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

Measurements of b -> s mu(+)mu(-) 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 -> s mu(+)mu(-) transitions). A 3B(3) - L-e - 2L(mu) 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.

Automated summary

Measurements of b -> s mu(+)mu(-) transitions suggest the existence of a new physics field coupling to di-muon pairs associated with the b to s flavor transition. The 2022 LHCb reanalysis of R-K and R-K* indicates the possibility of associated new physics in b -> se(+)e(-) transitions as well. This study examines the statistical preference for Z ' models coupling to di-electron pairs, considering relevant constraints from LEP-2 experiments. A set of anomaly-free models that interpolate between no coupling of Z ' to electrons and equal coupling to muons and electrons, while allowing Z ' boson mediation in b -> s mu(+)mu(-) transitions, is identified. The 3B(3) - L-e - 2L(mu) model provides a nearly optimal fit to the measurements. The predictions for relevant LEP-2 observables, calculated using dimension 6 SMEFT operators and implemented in the flavio computer program, are made available for global fits.