Search for A′→μ+μ− decays
Searches are performed for both prompt-like and long-lived dark photons, A′, produced in proton-proton collisions at a center-of-mass energy of 13 TeV. These searches look for A′→μ+μ− decays using a data sample corresponding to an integrated luminosity of 5.5/fb collected with the LHCb detector. Neither search finds evidence for a signal, and 90 confidence-level exclusion limits are placed on the γ-A′ kinetic-mixing strength. The prompt-like A′ 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<m(A′)≲740 MeV and 10.6<m(A′)≲30 GeV. The search for long-lived A′→μ+μ− decays places world-leading constraints on low-mass dark photons with lifetimes O(1) ps.