A Precise Study Of The Decay Of A Double Meson Containing A Bottom Quark To A Charm Quark

In the framework of nonrelativistic quantum chromodynamics（NRQCD）factorization theory,several processes of heavy quarkonium decay are discussed in detail and further studied,including theΥradiative decays to charmonium and the form factors of B_cmeson decays into charmonium.In 2019,Belle experimental group first observed the process of radiative decay fromΥ（1S）to charmonium,and gave a first measurement of the decay branching ratio of the process,which is an important progress in the experimental flavor physics.However,the experimental measurement ofΥ（1S）→χ_c1+γis quite different from the previous theoretical predictions,and there is an order of magnitude difference.Such a gap is a great challenge to the theoretical research,and it is in urgent need of a reasonable theoretical explanation.Solving the gap between theoretical prediction and experimental data is helpful to understand the heavy quarkonium effective theory and hadronization mechanism.In this paper,we analytically calculate the radiative decays of bottomonium into S-wave and P-wave charmonium,i.e.Υ（n S）→η_c（n S）+γandΥ（n S）→χ_{c J}+γ.Compared with the previous calculation,we obtained the analytical expression of the decay widths and considered the color-octet contributions.For the process ofΥ（n S）→η_c（n S）+γ,the relativistic corrections are also obtained.On the other hand,the LHCb collaboration also found the anomalous signal of the transition from bottom quark to charm quark in the semi-leptonic decays from B_cmeson to charmonium,in there it shows that the measured J/ψhas about two standard deviations from the prediction of the Standard Model.In this process,the form factor of B_cmeson to charmonium plays an important role.In this paper,we also systematically discuss the different form factors of B_cmeson decay to charmonium,and analytically compute the tensor form factors of B_cmeson decay to charmonium,which improves the theoretical study of B_cmeson decay to charmonium form factors.The relevant results have important scientific significance for extracting the nonperturbative parameters of B_cmeson.