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Generalized parton distributions (GPDs) are three-dimensional structurefunctions of hadrons, and they can reveal the orbital-angular-momentumcontributions to the nucleon spin. Therefore, GPDs are important for solvingthe proton spin puzzle. The generalized distribution amplitudes (GDAs) are the$s$-$t$ crossed quantities of the GPDs, and the GDAs can be investigated intwo-photon process ($\gamma^* \gamma \to h\bar h$) which is accessible at KEKB.The pion GDAs were obtained by analyzing the Belle measurements for $\pi^0\pi^0$ production in the $e^+ e^-$ collision. From the obtained GDAs, the formfactors of energy-momentum tenor were calculated for pion in the timelikeregion. In order to study the gravitational radii for the pion, the formfactors of energy-momentum tenor were obtained in the spacelike region by usingthe dispersion relation. Then, the mass radius was calculated as 0.32 $\sim$0.39 fm and the mechanical radius was also estimated for the pion as 0.82$\sim$ 0.88 fm by using the spacelike form factors. This is the first findingon gravitational form factors and radii of hadrons from actual experimentalmeasurements. In the near future we can expect more precise measurements of$\gamma^* \gamma \to h\bar h$ as the Belle II started data taking by the higherluminosity Super KEKB, so that the GDAs of other hadrons could be studied aswell.

Hadron tomography for pion and its gravitational form factors