Semileptonic and radiative decays of the B_c meson in light-front quark model

Abstract

We investigate the exclusive semileptonic $B_c\to (D,\eta_c,B,B_s)\ell\nu_\ell$, $\eta_b\to B_c\ell\nu_\ell$($\ell=e,\mu,\tau$) decays using the light-front quark model constrained by the variational principle for the QCD motivated effective Hamiltonian. The form factor $f_+(q^2)$, which is free from the zero mode, can be obtained from the analytic continuation method in the $q^+=0$ frame. However, the form factor $f_-(q^2)$, which is not free from the zero mode in the $q^+=0$ frame, needs to be obtained in the $q^+\neq 0$ frame with the effective treatment based on the Bethe-Salpeter formalism in handling the nonvalence (higher Fock states) contribution. We verify the covariance of our model calculation by checking the consistency between the two results of $f_+(q^2)$; one from the analytic continuation and the other from the effective treatment of handling the nonvalence contribution. Our results are then compared to the available experimental data and the results from other theoretical approaches. Since the prediction on the magnetic dipole $B^*_c\to B_c+\gamma$ decay turns out to be very sensitive to the mass difference between $B^*_c$ and $B_c$ mesons, our prediction both on the unmeasured $B^*_c$ meson mass and the decay width $\Gamma(B^*_c \to B_c \gamma)$ may help in determining the mass of $B^*_c$ experimentally.