Pseudoscalar quarkonium+γ production at NLL+NLO accuracy

We consider the exclusive pseudoscalar heavy-quarkonium (η_{b, c}) production in association with a photon at future lepton colliders where the collider energies of O(10²) GeV are far greater than the quarkonium mass. At these energies, the logarithm of mass to collision energy becomes increasingly large hence its resummation becomes particularly important. By making use of the light-cone-distribution factorization formula, we resum the logarithms up to next-to-leading-logarithmic accuracy (NLL) that corresponds to order-α_s accuracy. We combine the resummed result with a known fixed-order result at next-to-leading order (NLO) such that both resummed-logarithmic terms and non-logarithmic terms are included at the same order in α_s. This allowed us to provide reliable predictions at accuracies of order α_s ranging from relatively low energies near quarkonium mass to the collider energies of O(10²) GeV. We also include the leading relativistic corrections resummed at leading-logarithmic accuracy. Our prediction at the Belle energy is comparable with fixed-order predictions in literatures while it shows a large deviation from a recent Belle’s upper limit by about 4 σ. Finally, we make predictions for the energies of future Z - and Higgs factories.