Abstract
Within the framework of nonrelativistic QCD (NRQCD) factorization, we calculate the next-to-leading-order (NLO) perturbative corrections to the radiative decay Υ → ηc(χcJ) + γ. Both the helicity amplitudes and the helicity decay widths are obtained. It is the first computation for the processes involving both bottomonium and charmonium at two-loop accuracy. By employing the Cheng-Wu theorem, we are able to convert most of complex-valued master integrals (MIs) into real-valued MIs, which makes the numerical integration much efficient. Our results indicate the $$ \mathcal{O}\left({\alpha}_s\right) $$
O
α
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corrections are moderate for ηc and χc2 production, and are quite marginal for χc0 and χc1 production. It is impressive to note the NLO corrections considerably reduce the renormalization scale dependence in both the decay widths and the branching fractions for χcJ, and slightly improve that for ηc. With the NRQCD matrix elements evaluated via the Buchmüller-Tye potential model, we find the decay width for ηc production is one-order-of-magnitude larger than χcJ production, which may provide a good opportunity to search for Υ → ηc + γ in experiment. In addition, the decay width for χc1 production is several times larger than those for χc0,2. Finally, we find the NLO NRQCD prediction for the branching fraction of Υ → χc1 + γ is only half of the lower bound of the experimental data measured recently by Belle. Moreover, there exists serious contradiction between theory and experiment for Υ → ηc + γ. The discrepancies between theory and experiment deserve further research efforts.