Kinematical higher-twist corrections in ${\gamma }^{\ast }\gamma \to \pi \pi$

We apply the Braun-Manashov technique to improve the description of ${\gamma }^{\ast }(q_1)\gamma (q_2)\to M(p_1)M(p_2)$ 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 $s\mathrm{/}{Q}^2$ and $m^2\mathrm{/}{Q}^2$ 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, $\pi \pi$ GDAs are the best way to investigate the energy-momentum tensor form factors for pions.