Performance of the CMS muon trigger system in proton-proton collisions at $\sqrt{s} =$ 13 TeV

TL;DR

This paper evaluates the CMS muon trigger system's performance during Run 2 at 13 TeV, highlighting improvements in algorithms that maintained high efficiency and purity despite increased collision rates and pileup.

Contribution

It details the development and implementation of improved muon reconstruction algorithms that sustain trigger performance at high luminosities during Run 2.

Findings

Muon trigger efficiency remains high across a wide pT range.

Trigger rates are effectively managed despite increased pileup.

Muon sample purity is maintained at high levels.

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 $\times$ 10$^{34}$cm$^{-2}$s$^{-1}$ while delivering proton-proton collisions at $\sqrt{s} =$ 13 TeV. The challenge for the trigger system of the CMS experiment is to reduce the registered event rate from about 40 MHz to about 1 kHz. 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 GeV.