Abstract
At the start of Run 2 in 2015, the LHC delivered proton-proton collisions at a center-of-mass energy of 13\\TeV. During Run 2 (years 2015–2018) the LHC eventually reached a luminosity of 2.1× 1034 cm-2s-1, almost three times that reached during Run 1 (2009–2013) and a factor of two larger than the LHC design value, leading to events with up to a mean of about 50 simultaneous inelastic proton-proton collisions per bunch crossing (pileup). The CMS Level-1 trigger was upgraded prior to 2016 to improve the selection of physics events in the challenging conditions posed by the second run of the LHC. This paper describes the performance of the CMS Level-1 trigger upgrade during the data taking period of 2016–2018. The upgraded trigger implements pattern recognition and boosted decision tree regression techniques for muon reconstruction, includes pileup subtraction for jets and energy sums, and incorporates pileup-dependent isolation requirements for electrons and tau leptons. In addition, the new trigger calculates high-level quantities such as the invariant mass of pairs of reconstructed particles. The upgrade reduces the trigger rate from background processes and improves the trigger efficiency for a wide variety of physics signals.
Highlights
The CERN LHC collides bunches of particles in the CMS and ATLAS experiments at a maximum rate of about 40 MHz, where the bunches are spaced 25 ns apart
This paper describes the performance of the CMS Level-1 trigger upgrade during the data taking period of 2016–2018
The efficiencies are calculated for different types of trigger seeds using a tag-and-probe method, and the resolutions are determined by comparing the triggerlevel kinematic variables with their offline reconstructed counterparts, to the performance studies presented in this paper
Summary
The CERN LHC collides bunches of particles in the CMS and ATLAS experiments at a maximum rate of about 40 MHz, where the bunches are spaced 25 ns apart. The instantaneous luminosity steadily increased throughout Run 2, which ended in 2018 These changes were designed to provide a larger data set for studies of rare interactions and searches for new physics, but they presented several challenges to the trigger system. Improved trigger algorithms were needed to enhance the separation of signal and background events and to provide more accurate energy reconstruction in the presence of a larger number of simultaneous collisions per LHC bunch crossing (pileup). Energy, and pileup, the upgraded Level-1 trigger maintained or increased its efficiency to separate the chosen signal events from background, because of finer detector input granularity, enhanced object reconstruction (e.g., μ, e/γ , jet, τ, and energy sums), and correlated multi-object triggers targeting specific physics signatures. This paper describes the trigger algorithms of the Phase 1 Level-1 trigger upgrade and reports their performance, measured using Run 2 data.
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