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

We have presented a theoretical study of exclusive rare $W$-boson decays, $W\to V\mathrm{\ell }{\bar{\nu }}_{\mathrm{\ell }}$ with $V$ denoting a neutral vector meson and $\mathrm{\ell }=e$ or $\mu$, in the standard model. The leading-order contributions to these processes are given by $W\to {\gamma }^{\ast }\mathrm{\ell }{\bar{\nu }}_{\mathrm{\ell }}$ with the subsequent ${\gamma }^{\ast }\to V$ transition. Branching fractions of these decay modes, for $V=\rho$, $\omega$, $\phi$, and $J\mathrm{/}\mathrm{\Psi }$, 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.