Performance of the CMS Level-1 trigger in proton-proton collisions at $\sqrt{s} =$ 13 TeV

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 $\times$ 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.

• The paper demonstrates significant performance enhancements in the CMS Level-1 trigger system for high-luminosity 13 TeV collisions.
• It details advanced methodologies such as boosted decision tree regression and pattern recognition for improved muon, electron, and photon triggering.
• The upgraded trigger reduces false positives and enhances energy resolution, bolstering precise event selection for high-energy physics analyses.

Performance of the CMS Level-1 Trigger in Proton-Proton Collisions at 13 TeV

The CMS collaboration at the Large Hadron Collider (LHC) has achieved significant advancements in their particle physics experiments through the comprehensive upgrade of the Level-1 trigger system during Run 2, from 2016 to 2018. This paper explores the performance enhancements of the CMS Level-1 trigger, which are pivotal for managing the complex conditions of high-energy proton-proton collisions at a center-of-mass energy of 13 TeV.

Overview of Upgraded Trigger System

The Level-1 trigger system benefits from sophisticated algorithmic approaches and enhanced hardware, designed to maintain high efficiency and reduce the rate of false positives despite increasing luminosity and pileup effects. As the LHC's luminosity reached levels two times higher than its design value, the upgraded trigger's latency and throughput capabilities were critical to handle up to 2.1 x 10^34 cm^-2 s^-1.

Key upgrades include the application of pattern recognition and boosted decision tree (BDT) regression techniques for muon reconstruction. These techniques help improve the discrimination between signal and background events, crucial for accurate selection and analysis of physics events.

Detailed Improvements in Muon and Calorimeter Triggers

The paper details several significant advancements across various elements of the Level-1 trigger:

• Muon Trigger: The BMTF, OMTF, and EMTF provide refined muon detection with enhanced transverse momentum resolution via optimized track-finding and BDT algorithms. These improvements yielded a reduction in trigger rates, enabling more precise capture of genuine physics events.
• Electron, Photon, and Tau Triggers: Refined clustering algorithms in the calorimeter trigger system, for instance, improved electron/photon identification through dynamic energy summation across trigger towers, also enabling more granular pileup-independent isolation thresholds.
• Jet and Energy Sum Triggers: These now employ pileup subtraction algorithms to ensure consistent energy resolution and more accurate \MET calculations. This advancement helps in making the trigger system more robust against the increased pileup common in Run 2 conditions.
• Global Trigger Enhancements: New capabilities like invariant mass computation and cross-object correlations allow for more precise event selections, essential for inclusive physics studies and rare event searches.

Impact on Physics Analysis and Future Prospects

The upgraded Level-1 trigger has shown substantial improvements in efficiency and resolution across various detection channels while maintaining overall robustness in increasingly demanding conditions. Moving forward, these advances contribute substantially to the CMS's capacity to study Standard Model physics and search for new phenomena such as supersymmetry and dark matter candidates.

The practical implications of these improvements are evident in increased data quality and analysis potential. For example, low-mass dimuon resonance searches benefit from finely tuned dimuon invariant mass calculations at the trigger level, enhancing the discovery potential for rare processes.

Conclusion

In summary, the CMS Level-1 trigger upgrade has successfully adapted the system to the LHC's advanced operational requirements in Run 2, underlining the CMS collaboration's commitment to pushing the frontiers of high-energy physics. As the LHC advances to future runs with expected even higher luminosities, such foundational improvements will be indispensable for CMS to meet its physics goals and extend our understanding of fundamental particle interactions.