Bc → J/ψ helicity form factors and the Bc+ → J/ψ plus (P, V, l+νl) decays

In this paper, we calculate the B-c -> J/psi helicity form factors (HFFs) up to twist-4 accuracy by using the light-cone sum rules (LCSR) approach. After extrapolating those HFFs to the physically allowable q(2) region, we investigate the B-c(+)-meson two-body decays and semi-leptonic decays B-c(+) -> J/psi (P, V, l(+)nu(l)), where P/V stand for light pseudoscalar and vector mesons, respectively. The branching fractions can be derived using the CKM matrix element and the B-c lifetime from the Particle Data Group, and we obtain B(B-c(+) -> J/psi pi(+)) = (0.1361(-0.002)(+0.002))%, B(B-c(+) -> J/psi K+) = (0.0101(-0.000)(+0.000))%, B(B-c(+) -> J/psi rho(+)) = (0.768(-0.033)(+0.029))%, B(B-c(+) -> J/psi K*(+)) = (0.043(-0.001)(+0.001))% , B(B-c(+) -> J/psi mu(+)nu(mu)) = (2.802(-0.675)(+0.526))% and B(B-c(+) -> J/psi tau(+) nu(tau)) = (0.559(-0.170)(+0.131))%. We then obtain R pi+/mu+nu mu =( )0.048(-0.012)(+0.009) and RK+/pi+ = 0 .075(-0.005)(+0.005), which agree with the LHCb measured value within 1 sigma-error. We also obtain R-J/psi = 0.199(-0.077)(+0.060), which like other theoretical predictions, is consistent with the LHCb measured value within 2 sigma-error. These imply that the HFFs under the LCSR approach are also applicable to the B-c(+) meson two-body decays and semi-leptonic decays B-c(+) -> J/psi + (P, V, l(+)nu(l)), and the HFFs obtained using LCSR in a new way implies that there may be new physics in the B-c -> J/psi l(+)nu(l) semi-leptonic decays.

Automated summary

This paper calculates the helicity form factors (HFFs) for the B-c -> J/psi decay and investigates the decays of B-c(+) meson into J/psi and other particles using the light-cone sum rules approach. The branching fractions obtained are consistent with experimental measurements, indicating the applicability of the method. The results also suggest the possibility of new physics in the B-c -> J/psi l(+)nu(l) semi-leptonic decays.