Kinematical higher-twist corrections in γ*γ → M(M)over-bar

We estimate kinematical higher-twist (up to twist 4) corrections to the gamma*(q(1))gamma(q(2)) -> M(p(1))(M) over bar (p(2)) amplitudes at large Q(2) = -q(1)(2) and small s = (q(1) + q(2))(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/Q(2) and m(2)/Q(2) 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 gamma*gamma -> pi(0)pi(0) with the pi pi GDA extracted from Belle measurements and with the asymptotic pi pi GDA as inputs to study the magnitude of the kinematical corrections. To see how the target mass corrections of order m(2)/Q(2) affect the cross section, we also perform the calculation for gamma*gamma -> eta eta by using a model eta eta GDA. In the range s > 1 GeV2, the kinematical higher-twist corrections account for similar to 15% of the total cross section, an effect which is not negligible. Since pi pi 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.

Automated summary

We estimated the kinematical higher-twist corrections to the gamma*(q(1))gamma(q(2)) -> M(p(1))(M) over bar (p(2)) amplitudes, and calculated the cross sections for gamma*gamma -> pi(0)pi(0) and gamma*gamma -> eta eta. The results show that the kinematical higher-twist corrections play an important role in the total cross section, and considering these corrections is necessary for an accurate evaluation of the meson energy-momentum tensor form factors.