Search for Long-lived Particles Decaying in the CMS Endcap Muon System in Proton-Proton Collisions at $\sqrt{s}=13~\mathrm{TeV}$

A search for long-lived particles (LLPs) produced in decays of standard model (SM) Higgs bosons in $137~\mathrm{fb}^{-1}$ of proton-proton collisions at $\sqrt{s}=13~\mathrm{TeV}$ recorded by the CMS experiment during $2016$$-$$2018$ is presented. The search employs a novel technique to reconstruct hadronic decays of LLPs in the endcap muon system. The search is sensitive to a broad range of LLP decay modes including $\tau^{-}\tau^{+}$, LLP masses as low as a few GeV, and is largely model-independent. No excess of events above the SM background is observed and stringent limits on the Higgs boson ($\mathrm{h}^0$) branching fraction to LLPs ($\mathrm{S}$) are obtained, particularly for proper decay lengths greater than a few meters. This search result represents the most stringent limits on the branching fraction $\mathcal{B}(\mathrm{h}^0 \rightarrow\mathrm{S}\mathrm{S})$ for proper decay lengths greater than $6$$-$$40$ m for S masses between $7$$-$$40$ GeV.