Abstract
A massless dark photon could affect standard-model particles only via higher-dimensional operators and would therefore have eluded recent searches for its massive counterpart, which were based on the assumption that the latter had renormalizable interactions with known fermions due to gauge kinetic mixing. In this study we entertain the possibility that the massless dark photon has nonnegligible flavor-changing dipole-type couplings with the $u$ and $c$ quarks, giving rise to the decays of charmed hadrons into a lighter hadron plus missing energy carried away by the dark photon. We propose to investigate decays of this kind, especially those in which the parents are the charmed pseudoscalar-mesons $D^+$, $D^0$, and $D_s^+$ and singly charmed baryons $\Lambda_c^+$, $\Xi_c^+$, and $\Xi_c^0$. Employing a simplified new-physics model satisfying the relevant constraints, we find that the branching fractions of these processes could be as large as several times $10^{-5}$. This suggests that one or more of them might in the near future fall within reach of the ongoing Belle II and BESIII experiments. Since the same underlying operators are responsible for all of these transitions, detecting one of them automatically implies particular predictions for the others, allowing for additional experimental checks on the massless-dark-photon scenario.
Highlights
Attempts to address longstanding open questions in physics, such as the nature and origin of neutrino mass and the particle identity of cosmic dark matter, have increasingly postulated the existence of a dark sector beyond the standard model (SM)
It is crucial that upcoming endeavors to seek dark photons take into account the possibility that they are massless, in which case they may have consequential flavor-changing neutral current (FCNC) interactions with SM fermions
We explore the latter scenario, that where the massless dark photon has nonnegligible flavor-changing dipole-type couplings to the u and c quarks
Summary
This symmetry may be spontaneously broken or stay unbroken, causing the associated gauge boson, the dark photon, to gain mass or remain massless, respectively Whether it is massive or massless, the hope is that the dark photon can somehow communicate with the SM as well as connect it to other constituents of the dark side, leading to interesting and potentially observable consequences. The implications are that it has no direct couplings to SM fermions, in contrast to its massive counterpart, and that restrictions inferred from the aforesaid quests for A0 do not apply to γ The latter can still affect the SM sector through higherdimensional operators induced by loop diagrams containing particles charged under Uð1ÞD and coupled to SM fields [6,7,8]. The couplings of γto quarks are of dipole type and given by the gauge-invariant
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