Extra gauge bosons and lepton flavor universality violation in ϒ and B meson decays

Abstract

Lepton flavor universality can be tested through the ratio of semileptonic $B$ meson decays and leptonic $\mathrm{\ensuremath{\Upsilon}}$ meson decays, with $\mathrm{\ensuremath{\Upsilon}}\ensuremath{\equiv}\mathrm{\ensuremath{\Upsilon}}(nS)$ ($n=1$, 2, 3). For the charged-current transitions $b\ensuremath{\rightarrow}c\ensuremath{\tau}{\overline{\ensuremath{\nu}}}_{\ensuremath{\tau}}$, 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 $b\overline{b}\ensuremath{\rightarrow}\ensuremath{\tau}\overline{\ensuremath{\tau}}$, the BABAR experiment reported recently a new measurement of leptonic decay ratio ${R}_{\mathrm{\ensuremath{\Upsilon}}(3S)}=\mathrm{BR}(\mathrm{\ensuremath{\Upsilon}}(3S)\ensuremath{\rightarrow}\phantom{\rule{0ex}{0ex}}{\ensuremath{\tau}}^{+}{\ensuremath{\tau}}^{\ensuremath{-}})/\mathrm{BR}(\mathrm{\ensuremath{\Upsilon}}(3S)\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}})$, showing an agreement with the SM at the $1.8\ensuremath{\sigma}$ level. In light of this new BABAR result and regarding the connection between new physics (NP) interpretations to the charged-current $b\ensuremath{\rightarrow}c\ensuremath{\tau}{\overline{\ensuremath{\nu}}}_{\ensuremath{\tau}}$ anomalies and neutral-current $b\overline{b}\ensuremath{\rightarrow}\ensuremath{\tau}\overline{\ensuremath{\tau}}$ 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 ${R}_{\mathrm{\ensuremath{\Upsilon}}(3S)}$ cannot easily be accommodated (within its experimental $1\ensuremath{\sigma}$ range) together with the other $b\ensuremath{\rightarrow}c\ensuremath{\tau}{\overline{\ensuremath{\nu}}}_{\ensuremath{\tau}}$ data, hinting toward a new anomalous observable.