Implementation and Performance of a High Multiplicity Trigger for Long-Lived Particles in the CMS Detector

Many well-motivated extensions to the Standard Model predict the existence of long-lived particles (LLPs), whose lifetime and production cross-section may enable LHC experiments to be sensitive to their decays. An LLP that decays in the CMS Muon System may induce a particle shower, resulting in a localized cluster of hits known as a Muon Detector Shower (MDS). A dedicated trigger that selects events with a high multiplicity MDS may improve signal acceptance by a factor of 30 for MDS-based analyses. We present the design and implementation of a dedicated Level 1 Trigger and High Level Trigger in CMS that targets LLP-induced showers in the Cathode Strip Chambers (CSCs). At Level 1, the trigger targets events with a larger number of anode and/or cathode hits within a chamber of the CSCs, and we demonstrate that this method efficiently selects events with a high multiplicity cluster. After initializing the HMT trigger in July 2022, the performance is assessed using data from 2022 and 2023.