Only a fine-tuned dark Z model with cancelled electron couplings survives the combined constraints, but its muon g-2 contribution is orders of magnitude too large.
Measurement of the $B^0 \rightarrow K^{*0}e^+e^-$ branching fraction at low dilepton mass
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abstract
The branching fraction of the rate decay $B^0 \rightarrow K^{*0}e^+e^-$ in the dilepton mass region from 30 to 1000 MeV$/c^2$ has been measured by the LHCb experiment, using $pp$ collision data, corresponding to an integrated luminosity of 1.0 fb$^{-1}$, at a centre-of-mass energy of 7 TeV. The decay mode $B^0 \rightarrow J/\psi(e^+e^-) K^{*0}$ is utilized as a normalization channel. The branching fraction $B^0 \rightarrow K^{*0}e^+e^-$ is measured to be $$ B(B^0 \rightarrow K^{*0}e^+e^-)^{30-1000 MeV/c^2}= (3.1\, ^{+0.9\mbox{} +0.2}_{-0.8\mbox{}-0.3} \pm 0.2)\times 10^{-7}, $$ where the first error is statistical, the second is systematic, and the third comes from the uncertainties on the B^0 \rightarrow J/\psi K^{*0}$ and $J/\psi \rightarrow e^+e^- $ branching fractions.
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Constraints on Dark Photon and Dark $Z$ Model Parameters in the $B$ and $K$ Meson Decays
Only a fine-tuned dark Z model with cancelled electron couplings survives the combined constraints, but its muon g-2 contribution is orders of magnitude too large.