Kinematical higher-twist corrections in γ∗γ →m M ¯

We estimate kinematical higher-twist (up to twist 4) corrections to the γ∗(q1)γ(q2)→M(p1)M¯(p2) amplitudes at large Q2=-q12 and small s=(q1+q2)2, where M is a scalar or pseudoscalar meson. This process is known to factorize at leading twist into a perturbatively calculable coefficient function and generalized distribution amplitudes (GDAs). The kinematical higher-twist contributions of order s/Q2 and m2/Q2 turn out to be important in the cross section, considering the kinematics accessible at Belle and Belle II. We present numerical estimates for the cross section for γ∗γ→π0π0 with the ππ GDA extracted from Belle measurements and with the asymptotic ππ GDA as inputs to study the magnitude of the kinematical corrections. To see how the target mass corrections of order m2/Q2 affect the cross section, we also perform the calculation for γ∗γ→ηη by using a model ηη GDA. In the range s>1 GeV2, the kinematical higher-twist corrections account for ∼15% of the total cross section, an effect which is not negligible. Since ππ GDAs are the best way to access the pion energy-momentum tensor (EMT), our study demonstrates that an accurate evaluation of EMT form factors requires the inclusion of kinematical higher-twist contributions.