Search for Hidden Valley dark showers with displaced muons with the CMS experiment

A search for signatures of a dark analog to quantum chromodynamics is performed. The analysis targets long-lived dark mesons that decay into Standard Model particles with a high branching fraction to muons. It is the first search at the Large Hadron Collider (LHC) that targets the decay of Hidden Valley dark showers into muons. A unique dataset with $10^{10}$ B meson events is used. It was collected by the CMS experiment at the CERN LHC in 2018 using displaced muon triggers, which have high efficiency for the signal models. Resonant dimuon signatures are searched for, with both pointing and non-pointing topologies. No significant excess is observed beyond the Standard Model expectation. Upper limits on the branching ratio of the Higgs boson decays to dark partons are determined to be as low as about $10^{-4}$, at $95\%$ confidence level, surpassing and extending existing limits for the mean proper lifetime of less than approximately $0.1~\mathrm{m}$ and for a mass as low as $2~\mathrm{GeV}$. First limits are set for extended dark shower models, probing the low-mass region down to $0.33~\mathrm{GeV}$.