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Power expansion for heavy quarkonium production at next-to-leading order in $\rm e^+e^-$ annihilation

We study heavy quarkonium production associated with gluons in $\rm e^+e^-$ annihilation as an illustration of the perturbative QCD (pQCD) factorization approach, which incorporates the first nonleading power in the energy of the produced heavy quark pair. We show how the renormalization of the four-quark operators that define the heavy quark pair fragmentation functions using dimensional regularization induces "evanescent" operators that are absent in four dimensions. We derive closed forms for short-distance coefficients for quark pair production to next-to-leading order ($α_s^2$) in the relevant color singlet and octet channels. Using non-relativistic QCD (NRQCD) to calculate the heavy quark pair fragmentation functions up to $v^4$ in the velocity expansion, we derive analytical results for the differential energy fraction distribution of the heavy quarkonium. Calculations for ${}^3S_1^{[1]}$ and ${}^1S_0^{[8]}$ channels agree with analogous NRQCD analytical results available in the literature, while several color-octet calculations of energy fraction distributions are new. We show that the remaining corrections due to the heavy quark mass fall off rapidly in the energy of the produced state. To explore the importance of evolution at energies much larger than the mass of the heavy quark, we solve the renormalization group equation perturbatively to two-loop order for the ${}^3S_1^{[1]}$ case.