Search for dark photons produced in 13 TeV $pp$ collisions

Searches are performed for both prompt-like and long-lived dark photons, $A^{\mathrm{\prime }}$, produced in proton-proton collisions at a center-of-mass energy of 13 TeV, using $A^{\mathrm{\prime }}\to {\mu }^{+}{\mu }^{-}$ decays and a data sample corresponding to an integrated luminosity of 1.6 fb${}^{-1}$ collected with the LHCb detector. The prompt-like $A^{\mathrm{\prime }}$ search covers the mass range from near the dimuon threshold up to 70 GeV, while the long-lived $A^{\mathrm{\prime }}$ search is restricted to the low-mass region $214<m(A^{\mathrm{\prime }})<350$ MeV. No evidence for a signal is found, and 90% confidence level exclusion limits are placed on the $\gamma$-$A^{\mathrm{\prime }}$ kinetic-mixing strength. The constraints placed on prompt-like dark photons are the most stringent to date for the mass range $10.6<m(A^{\mathrm{\prime }})<70$ GeV, and are comparable to the best existing limits for $m(A^{\mathrm{\prime }})<0.5$ GeV. The search for long-lived dark photons is the first to achieve sensitivity using a displaced-vertex signature.