Semi-analytical two-loop QCD corrections to $e^{+}{e}^{-}\to J/\psi +{\chi }_{cJ}$ at B factories

Abstract

In this work, we compute the next-to-next-to-leading-order (NNLO) QCD corrections to the process $e^{+}{e}^{-}\to J/\psi +{\chi }_{cJ}$ at B factories within the NRQCD factorization framework. The helicity amplitudes are obtained via asymptotic expansions around $r=0$ and $r=1$, with $r=16m_c^2/s$. Our asymptotic expressions reproduce the exact numerical results with high accuracy across the kinematic range $0\le r\le 1$, achieving a relative error below $10^{-5}$, which is sufficient for phenomenological applications. Notably, the large logarithmic terms are obtained analytically, and the explicit expressions established in this work provide a useful basis for further theoretical investigations. We compute the unpolarized cross sections. The $\mathcal{O}({\alpha }_s)$ correction is found to be large, while the $\mathcal{O}({\alpha }_s^2)$ correction for ${\chi }_{c0}$ production amounts to $33\%$ of the leading-order (LO) cross section, significantly reducing the scale uncertainties. For ${\chi }_{c1}$, the $\mathcal{O}({\alpha }_s)$ and $\mathcal{O}({\alpha }_s^2)$ corrections correspond to $35\%$ and $-15\%$, respectively. For ${\chi }_{c2}$, the corresponding corrections are $24\%$ and $-38\%$. The large cancellation between the corrections for ${\chi }_{c2}$ brings the NNLO cross section close to the LO prediction. Our prediction for ${\chi }_{c0}$ is consistent with both the {\tt Belle} and {\tt BaBar} data within uncertainties. We also predict the angular distribution parameters ${\alpha }_{\theta }^J$, which are independent of nonperturbative inputs. A sharp discrepancy between the theory and the {\tt Belle} measurement is observed for ${\alpha }_{\theta }^0$, calling for further experimental and theoretical investigations. Moreover, future measurements of the angular distribution parameters for ${\chi }_{c1}$ and ${\chi }_{c2}$ will provide important tests of the theoretical framework.

arXiv comment34 pages, 5 figures, 2 tables; Version accepted for publication in JHEP