Abstract

The muon trigger system of the CMS experiment uses a combination of hardware and software to identify events containing a muon. During Run 2 (covering 2015–2018) the LHC achieved instantaneous luminosities as high as 2 × 1034 while delivering proton-proton collisions at √(s) = 13. The challenge for the trigger system of the CMS experiment is to reduce the registered event rate from about 40MHz to about 1kHz. Significant improvements important for the success of the CMS physics program have been made to the muon trigger system via improved muon reconstruction and identification algorithms since the end of Run 1 and throughout the Run 2 data-taking period. The new algorithms maintain the acceptance of the muon triggers at the same or even lower rate throughout the data-taking period despite the increasing number of additional proton-proton interactions in each LHC bunch crossing. In this paper, the algorithms used in 2015 and 2016 and their improvements throughout 2017 and 2018 are described. Measurements of the CMS muon trigger performance for this data-taking period are presented, including efficiencies, transverse momentum resolution, trigger rates, and the purity of the selected muon sample. This paper focuses on the single- and double-muon triggers with the lowest sustainable transverse momentum thresholds used by CMS. The efficiency is measured in a transverse momentum range from 8 to several hundred.

Highlights

  • We summarize the reconstruction algorithms deployed in the high-level trigger (HLT) throughout the data-taking period, focusing on the improvements made since the start of Run 2

  • Three types of gas-ionization chambers make up the CMS muon system: drift tube chambers (DTs), cathode strip chambers (CSCs), and resistive-plate chambers (RPCs)

  • To maintain the excellent performance of the CMS muon trigger system in the much harsher running conditions of LHC Run 2, especially the high-pileup environment in 2017 and 2018, the reconstruction algorithms and identification criteria for muons in the high-level trigger system were continually improved during the Run 2 data-taking period

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Summary

The CMS detector

The central feature of the CMS apparatus is a superconducting solenoid of 6 m internal diameter, providing a magnetic field of 3.8 T. During the data-taking period in 2016, the silicon tracker consisted of 1440 silicon pixel and 15 148 silicon strip detector modules. They were arranged in concentric layers, three pixel and 10 strips, around the beam axis in the central region of the detector, and in disks, two pixel and 12 strips, perpendicular to it in the forward directions. Before the data-taking period in 2017, an upgraded pixel detector consisting of 1856 modules was installed, adding an additional barrel layer closer to the interaction point and additional disks in the two forward parts of the detector [3].

Wheel 0
Data samples and trigger efficiencies
The Level-1 trigger
Muon reconstruction and selection in the high-level trigger
Level-2 muon reconstruction
Level-3 muon reconstruction
L3 muon seeded by L2: the cascade algorithm
L3 muon seeded by L1: the tracker muon algorithm
Performance of the cascade and tracker muon algorithms
L3 muon seeded by L2 and L1: the iterative algorithm
L3 muon reconstruction and identification performance
Muon isolation
Performance of selected single- and double-muon reference triggers
Trigger efficiency
Trigger rate
Processing time
Findings
Summary
Full Text
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