Constraints on Dark Photon and Dark $Z$ Model Parameters in the $B$ and $K$ Meson Decays

The study investigates flavor-changing neutral current (FCNC) decays of $B$ and $K$ mesons in the context of a dark $U(1)_D$ model with a dark photon/dark $Z$ mass between 10 MeV and 2 GeV. While the model improves the fit to certain decay distributions, such as $B \to K^{(*)} \ell^+ \ell^-$ and $B_s \to \phi \mu^+ \mu^-$, it is ruled out by stringent experimental constraints, including atomic parity violation, $K^+ \to \mu^+ + \text{invisible}$, and $B_s - \overline{B}_s$ mixing. To address these constraints, the model is extended with three modifications; allowing additional invisible decays of $Z_D$, introducing a direct vector coupling of $Z_D$ to muons, and including a direct coupling of $Z_D$ to both muons and electrons, with fine-tuning to cancel the mixing-induced coupling to electrons. Among these extensions, only the third scenario, involving fine-tuned electron coupling, remains consistent with all experimental constraints.