Plan B: new Z ′ models for b → sℓ + ℓ − anomalies

Measurements of b → sμ + μ − transitions indicate that there may be a new physics field coupling to di-muon pairs associated with the b to s flavour transition. Including the 2022 LHCb reanalysis of R K and R_K^∗ , one infers that there may also be associated new physics in b → se + e − transitions. Here, we examine the extent of the statistical preference for Z ′ models coupling to di-electron pairs taking into account the relevant constraints, in particular from experiments at LEP-2. We identify an anomaly-free set of models which interpolates between the Z ′ not coupling to electrons at all, to one in which there is an equal Z ′ coupling to muons and electrons (but where in all models in the set, the Z ′ boson can mediate b → sμ + μ − transitions). A 3 B 3 − L e − 2 L μ model provides a close-to-optimal fit to the pertinent measurements along the line of interpolation. We have (re-)calculated predictions for the relevant LEP-2 observables in terms of dimension-6 SMEFT operators and put them into the flavio computer program, so that they are available for global fits.