The authors present the first analytical two-loop NRQCD amplitude for e+e- to J/psi+eta_c as an expansion in m_c^2/s, with cross-section predictions consistent (within uncertainties) with B-factory data.
This contribution is propor- tional to the color charge of the external collinear particle and thus vanishes after summing over all diagrams, since the mesons are color neutral
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Analytical two-loop amplitudes of $e^{+} e^{-} \longrightarrow \boldsymbol{J} / \boldsymbol{\psi}+\boldsymbol{\eta}_c$ at $B$ factories
The authors present the first analytical two-loop NRQCD amplitude for e+e- to J/psi+eta_c as an expansion in m_c^2/s, with cross-section predictions consistent (within uncertainties) with B-factory data.