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

A search for long-lived particles (LLPs) decaying in the CMS muon detectors is presented. The data sample consists of $138~\mathrm{fb}^{-1}$ of proton-proton collisions at $\sqrt{s}=13~\mathrm{TeV}$, recorded at the LHC in $2016$$-$$2018$. A unique technique is employed to reconstruct decays of LLPs in the muon detectors. The search is sensitive to LLP masses from 0.4 to $55~\mathrm{GeV}$ and a broad range of LLP decay modes, including decays to a pair of quarks, kaons, pions, electrons, photons, or $\tau$ leptons. No excess of events above the standard model background is observed. The most stringent LHC limits to date on the branching fraction of the Higgs boson to a pair of LLPs with masses below $10~\mathrm{GeV}$ are set. This search also provides the most stringent limit for proper decay lengths in the range $0.04$$-$$0.4~\mathrm{m}$ and above $4~\mathrm{m}$ for an LLP mass of $15~\mathrm{GeV}$, in the range $0.3$$-$$0.9~\mathrm{m}$ and above $3~\mathrm{m}$ for an LLP mass of $40~\mathrm{GeV}$, and above $0.8~\mathrm{m}$ for an LLP mass of $55~\mathrm{GeV}$. Finally, this search sets the first LHC limits on a dark quantum chromodynamic sector that couples to the Higgs boson through gluon, Higgs, photon, vector, and dark photon portals, and is sensitive to branching fractions of the Higgs boson to dark quarks as low as $10^{-3}$.