Search for light long-lived neutral particles that decay to collimated pairs of leptons or light hadrons in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

Abstract

A search for light long-lived neutral particles with masses in the $O$(MeV-GeV) range is presented. The analysis targets the production of long-lived dark photons in the decay of a Higgs boson produced via gluon-gluon fusion or in association with a $W$ boson. Events that contain displaced collimated Standard Model fermions reconstructed in the calorimeter or muon spectrometer are selected in 139 fb${}^{-1}$ of $\sqrt{s}=13$ TeV $pp$ collision data collected by the ATLAS detector at the LHC. Background estimates for contributions from Standard Model processes and instrumental effects are extracted from data. The observed event yields are consistent with the expected background. Exclusion limits are reported on the production cross-section times branching fraction as a function of the mean proper decay length $c\tau$ of the dark photon, or as a function of the dark-photon mass and kinetic mixing parameter that quantifies the coupling between the Standard Model and potential hidden (dark) sectors. A Higgs boson branching fraction above 1% is excluded at 95% CL for a Higgs boson decaying into two dark photons for dark-photon mean proper decay lengths between 10 mm and 250 mm and dark photons with masses between 0.4 GeV and 2 GeV.

52 pages in total, 13 figures, 5 tables. All figures including auxiliary figures are available at http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/EXOT-2019-05

References in corpus (18)

• The automated computation of tree-level and next-to-leading order differential cross sections, and their matching to parton shower simulations
• An Introduction to PYTHIA 8.2
• Matching NLO QCD computations with Parton Shower simulations: the POWHEG method
• Event generation with SHERPA 1.1
• Echoes of a Hidden Valley at Hadron Colliders
• Secluded U(1) below the weak scale
• New Fixed-Target Experiments to Search for Dark Gauge Forces
• Illuminating Dark Photons with High-Energy Colliders
• Non-Abelian Dark Sectors and Their Collider Signatures
• Probing a Secluded U(1) at B-factories
• Search for $A^{\prime }\to {\mu }^{+}{\mu }^{-}$ decays
• Kinetic Mixing as the Origin of Light Dark Scales
• Discovering New Light States at Neutrino Experiments
• Constraints on sub-GeV hidden sector gauge bosons from a search for heavy neutrino decays
• Constraints on Hidden Photon Models from Electron g-2 and Hydrogen Spectroscopy
• Discovering Higgs Decays to Lepton Jets at Hadron Colliders
• Monitoring and data quality assessment of the ATLAS liquid argon calorimeter
• Exotic Higgs decays in the golden channel

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