$W^- \to \tau \bar \nu_\tau$ 3-sigma anomaly in new physics beyond the standard model

Abstract

Among so-called three 3-sigma anomalies in high energy physics, the excess of the branching ratio $W^- \to \tau \bar \nu_\tau$ with respect to the electrons and muons is especially interesting because (1) in the standard model (SM), $W^-\ell\bar \nu_\ell$ is the pure left-handed charge-current which has been tested precisely already, at least for the first two generation fermions, and (2) the $W^\pm$ two-body leptonic decay is the cleanest one among three anomalies due to its simpler kinematics and less hadronic uncertainties. In this paper, we explore the possibilities to account for the anomaly in type II two-Higgs-doublet model (2HDM) and minimal supersymmetric model (MSSM), as well as effective lagrangian approach by introducing anomalous left- and right-handed $W^-\tau \bar\nu_\tau$ couplings. Our results show that 2HDM and MSSM can hardly accommodate $W^- \to \tau \nu_\tau$ anomaly, and the anomaly is only marginally consistent to the measurements of $\tau \to \nu_\tau \ell \bar \nu_\ell$ at 95% confidence level with the presence of anomalous couplings. In the allowed regions, the right-handed coupling of $W^-\tau \nu_\tau$ shifts from 0 in SM to $\sim 0.12$ while the left-handed one from 1 to $\sim 1.005$.