  Generalized parton distributions (GPDs) are three-dimensional structure
functions of hadrons, and they can reveal the orbital-angular-momentum
contributions to the nucleon spin. Therefore, GPDs are important for solving
the proton spin puzzle. The generalized distribution amplitudes (GDAs) are the
$s$-$t$ crossed quantities of the GPDs, and the GDAs can be investigated in
two-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 form
factors of energy-momentum tenor were calculated for pion in the timelike
region. In order to study the gravitational radii for the pion, the form
factors of energy-momentum tenor were obtained in the spacelike region by using
the 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 finding
on gravitational form factors and radii of hadrons from actual experimental
measurements. 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 higher
luminosity Super KEKB, so that the GDAs of other hadrons could be studied as
well.
