Bc → J/ψ helicity form factors and the B+c→J/ψ+(P,V,l+vl)decays

In this paper,we calculate the Bc → J/ψ 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 q2 region,we in-vestigate the B+c-meson two-body decays and semi-leptonic decays B+c→ J/ψ+(P,V,l+vl),where P/V stand for light pseudoscalar and vector mesons,respectively.The branching fractions can be derived using the CKM matrix element and the Bc lifetime from the Particle Data Group,and we obtain B(B+c → J/ψπ+)=(0.136+0.002-0.002)％,B(B+t→J/ψK+)=(0.010+0.000-0.000)％,B(B+c → J/ψρ+)=(0.768+0.029-0.033)％,B(B+c→ J/ψK*+)=(0.043+0.001-0.001)％,B(B+c →J/ψμ+vμ)=(2.802+0.526-0.675)％and B(B+c → J/ψτ+vτ)=(0.559+0.131-0.170)％.We then obtain Rπ+/μ+vμ=0.048+0.009-0.012 and RK+/π+=0.075+0.005-0.005,which agree with the LHCb measured value within 1σ-error.We also obtain RJ/ψ=0.199+0.060-0.077,which like other theoretical predictions,is consistent with the LHCb measured value within 2σ-error.These imply that the HFFs under the LCSR approach are also applicable to the B+meson two-body decays and semi-leptonic decays Bc → J/ψ+(P,V,l+vl),and the HFFs obtained using LCSR in a new way implies that there may be new physics in the Bc → J/ψl+vl semi-leptonic decays.