Rare $W$-boson decays into a vector meson and lepton pair

We have presented a theoretical study of exclusive rare $W$-boson decays, $W\to V\ell\bar{\nu}_\ell$ with $V$ denoting a neutral vector meson and $\ell=e$ or $\mu$, in the standard model. The leading-order contributions to these processes are given by $W\to \gamma^*\ell\bar{\nu}_\ell$ with the subsequent $\gamma^*\to V$ transition. Branching fractions of these decay modes, for $V=\rho$, $\omega$, $\phi$, and $J/\Psi$, respectively, have been calculated and predicted around $10^{-6}\sim 10^{-7}$, which are surprisingly larger than those of two-body hadronic radiative decays $W^\pm\to M^\pm \gamma$ with $M$ denoting a pseudoscalar or vector meson. Thus it is expected that rare $W$ decays into a neutral vector meson plus lepton pair may be the promising channels in future experimental facilities with a large number of $W$-boson events produced.