Naturalising the Third Family Hypercharge Model for Neutral Current $B-$Anomalies

Abstract

We consider a deformation of the Third Family Hypercharge Model, which arguably makes the model more natural. Additional non-zero charges of the spontaneously broken, family-dependent $U(1{)}_F$ gauge symmetry are assigned to the second family leptons, and the third family leptons' charges are deformed away from their hypercharges in such a way that the $U(1{)}_F$ gauge symmetry remains anomaly-free. Second family $U(1{)}_F$ lepton charges allow a $Z^{\prime }$ coupling to muons without having to assume large charged lepton mixing, which risks violating tight lepton flavour violation bounds. In this deformed version, only the bottom and top Yukawa couplings are generated at the renormalisable level, whereas the tauon Yukawa coupling is absent. The $Z^{\prime }$ mediates a beyond the Standard Model contribution to an effective $(\bar{b}s)(\bar{\mu }\mu )$ vertex in the combination $C_9=-9C_{10}$ and is able to fit the apparent discrepancy between Standard Model predictions in flavour changing neutral-current $B-$meson decays and their measurements, whilst simultaneously avoiding current constraints from direct $Z^{\prime }$ searches and other measurements, when $1.2\text{TeV}<M_{Z^{\prime }}<12.5\text{TeV}$.

Comments: 13 pages, 6 figures. More accurate cross-section calculation taking off-shell Z' effects into account. Little qualitative difference, although the lower limit for the Z prime mass is slightly larger (now 1.2 TeV rather than 0.8 TeV as in the published version). Two minus sign typos in the definition of the Lagrangian have also been corrected. (Previous version accepted for publication in EPJC.)