Search for Neutral Long-lived Particles Decaying in the CMS Muon Detectors with Run3

Many searches for beyond the standard model (SM) physics predict the existence of "long-lived particles" (LLPs). These LLPs are neutral, weakly-coupled, and have a long lifetime. Moreover, LLPs often have large displacement signatures allowing them to go undetected in conventional searches for prompt particles and thus remain largely unexplored at the LHC. We present an extension of a previous search at the LHC that used the CMS muon detectors (MD) as a sampling calorimeter capable of detecting displaced showers produced by LLP decays. The MD are composed of detector planes interleaved with the steel layers of the magnet flux return yoke. Decays of LLPs in the MD induce hadronic and electromagnetic showers, giving rise to a high hit multiplicity in localized detector regions that can be efficiently identified with a novel reconstruction technique. Additionally, the steel layers allow for exceptional background shielding from the SM which dominates existing LLP searches. This search is largely model-independent, can detect LLP masses as low as a few GeV, and is sensitive to many final states including hadrons, taus, electrons, and photons. Starting in Run3, a new high level trigger was developed for LLP searches allowing for a higher event rate and access to a larger kinematic regime. Using a partial dataset from Run3 and the new trigger, we present a sensitive measurement of the LLPs proper lifetime from 0.1m to 1000m.