Pion Polarizabilities from $\gamma\gamma\to\pi\pi$ Analysis
  We present results for pion polarizabilities predicted using dispersion relations from our earlier Amplitude Analysis of world data on two photon production of meson pairs. The helicity-zero polarizabilities are rather stable and insensitive to uncertainties in cross-channel exchanges. The need is first to confirm the recent result on $(\alpha_1-\beta_1)$ for the charged pion by COMPASS at CERN to an accuracy of 10\% by measuring the $\gamma\gamma\to\pi^+\pi^-$ cross-section to an uncertainty of ~1\%. Then the same polarizability, but for the $\pi^0$, is fixed to be $(\alpha_1-\beta_1)_{\pi^0}=(-0.9\pm0.2)\times 10^{-4}$ fm$^{3}$. By analyzing the correlation between uncertainties in the meson polarizability and those in $\gamma\gamma$ cross-sections, we suggest experiments need to measure these cross-sections between $\sqrt{s}\simeq 350$ and 600 MeV. The $\pi^0\pi^0$ cross-section then makes the $(\alpha_2-\beta_2)_{\pi^0}$ the easiest helicity-two polarizability to determine. 
Ling-Yun Dai and Michael R. Pennington
hep-ph