  Three-dimensional tomography of hadrons can be investigated by generalized
parton distributions (GPDs), transverse-momentum-dependent parton distributions
(TMDs), and generalized distribution amplitudes (GDAs). The GDA studies had
been only theoretical for a long time because there was no experimental
measurement until recently, whereas the GPDs and TMDs have been investigated
extensively by deeply virtual Compton scattering and semi-inclusive deep
inelastic scattering. Here, we report our studies to determine pion GDAs from
recent KEKB measurements on the differential cross section of $\gamma^* \gamma
\to \pi^0 \pi^0$. Since an exotic-hadron pair can be produced in the final
state, the GDAs can be used also for probing internal structure of exotic
hadron candidates in future. The other important feature of the GDAs is that
the GDAs contain information on form factors of the energy-momentum tensor for
quarks and gluons, so that gravitational form factors and radii can be
calculated from the determined GDAs. We show the mass (energy) and the
mechanical (pressure, shear force) form factors and radii for the pion. Our
analysis should be the first attempt for obtaining gravitational form factors
and radii of hadrons by analysis of actual experimental measurements. We
believe that a new field of gravitational physics is created from the
microscopic level in terms of elementary quarks and gluons.
