The electric alpha and magnetic beta pion Compton polarizabilities
characterize the pion's deformation in the electromagnetic field of the gamma
during gamma-pi Compton scattering. The pion polarizabilities are key
observables, and provide stringent tests of our understanding of chiral
symmetry, its spontaneous breakdown, the role of explicit symmetry breaking in
QCD. The chi_PT effective Lagrangian, using data from radiative pion beta
decay, predicts the pion electric and magnetic polarizabilities alpha = -beta =
2.7 +- 0.4, in standard polarizability units. The polarizabilities deduced by
Antipov et al. in their low statistics Primakoff experiment (~ 7000 events)
were about three times larger than this prediction. For pion polarizability,
gamma-pi scattering will be measured in CERN COMPASS via radiative pion
Primakoff scattering (pion Bremsstrahlung) in the nuclear Coulomb field: pi + Z
--> pi' + gamma + Z; equivalent to gamma + pi --> gamma + pi Compton scattering
for laboratory gamma's of order 1 GeV incident on a target pion at rest. The
pion polarizabilities are determined by their effect on the shape of the
measured gamma-pi Compton scattering angular distribution. One can expect from
COMPASS a total measurement uncertainty for alpha of order 0.4 for the pion
polarizability. COMPASS can also carry out a first measurement of Kaon
ploarizabilities. New high precision pion and Kaon polarizability measurements
via radiative pion and Kaon scattering data from COMPASS will provide important
new tests of the QCD chiral dynamics polarizability prediction.Comment: Contribution to Advanced Study Institute, "Symmetries and Spin" -
Praha-SPIN-2002, Workshop Chairman, M. Finger,
http://mfinger.home.cern.ch/mfinger/praha2002/ Prague, Czech Republic, July
2002, 15 pages, 9 figure
