Search for $A!to!mu^+mu^-$ 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 $A!to!mu^+mu^-$ 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 $gamma$-$A$ 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 $214 < m(A) lesssim 740$ MeV and $10.6 < m(A) lesssim 30$ GeV. The search for long-lived $A!to!mu^+mu^-$ decays places world-leading constraints on low-mass dark photons with lifetimes $mathcal{O}(1)$ ps.