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

Abstract

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 RK and RK∗\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {R}_{K^{\\ast }} $$\\end{document}, 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 3B3 − Le − 2Lμ 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.