Search for long-lived heavy neutral leptons decaying in the CMS muon detectors in proton-proton collisions at $\sqrt{s}=13~\mathrm{TeV}$

A search for heavy neutral leptons (HNLs) decaying in the CMS muon system is presented. The data sample consists of 137$~\mathrm{fb}^{-1}$ of proton-proton collisions at $\sqrt{s}=13~\mathrm{TeV}$, recorded at the LHC in 2016--2018. Decay products of long-lived HNLs could interact with the shielding materials in the CMS muon system and create hadronic and electromagnetic showers detected by the muon chambers. This distinctive signature provides a unique handle to search for HNLs with masses below 10 GeV and proper decay lengths of the order of meters. The signature is sensitive to HNL couplings to all three generations of leptons. Candidate events are required to contain a prompt electron or muon and a shower in the muon chambers. No significant deviations from the standard model background expectation are observed, and the most stringent limits to date are found for HNLs in the mass range of 2.1--3.0 (1.9--3.3) GeV, reaching squared mixing parameter values as low as $8.9\times10^{-6}$ ($4.6\times10^{-6}$) in the electron (muon) channel.