According to the Particle Data Group, the measurements of ${cal B}(W^+ to tau^+ u_tau)$ and ${cal B}(W^+ to ell^+ u_ell)$ ($ell = e,mu$) disagree with one another at the $2.3sigma$ level. In this paper, we search for a new-physics (NP) explanation of this $W to tau u$ puzzle. We consider two NP scenarios: (i) the $W$ mixes with a $W$ boson that couples preferentially to the third generation, (ii) $tau_{L,R}$ and $ u_{tau L}$ mix with isospin-triplet leptons. Unfortunately, once other experimental constraints are taken into account, neither scenario can explain the above experimental result. Our conclusion is that the $W to tau u$ puzzle is almost certainly just a statistical fluctuation.
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