Extra gauge bosons and lepton flavor universality violation in $\Upsilon$ and $B$ meson decays

TL;DR

This paper investigates how new massive gauge bosons could explain lepton flavor universality violations observed in B and Upsilon meson decays, highlighting challenges in reconciling recent experimental results with theoretical models.

Contribution

It analyzes the implications of a recent BABAR measurement on Upsilon decays within a simplified new physics model involving extra gauge bosons, revealing tensions with charged-current anomalies.

Findings

BABAR measurement of $R_{5S}$ cannot be easily explained alongside other $b o c au ar{ u}$ data.

The study suggests the need for a new observable to better understand lepton flavor universality violations.

Revisits the impact of extra gauge bosons on meson decay ratios and their compatibility with experimental data.

Abstract

Lepton flavor universality can be tested through the ratio of semileptonic $B$ meson decays and leptonic $\Upsilon$ meson decays, with $\Upsilon \equiv \Upsilon(nS)$ ($n=1,2,3$). 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 $R_{\Upsilon(3S)} = {\rm BR}(\Upsilon(3S) \to \tau^+\tau^-)/{\rm BR}(\Upsilon(3S) \to \mu^+\mu^-)$, 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 $R_{\Upsilon(3S)}$ 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.