  Generalized parton distributions (GPDs) have been investigated in the deeply
virtual Compton scattering (DVCS) to solve the proton spin puzzle. On the other
hand, the generalized distribution amplitudes (GDAs) can be studied in the
two-photon process $\gamma^* \gamma \rightarrow h \bar{h}$ which is accessible
at KEKB. Namely, the GDAs are the $s$-$t$ crossed quantities of the GPDs. In
2016, the differential cross section of the process $\gamma^* \gamma
\rightarrow \pi^0 \pi^0$ was measured by the Belle collaboration in the
$e^+e^-$collision , so that the pion GDAs can be obtained by analyzing the
Belle data. Expressing the GDAs with a few parameters, we determined the GDAs
by a $\chi^2$ analysis . The form factors of the quark energy-momentum tensor
are obtained from the determined GDAs. Then, we calculated the mass radius as
0.56-0.69 fm for the pion by using the form factor. This is the first study on
gravitational form factors and radius of hadrons from actual experimental
measurements . The Belle II will start taking data in 2018 by the upgraded
SuperKEKB. Therefore, much accurate data are expected for the pion, and other
hadron-pair productions will be measured in the near future. The GDA studies
are valuable for understanding not only the 3D structure but also gravitational
properties of hadrons.
