Search for $A^{\prime }\to \hat{I}{¼}^{+}\hat{I}{¼}^{-}$ decays

Abstract

Searches are performed for both prompt-like and long-lived dark photons, $A^{\prime }$, produced in proton-proton collisions at a center-of-mass energy of 13 TeV. These searches look for $A^{\prime }\to \hat{I}{¼}^{+}\hat{I}{¼}^{-}$ 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 ${\hat{I}}^3$-$A^{\prime }$ kinetic-mixing strength. The prompt-like $A^{\prime }$ 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 $214<m(A^{\prime })\lesssim 740$ MeV and $10.6<m(A^{\prime })\lesssim 30$ GeV. The search for long-lived $A^{\prime }\to \hat{I}{¼}^{+}\hat{I}{¼}^{-}$ decays places world-leading constraints on low-mass dark photons with lifetimes $\mathcal{O}(1)$ ps.

All figures and tables, along with any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2019-031.html (LHCb public pages)