Search for $A^{\mathrm{\prime }}\text{\hspace{0.17em}⁣}\to \text{\hspace{0.17em}⁣}{\mu }^{+}{\mu }^{-}$ decays

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. These searches look for $A^{\mathrm{\prime }}\text{\hspace{0.17em}⁣}\to \text{\hspace{0.17em}⁣}{\mu }^{+}{\mu }^{-}$ 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 $\gamma$-$A^{\mathrm{\prime }}$ kinetic-mixing strength. The prompt-like $A^{\mathrm{\prime }}$ 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 $214<m(A^{\mathrm{\prime }})\lesssim 740$ MeV and $10.6<m(A^{\mathrm{\prime }})\lesssim 30$ GeV. The search for long-lived $A^{\mathrm{\prime }}\text{\hspace{0.17em}⁣}\to \text{\hspace{0.17em}⁣}{\mu }^{+}{\mu }^{-}$ decays places world-leading constraints on low-mass dark photons with lifetimes $\mathcal{O}(1)$ ps.