Gauging the accidental symmetries of the standard model, and implications for the flavor anomalies

Abstract

We explore the possibility that lepton family numbers and baryon number are such good symmetries of Nature because they are the global remnant of a spontaneously broken gauge symmetry. An almost arbitrary linear combination of these symmetries (together with a component of global hypercharge) can be consistently gauged, if the Standard Model (SM) fermion content is augmented by three chiral SM singlet states. Within this framework of $U(1)$ extensions of the SM one generically expects flavor nonuniversality to emerge in the charged leptons, in such a way that naturally prevents lepton flavor violation, by aligning the mass and weak eigenbases. For quarks, all the SM Yukawa couplings responsible for their observed masses and mixings arise at the renormalizable level. We perform fits to show that models in this class can explain ${R}_{{K}^{(*)}}$ and the other neutral current $B$ anomaly data if we introduce a heavy vectorlike quark to mediate the required quark flavor violation, while simultaneously satisfying other constraints from direct ${Z}^{\ensuremath{'}}$ searches at the LHC, ${B}_{s}$ meson mixing, a number of electroweak precision observables, and neutrino trident production. Within this symmetry-motivated framework of models, we find interesting implications for the flavor anomalies; notably, any axial couplings of the ${Z}^{\ensuremath{'}}$ to electrons and muons must be flavor universal, with the flavor universality violation arising solely from the vectorlike couplings. We also comment on the generation of neutrino masses in these models.