  The electric ${\alpha}_{\pi}$ and magnetic $\beta_{\pi}$ charged pion Compton
polarizabilities are of fundamental interest in the low-energy sector of
quantum chromodynamics (QCD).They are directly linked to the phenomenon of
spontaneously broken chiral symmetry within QCD and to the dynamics of the
pion-photon interaction.The combination (${\alpha}_{\pi}-\beta_{\pi}$) was
measured by:(1) CERN COMPASS via radiative pion Primakoff scattering
(Bremsstrahlung) in the nuclear Coulomb field, ${\pi}^- Z \rightarrow {\pi}^-Z
{\gamma}$, (2) SLAC PEP Mark-II via two-photon production of pion pairs,
${\gamma}{\gamma}\rightarrow{\pi}^+{\pi}^-$, and (3) Mainz Microtron via
radiative pion photoproduction from the proton,
${\gamma}p\rightarrow{\gamma}{\pi}^+n$. COMPASS and Mark-II agree with one
another:(1) ${\alpha}_{\pi}-\beta_{\pi}=4.0\times 10^{-4}fm^3$, (2)
${\alpha}_{\pi}-\beta_{\pi}=4.4 \times 10^{-4}fm^3$. The Mainz value (3)
${\alpha}_{\pi}-\beta_{\pi}=11.6 \times 10^{-4}fm^3$ is excluded here because a
dispersion relations calculation using the Mainz value as input gives
significantly too large ${\gamma}{\gamma} \rightarrow {\pi}^0{\pi}^0$ cross
sections compared to DESY Crystal Ball data. COMPASS and Mark-II values agree
well with the two-loop chiral perturbation theory ChPT prediction
${\alpha}_{\pi}-\beta_{\pi}=5.7\times10^{-4}fm^3$, thereby strengthening the
identification of the pion with the Goldstone boson of QCD.
