Impact of the GE1/1 station on the performance of the muon system in CMS

Highlights

• In 2018 the forward CMS muon system will be upgraded with GEM detectors.

• The detectors have high rate capability to sustain the expected background.

• They will allow to reduce muon trigger rates at high luminosity scenarios.

• They will provide detection redundancy in case of ageing damages on CSC chambers.

Abstract

In 2018 a new muon station based on GEM detectors will be installed in the CMS experiment. The present work sums up the main results obtained to evaluate the impact of this upgrade on the muon trigger rate and on muon reconstruction.

Introduction

The muon spectrometer of the Compact Muon Solenoid experiment was designed as an almost hermetic and redundant system relying on different ionization detectors. Four stations of Drift Tubes (DT) in the barrel and Cathode Strip Chambers (CSC) in the endcaps provide position measurement and triggering. In addition, Resistive Plate Chambers (RPC), with their coarser granularity but fast timing properties, guarantee trigger redundancy in all the spectrometer, apart from the forward endcap region that covers the pseudorapidity range $1.6<\left|\eta \right|<2.4$ [1]. As a matter of fact, this region has background rates at the limit of the RPC rate capability (up to 1 kHz/cm²), therefore it is equipped solely with CSC chambers. The high η region not only has the least redundancy, but it is also the most challenging for muon triggering and reconstruction [2] due to the exceptionally high rates and the reduced magnetic field.

After the upgrades planned for the Long Shutdowns LS2 and LS3 foreseen in 2018 and 2023, the Large Hadron Collider luminosity will exceed the design value by a factor two and five respectively. Maintaining efficient muon triggering in the forward region after the LHC upgrades represents a particular challenge. Already at an instantaneous luminosity of $1.7\times {10}^{34}$ cm⁻² s⁻¹ the rate of the single muon trigger in this restricted part of the muon system is expected to approach values of 10 kHz, corresponding to one tenth of the entire Level-1 trigger bandwidth [3], [4]. The trigger rate could be reduced simply by increasing the muon transverse momentum p_T threshold, but this would deeply affect the sensitivity of all the physics analyses that include soft muons in their signatures.