Search for low-mass hidden-valley dark showers with non-prompt muon pairs in proton-proton collisions at $\sqrt{s}$ = 13 TeV

TL;DR

This paper searches for long-lived dark mesons from a hidden-valley sector produced by Higgs decays, using CMS data and machine learning to set new limits on dark-shower models with muon signatures.

Contribution

It introduces a novel search for low-mass, long-lived dark mesons decaying into muons, employing machine learning and setting the first limits on extended dark-shower models.

Findings

No significant excess observed above standard model expectations.

Upper limits on Higgs to dark partons branching fraction as low as 10^{-4}.

First constraints on dark-shower models with two dark flavors and dark photons.

Abstract

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 of the dark mesons decaying into muons. The dark mesons are formed by the hadronisation of dark partons, which are produced by a decay of the Higgs boson. The search is performed using a data set corresponding to an integrated luminosity of 41.6 fb$^{-1}$, which was collected in proton-proton collisions at $\sqrt{s}$ = 13 TeV by the CMS experiment at the CERN LHC in 2018 using non-prompt muon triggers. The search is based on resonant muon pair signatures. Machine-learning techniques are employed in the analysis, utilising boosted decision trees to discriminate between signal and background. No significant excess is observed above the standard model expectation. Upper limits on the branching fraction of the Higgs boson decaying to dark partons are determined to be as low as 10$^{-4}$ at 95% confidence level, surpassing and extending the existing limits on models with dark $\tilde{\omega}$ mesons for mean proper decay lengths of less than 500 mm and for $\tilde{\omega}$ masses down to 0.3 GeV. First limits are set for extended dark-shower models with two dark flavours that contain dark photons, probing their masses down to 0.33 GeV.