Form Factors for $B_c^{\ast }\to {\eta }_c+l{\nu }_l$ at NLO in QCD

Abstract

We present the Non-Relativistic QCD (NRQCD) calculations at the next-to-leading order (NLO) of ${\alpha }_s$ for $B_c^{\ast }\to {\eta }_c$ vector, axial-vector, tensor and axial-tensor form factors, and obtain complete analytical expressions for the form factors, along with their asymptotic forms in the hierarchical heavy quark limit. Our results show that the NLO corrections are both sizable and well-behaved in the low squared transfer momentum $(q^2)$ region. Using the NRQCD + lattice + $z$-series method, we further provide theoretical predictions for $B_c^{\ast }\to {\eta }_c$ form factors across the full physical $q^2$ range. Based on these predicted form factors, we finally compute the decay widths and branching fractions for the semileptonic decays $B_c^{\ast }\to {\eta }_c+l{\nu }_l$.

Comments: to be published in PRD