Kinematical higher-twist corrections in $γ^* γ\to ππ$

We apply the Braun-Manashov technique to improve the description of $γ^*(q_1) γ(q_2) \to M(p_1) M(p_2)$ amplitudes at large $Q^2=-q_1^2$ and small $s=(q_1+q_2)^2$. We derive the kinematical higher-twist contributions of order $s/Q^2$ and $m^2/Q^2$ to the helicity amplitudes and estimate their sizes in the kinematics accessible at Belle and Belle II.Since pion GPDs cannot be directly measured by experiment, $ππ$ GDAs are the best way to investigate the energy-momentum tensor form factors for pions.

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