Relativistic corrections to the form factors of $B_c$ into $P$-wave orbitally excited charmonium
We investigated the form factors of the $B_{c}$ meson into $P$-wave orbitally excited charmonium using the nonrelativistic QCD effective theory. Through the analytic computation, the next-to-leading order relativistic corrections to the form factors were obtained, and the asymptotic expressions were studied in the infinite bottom quark mass limit. Employing the general form factors, we discussed the exclusive decays of the $B_{c}$ meson into $P$-wave orbitally excited charmonium and a light meson. We found that the relativistic corrections lead to a large correction for the form factors, which makes the branching ratios of the decay channels $\mathcal{B}(B_{c} ^{\pm }\to \chi _{cJ}(h_c) +\pi ^{\pm }(K^{\pm }))$ larger. These results are useful for the phenomenological analysis of the $B_{c}$ meson decays into $P$-wave charmonium, which shall be tested in the LHCb experiments.