Search for A 0 → μ + μ − Decays

Searches are performed for both promptlike and long-lived dark photons, A0, produced in proton-proton collisions at a center-of-mass energy of 13 TeV. These searches look for A0 → μþμ− decays using a data sample corresponding to an integrated luminosity of 5.5 fb−1 collected with the LHCb detector. Neither search finds evidence for a signal, and 90% confidence-level exclusion limits are placed on the γ–A0 kinetic mixing strength. The promptlike A0 search explores the mass region from near the dimuon threshold up to 70 GeV and places the most stringent constraints to date on dark photons with 214 < mðA0Þ≲ 740 MeV and 10.6 < mðA0Þ≲ 30 GeV. The search for long-lived A0 → μþμ− decays places world-leading constraints on low-mass dark photons with lifetimes Oð1Þ ps.