Pion Polarizability 2022 Update

 The electric α_π and magnetic β_π charged and neutral pion polarizabilities characterize the induced oscillating dipole moments of the pion during γπ Compton scattering via the interaction of the γ’s electromagnetic field with the quark substructure of the pion. In particular, α_π is the proportionality constant between the γ’s electric field and the electric dipole moment, while β_π is similarly related to the γ’s magnetic field and the induced magnetic dipole moment. Pion polarizabilities affect the shape of the γπ Compton scattering angular distribution. By crossing symmetry, the γπ→γπ  amplitudes are related to the γγ→ππ amplitudes. The polarizabilities are basic characteristics of the pion, and are therefore of fundamental interest in the low-energy sector of quantum chromodynamics. A stringent test of chiral perturbation theory (ChPT) is possible based on comparisons of precision experimental pion polarizabilities with ChPT predictions. The combination (α_π-β_π) has been measured by: (1) radiative pion Primakoff scattering (Bremsstrahlung of 190 GeV/c negative pions) in the nuclear Coulomb field of the Ni nucleus: π^- Ni → π^- Ni γ, (2) two-photon fusion production of pion pairs γγ→ππ via the e⁺e⁻ → e⁺e⁻π⁺π⁻  reaction at SLAC Mark-II, (3) radiative pion photoproduction from the proton γp→ γπn at MAMI in Mainz. Only the CERN COMPASS charged pion polarizability measurement has acceptably small uncertainties. The COMPASS polarizabilities are in good agreement with ChPT predictions; and by Dispersion Relations with DESY Crystal Ball γγ → π⁰π⁰ data; strengthening the identification of the pion with the Goldstone boson of chiral symmetry breaking in QCD. This status report follows the review by Moinester and Scherer at IJMPA 34 (2019) 1930008, and includes a description of ongoing and planned pion polarizability experiments (COMPASS at CERN, BESIII at Beijing, JLab at Newport News).