Search prospects for dark-photons decaying to displaced collimated jets of muons at HL-LHC

Several models of new physics beyond the Standard Model predict the existence of neutral particles that decay to pairs of leptons. These particles can also be long-lived with decay length comparable to, or even larger than, the LHC detectors dimensions. The triggering and the standalone tracking capabilities of the ATLAS muon spectrometer have been exploited to search for neutral long-lived particles decaying to pairs of muons in LHC Run-2 13 TeV data and set exclusion limits on their mass and lifetime. The enormous amount of data that will be collected by ATLAS during the Run-3 (300 fb−1) and High-Luminosity (3000 fb−1) 14 TeV LHC phase, and the updated ATLAS detector setup, will offer a unique opportunity to probe unexplored regions of phase space in the context of such searches. This note presents sensitivity prospects for Run-3 and High Luminosity LHC discussed in the context of a Hidden Sector model predicting the decay of the Higgs boson to two neutral long-lived particles subsequently decaying into a pair of displaced muons. Two new muon trigger algorithms are studied to improve the selection efficiency of displaced muon pairs.