  Generalized parton distributions (GPDs) are 3-dimensional (3D) structure
functions for hadrons, and they are important for solving the proton spin
puzzle including partonic orbital-angular-momentum contributions. The $s$-$t$
crossed quantities of the GPDs are generalized distribution amplitudes (GDAs).
Here, $s$ and $t$ are Mandelstam variables. The GDAs can be studied in
two-photon processes ($\gamma^* \gamma\to h \bar h$) at KEKB. A GDA describes
the amplitude from quark and antiquark to the hadron pair $h \bar h$. In 2016,
the Belle collaboration reported measurements for pion-pair production in
electron-positron collision, and the pion GDAs were determined in this work by
analyzing the Belle data. In our analysis, the pion GDAs are expressed by a few
parameters, which are determined by analyzing the Belle data. From the obtained
GDAs, form factors of energy-momentum tenor, so called gravitational form
factors, are calculated for pion in the timelike region. The spacelike
gravitational form factors are calculated from the timelike ones by using the
dispersion relation. Then, the mass radius is calculated as 0.32-0.39 fm and
the mechanical radius, defined by the slope of the form factor $\Theta_1$, is
calculated as 0.82-0.88 fm for the pion by using the spacelike form factors.
This is the first study on gravitational form factors and radii of hadrons from
actual experimental measurements. In 2019, the Belle II collaboration will
start collecting data by the higher luminosity Super KEKB, so that the GDAs of
other hadrons should also be investigated in the near future. Our studies are
valuable in understanding 3D structure and gravitational properties of hadrons.
