Dark photons from charm mesons at LHCb

Abstract

We propose a search for dark photons $A^{\prime }$ at the LHCb experiment using the charm meson decay $D^{\ast }(2007{)}^0\to D^0{A}^{\prime }$. At nominal luminosity, $D^{\ast 0}\to D^0\gamma$ decays will be produced at about 700kHz within the LHCb acceptance, yielding over 5 trillion such decays during Run 3 of the LHC. Replacing the photon with a kinetically-mixed dark photon, LHCb is then sensitive to dark photons that decay as $A^{\prime }\to e^{+}{e}^{-}$. We pursue two search strategies in this paper. The displaced strategy takes advantage of the large Lorentz boost of the dark photon and the excellent vertex resolution of LHCb, yielding a nearly background-free search when the $A^{\prime }$ decay vertex is significantly displaced from the proton-proton primary vertex. The resonant strategy takes advantage of the large event rate for $D^{\ast 0}\to D^0{A}^{\prime }$ and the excellent invariant mass resolution of LHCb, yielding a background-limited search that nevertheless covers a significant portion of the $A^{\prime }$ parameter space. Both search strategies rely on the planned upgrade to a triggerless-readout system at LHCb in Run 3, which will permit identification of low-momentum electron-positron pairs online during data taking. For dark photon masses below about 100MeV, LHCb can explore nearly all of the dark photon parameter space between existing prompt-$A^{\prime }$ and beam-dump limits.

Comments: 19 pages, 11 figures, 1 table; v2: references added