Design and performance of the upgrade of the CMS L1 muon trigger

Abstract

After the Long Shutdown 1 (LS1) LHC will run at a center of mass energy of 13TeV, providing CMS with proton collisions at an expected luminosity which is almost double the LHC design value of 1034cm−2s−1, and almost three times the peak luminosity reached during Run1 of 7.7·1033cm−2s−1. The higher luminosity and center of mass energy of the LHC will raise the Level 1 (L1) muon trigger rate by almost a factor six for a given muon transverse momentum pT threshold. It is therefore necessary to increase the muon (pT) threshold to keep the trigger rate below 100kHz, the maximum sustainable rate for the CMS detectors. An increase of the L1 trigger thresholds implies a lowering of the efficiency in detecting signals from new physics. The CMS muon trigger is upgraded using custom designed AMC boards, with more powerful FPGAs and larger memories. The upgraded CMS muon trigger system implements pattern recognition and MVA (Boosted Decision Tree) regression techniques in the trigger boards for muon pT assignment, drastically reducing the trigger rate and improving the trigger efficiency. The upgraded system design exploits the redundancy of the CMS muon detectors at a very early stage merging different muon detector information already at L1. The pileup subtracted information from the upgraded calorimeter trigger allows to require isolated muons already in the L1 algorithms. The upgrade trigger is also designed to include inputs from GEM, the phase 2 muon detector upgrade in the very high pseudorapidity region.