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 $\times$ 10$^{34}$ cm$^{-2}$ s$^{-1}$, 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.

中文翻译：

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CMS Level-1 触发器在质子-质子碰撞中的性能，√s = 13 TeV

在 2015 年运行 2 开始时，大型强子对撞机以 13 TeV 的质心能量进行质子-质子碰撞。在 Run~2（2015--2018 年）期间，LHC 最终达到了 2.1 $\times$ 10$^{34}$ cm$^{-2}$ s$^{-1}$ 的光度，几乎是三倍在运行 1（2009-2013）期间达到的，并且比 LHC 设计值大两倍，导致每个束交叉（堆积）平均约 50 次同时发生的非弹性质子 - 质子碰撞事件。CMS Level-1 触发器在 2016 年之前进行了升级，以改进在 LHC 第二次运行所带来的挑战性条件下物理事件的选择。本文描述了2016-2018年数据采集期间CMS Level-1触发器升级的性能。升级后的触发器实现了用于 μ 子重建的模式识别和增强决策树回归技术，包括喷射和能量总和的堆积减法，并结合了电子和 tau 轻子的堆积依赖隔离要求。此外，新触发器计算高级量，例如重建粒子对的不变质量。升级降低了后台进程的触发率，提高了各种物理信号的触发效率。