CMS Run 2 high level trigger performance

Abstract

The Compact Muon Solenoid (CMS) experiment selects events with a two-level trigger system, the Level-1 (L1) trigger and the High Level trigger (HLT). The HLT is a farm of approximately 30K CPU cores that reduces the rate from 100 kHz to about 1 kHz. The HLT has access to the full detector readout and runs a streamlined version of the offline event reconstruction. In LHC Run 2 the peak instantaneous luminosity reached values above 2×1034cm−2s−1, posing a challenge to the online event selection. An overview of the object reconstruction and trigger selections used in the 2016-2018 data-taking period will be presented. The performance of the main trigger paths and the lessons learned will be summarised, also in view of the coming LHC Run 3.