  We study the semileptonic decays of the $B_c$ meson into final charmonium
states within the standard model and beyond. The relevant hadronic transition
form factors are calculated in the framework of the covariant confined quark
model developed by us. We focus on the tau mode of these decays, which may
provide some hints of new physics effects. We extend the standard model by
assuming a general effective Hamiltonian describing the $b\to c\tau\nu$
transition, which consists of the full set of the four-fermion operators. We
then obtain experimental constraints on the Wilson coefficients corresponding
to each operator and provide predictions for the branching fractions and other
polarization observables in different new physics scenarios.

High Energy Physics - Phenomenology