RPC muon trigger system needs coincidence
[1] CMS Collaboration, Journal of Instrumentation 3 (2008) S08004. [2] L. Evans, P. Bryant, Journal of Instrumentation 3 (2008) S08001. [3] R. Santonico, R. Cardarelli, Nuclear Instruments and Methods 187 (157) (1981) 377. [4] M. Bosteels, et al., CMS Gas System Proposal, CMS Note 018 (1999). [5] S. Costantini on behalf of the CMS Collaboration, Calibration of the RPC working voltage in the CMS experiment, CMS CR 055 (2012). [6] M. Abbrescia, et al., Nuclear Instruments and Methods A 359 (1995) 603. [7] M. Abbrescia, et al., Nuclear Instruments and Methods A 550 (2005) 116. [8] CMS Collaboration, RPC Trigger, in CMS TriDAS project : Technical Design Report; 1. The Trigger Systems. CERN, Geneva, 2000, p. 419. [9] M. Rodozov on behalf of the CMS Collaboration, Analysis of the radiation background in CMS RPCs, PoS(RPC2012)044.
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Operations and performance of the CMS RPC muon system at LHC Anna Cimmino,1 Universiteit Gent, Ghent, Belgium
a r t i c l e i n f o
a b s t r a c t
Available online 13 December 2012
The Compact Muon Solenoid (CMS) experiment is one of the two general-purpose detectors at the CERN Large Hadron Collider. CMS combines three different gaseous detector technologies to trigger and reconstruct muons: Drift Tubes in the barrel region (9Z9 o 1:2), Cathode Strip Chambers in the endcaps ð0:9 o 9Z9 o 2:4Þ , and Resistive Plate Chambers (RPCs) in both regions, as a dedicated muon trigger. We report on the operations and performance of the RPC system after 2 years of LHC running with increasing instantaneous luminosity. Special attention is given to the stability of the system and to the working point calibration procedures. & 2013 EURATOM. Published by Elsevier B.V. All rights reserved.
Keywords: Muon system RPC Gas detectors CMS
1. Introduction The Compact Muon Solenoid (CMS) [1] is a general-purpose particle physics detector built at the Large Hadron Collider (LHC) [2]. Its compact geometry is built around a 6 m diameter superconducting solenoid with a 3.8 T magnetic field. The iron return yoke of the magnet hosts the muon system, which combines three different gaseous detector technologies. In the barrel region, Drift Tube chambers (DTs) are arranged in coaxial layers to detect muons up to pseudorapidity 9Z9 o 1:2. The planar endcap regions are equipped with Cathode Strip Chambers (CSCs) ð0:9 o 9Z9 o2:4Þ to handle the higher rates and non-uniform magnetic field. Both detector regions are instrumented with Resistive Plate Chambers [3] (RPCs) ð9Z9 o1:6Þ, as a dedicated muon trigger. The RPC system is characterized by a time resolution t 3 ns and a spatial resolution of the order of 1 cm. Each of the 912 chambers has a double-gap (2 mm of width) design formed by two parallel bakelite electrodes (bulk resistivity 1010 O cm) and a common readout plane of copper strips. CMS RPCs are operated in avalanche mode, with a gas mixture composed of 95.2% C2H2F4, 4.5% C4H10 and 0.3% SF6, with a humidity of 40% at 21 1C [4]. This paper presents the results of performance studies of the RPC system related to 2011 operations. Focus is put on detector stability and working point calibration procedures.
2. High voltage scan A high voltage (HV) scan was carried out during early 2011 [5]. Collision data was recorded at 11 different HV settings, during a E-mail address: [email protected] & CERN for the benefit of the CMS Collaboration
1
series of dedicated runs, to define the optimal operating voltage for each chamber. Dependency of the applied HV on environmental pressure (P) and temperature (T) where taken into account [6], thus defining the effective HV : HVeff ¼ HV P=P 0 T 0 =T. P0 ¼965 mbar and T0 ¼293 K are the reference pressure and temperature respectively. RPC hits are predict by extrapolating, to the associated RPC strip plane, track segments reconstructed within the neighboring CSC or DT chamber. The extrapolated impact point is then matched to the closest cluster (set of adjacent strips fired by the passage of a ionizing particle). For each chambers partition, called roll, the measured efficiency as a function of HVeff , E, was fitted with a sigmoid function [7] and working points ðHVWP Þ were defined as: HVWP ¼ HVknee þ 100ð150Þ V for the barrel (endcap) region, where HVknee is the HVeff at E ¼ 95%. 3. Detector performance 3.1. Efficiency The agreement between efficiency measured in the subsequent runs (observed) and the predicted one measured using the fitting procedure confirmed the effectiveness of the technique. Average efficiency is found to be 94.9(93.8)% in the barrel (endcap) region. A maximum fluctuation of the efficiency of about 7 0:5% was observed, once the automatic online correction of the HV working point with the atmospheric pressure measured in the CMS cavern was applied. 3.2. Cluster size Pressure variations influence also the cluster size (CLS), defined as the number of continues strips fired at the passage of
0168-9002/ - see front matter & 2013 EURATOM. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2012.12.062
A. Cimmino / Nuclear Instruments and Methods in Physics Research A 718 (2013) 412–413
413
of fired strips in three or more planes within a 25 ns window and with hits matching predefined patterns. CMS Preliminary 2011
Without Pressure correction
The residuals (the distance in the RPC plane between the extrapolated impact point and the center of the RPC cluster) can be used as a measurement of the spatial resolution of the RPC system. The standard deviations of Gaussian fits to the distributions of the residuals are then taken as spatial resolutions. Results depend on strip width, CLS, and detector alignment. Measured resolutions goes from 0.81 to 1.32 cm in the barrel and from 0.86 to 1.28 cm in the endcap.
With Pressure correction
April 2011
3.4. Spacial resolution
October 2011
Fig. 1. CLS in the endcaps as a function of the run number (April and October 2011). The two regions, before and after the automatic pressure correction, are shown in the plot.
CMS Preliminary 2011
Outermost station
4. Background studies Single rate data were analyzed to study the radiation background level. The average background rate, measured in the RPC system at luminosity 3 1033 cm2 s1 , was 1:3 Hz=cm2 . The dependence between the background rate and luminosity was found to be linear as shown in Fig. 2. Linear extrapolation to 1034 cm 2 s 1 gives an average background of 6 Hz=cm2 and a maximum rate of 35240 Hz=cm2 that is still well below the limit of 100 Hz=cm2 used in the trigger design [9].
5. Conclusion Innermost station
Fig. 2. RPC background rate as a function of the instantaneous luminosity, for four radial stations of barrel region. Outermost station suffers from neutron background, while the innermost mainly affected by particles coming from the vertex.
Detector efficiency, cluster size, noise, spatial resolution, background rate have been studied. RPCs performance was well understood and tuned using regular and dedicated collision runs. The results show that the RPCs form a stable and reliable subdetector, effectively contributing to event triggering and reconstruction for the CMS physics programme. Data collected were also compared to Monte Carlo simulations. The agreement between the two confirms the excellent behavior of the RPC system. The HV scan procedure will be repeated at the beginning of each data taking year, to monitor in time the performance of the chambers and spot any aging effect. References
a ionizing particle. The stability of the system may be seen in Fig. 1. An overall average of 1.8 (RMS ¼0.3) was found, compatible with the trigger requirements of CLS o 2:0. A detailed description of the RPC trigger may be found in Ref. [8]. 3.3. Noise By measuring the hit rates during no-collision periods, the intrinsic noise per chamber was found to be well below 0.5 Hz cm2. These rates values are negligible for accidental muon triggering, since the RPC muon trigger system needs coincidence
[1] CMS Collaboration, Journal of Instrumentation 3 (2008) S08004. [2] L. Evans, P. Bryant, Journal of Instrumentation 3 (2008) S08001. [3] R. Santonico, R. Cardarelli, Nuclear Instruments and Methods 187 (157) (1981) 377. [4] M. Bosteels, et al., CMS Gas System Proposal, CMS Note 018 (1999). [5] S. Costantini on behalf of the CMS Collaboration, Calibration of the RPC working voltage in the CMS experiment, CMS CR 055 (2012). [6] M. Abbrescia, et al., Nuclear Instruments and Methods A 359 (1995) 603. [7] M. Abbrescia, et al., Nuclear Instruments and Methods A 550 (2005) 116. [8] CMS Collaboration, RPC Trigger, in CMS TriDAS project : Technical Design Report; 1. The Trigger Systems. CERN, Geneva, 2000, p. 419. [9] M. Rodozov on behalf of the CMS Collaboration, Analysis of the radiation background in CMS RPCs, PoS(RPC2012)044.