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Fermiophobic $Z'$ model for simultaneously explaining the muon anomalies $R_{K^{(*)}}$ and $(g-2)_μ$

We discuss a simple renormalisable, gauge invariant model with a fermiophobic $Z'$ boson: it has no couplings to the three Standard Model (SM) chiral families, but does couple to a fourth vector-like (VL) family. The SM Higgs couples to the fourth VL lepton, leading to an enhanced contribution to the muon anomalous magnetic moment $\left(g-2\right)_μ$. The latter contribution requires a non-vanishing coupling of $Z'$ to right-handed muons, which arises within this model due to mixing effects between the SM and VL fermions, along with $Z'$ couplings to the second generation SM lepton doublet and third generation SM quark doublet. This model can simultaneously account for the measured $B$-decay ratios $R_{K^{(*)}}$ and $\left(g-2\right)_μ$. We identify the parameter space where this explanation is consistent with existing experimental constraints coming from $B_{s}-\bar{B}_{s}$ mixing, neutrino trident production and collider searches. We also check that the SM Higgs coupling to the fourth VL lepton does not produce a dangerous contribution to the Higgs diphoton decay.