The performance of the CMS muon detector in proton-proton collisions at sqrt(s) = 7 TeV at the LHC

TL;DR

The CMS muon system—comprising DT, CSC, and RPC detectors—was evaluated using 7 TeV proton-proton data from 2010 (~40 pb^-1). The study demonstrates that all three subsystems meet or exceed design performance, including spatial and time resolutions, hit/segment reconstruction efficiencies, and L1 trigger efficiencies, with BX identification reaching >99% in time alignment. Calibrations, timing synchronization, and alignments were essential to achieving the reported performance, and the measured results agree well with detailed Monte Carlo simulations. Backgrounds were carefully measured and projected to higher luminosities, confirming robust operation and resilience to hardware failures, with reliable online/offline data quality monitoring ensuring high-quality physics data. Overall, the CMS muon system provides high-efficiency triggering and precise muon reconstruction across a wide eta range, enabling effective muon-based physics analyses at the LHC.

Abstract

The performance of all subsystems of the CMS muon detector has been studied by using a sample of proton--proton collision data at sqrt(s) = 7 TeV collected at the LHC in 2010 that corresponds to an integrated luminosity of approximately 40 inverse picobarns. The measured distributions of the major operational parameters of the drift tube (DT), cathode strip chamber (CSC), and resistive plate chamber (RPC) systems met the design specifications. The spatial resolution per chamber was 80-120 micrometers in the DTs, 40-150 micrometers in the CSCs, and 0.8-1.2 centimeters in the RPCs. The time resolution achievable was 3 ns or better per chamber for all 3 systems. The efficiency for reconstructing hits and track segments originating from muons traversing the muon chambers was in the range 95-98%. The CSC and DT systems provided muon track segments for the CMS trigger with over 96% efficiency, and identified the correct triggering bunch crossing in over 99.5% of such events. The measured performance is well reproduced by Monte Carlo simulation of the muon system down to the level of individual channel response. The results confirm the high efficiency of the muon system, the robustness of the design against hardware failures, and its effectiveness in the discrimination of backgrounds.

Figures (50)

• Figure 1: An $R$--$z$ cross section of a quadrant of the CMS detector with the axis parallel to the beam ($z$) running horizontally and radius ($R$) increasing upward. The interaction point is at the lower left corner. Shown are the locations of the various muon stations and the steel disks (dark grey areas). The 4 drift tube (DT, in light orange) stations are labeled MB ("muon barrel") and the cathode strip chambers (CSC, in green) are labeled ME ("muon endcap"). Resistive plate chambers (RPC, in blue) are in both the barrel and the endcaps of CMS, where they are labeled RB and RE, respectively.
• Figure 2: Photograph of a barrel wheel during the construction of CMS in June 2006. The 4 stations of DT chambers are separated by layers of the yoke steel (painted red). Several chambers had not yet been installed.
• Figure 3: Photograph of the ME1 muon station of the "plus" ($z >0$) endcap during the construction of CMS. Visible in concentric rings are the CSC types ME1/2 and ME1/3. The ME1/1 chambers are hidden behind the endcap calorimeters closest to the center. The endcap RPCs are in the layer behind the CSCs.
• Figure 4: Map of the $|B|$ field (left) and field lines (right) predicted for a longitudinal section of the CMS detector by a magnetic field model at a central magnetic flux density of 3.8$\text{\,T}$. Each field line represents a magnetic flux increment of 6$\text{\,Wb}$.
• Figure 5: Left: Schematic view of a DT chamber. Right: Section of a drift tube cell showing the drift lines and isochrones.