Extra gauge bosons and lepton flavor universality violation in ${\rm Y}$ and $B$ meson decays

Abstract

Lepton flavor universality can be tested through the ratio of semileptonic $B$ meson decays and leptonic ${\rm Y}$ meson decays, with ${\rm Y}\equiv {\rm Y}(nS)$ ($n=1,2,3$). For the charged-current transitions $b\to c\tau {\bar{\nu }}_{\tau }$, discrepancies between the experiment and the Standard Model (SM) have been observed in recent years by different flavor facilities such as BABAR, Belle, and LHCb. While for the neutral-current transitions $b\bar{b}\to \tau \bar{\tau }$, the BABAR experiment reported recently a new measurement of leptonic decay ratio $R_{{\rm Y}(3S)}=\mathrm{BR}({\rm Y}(3S)\to {\tau }^{+}{\tau }^{-})/\mathrm{BR}({\rm Y}(3S)\to {\mu }^{+}{\mu }^{-})$, showing an agreement with the SM at the $1.8\sigma$ level. In light of this new BABAR result and regarding the connection between new physics (NP) interpretations to the charged-current $b\to c\tau {\bar{\nu }}_{\tau }$ anomalies and neutral-current $b\bar{b}\to \tau \bar{\tau }$ processes, in this study, we revisit the NP consequences of this measurement within a simplified model with extra massive gauge bosons that coupled predominantly to left-handed leptons of the third-generation. We show that the BABAR measurement of $R_{{\rm Y}(3S)}$ cannot easily be accommodated (within its experimental $1\sigma$ range) together with the other $b\to c\tau {\bar{\nu }}_{\tau }$ data, hinting toward a new anomalous observable.

Comments: 8 pages, 3 figures. v2: Discussion improved, references added. v3: Matches with version published in Phys. Rev. D