Pion Polarizabilities at CERN COMPASS

Objective: The electric (ᾱ) and magnetic (β̄) pion Compton polarizabilities characterize the pion’s deformation in the electromagnetic field of the γ during γπ Compton scattering. They depend on the rigidity of the pion’s internal structure as a composite particle. The polarizabilities deduced by Antipov et al. in their low statistics Primakoff experiment (∼ 7000 events) were ᾱπ = −β̄π = 6.8± 1.4± 1.2, in units of 10−43 cm. This value, ignoring the large error bars, is about three times larger than the chiral perturbation theory (χPT) prediction. Taking into account the very high beam intensity, fast data acquisition, high acceptance and good resolution of the CERN COMPASS experiment, one can expect from COMPASS statistics a factor 6000 higher, a data sample that includes many tests to control systematic errors, and a significantly reduced total measurement uncertainty for ᾱπ , of order 0.4. Methodology: CERN COMPASS studies pion-photon interactions to achieve a unique Primakoff physics program centered on pion polarizability studies. We will use a 190 GeV pion beam and a virtual photon target, and magnetic spectrometers and calorimeters to measure the complete kinematics of pion-photon reactions. COMPASS was set up during 2000/01, including a successful Primakoff test run, and then began data taking with a muon beam for the proton spin physics component of its program. COMPASS will next run its spin physics program and Primakoff program preparations, followed by its pion beam physics program, including pion polarizability. For pion polarizability, γπ scattering will be measured via radiative pion scattering (pion Bremsstrahlung) in the nuclear Coulomb field: π + Z → π ′ + γ + Z. A virtual photon from the Coulomb field of the target nucleus is scattered from the pion and emerges as a real photon accompanying the pion at small forward angles in the laboratory frame, while the target nucleus (in the ground state) recoils with a small transverse momentum kick pt. The radiative pion scattering reaction is equivalent to γ + π → γ + π scattering for laboratory γ’s of order 1 GeV incident on a target π at rest. The pion polarizabilities are determined by their effect on the shape of the measured γπ Compton scattering angular distribution. Significance: 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, and consequently the very foundations of nuclear physics. The χPT effective Lagrangian, using data from radiative pion beta decay, predicts the pion electric and magnetic polarizabilities ᾱπ = -β̄π = 2.7 ± 0.4. New high precision pion polarizability measurements via radiative pion scattering data from COMPASS will provide important new tests of this QCD chiral dynamics prediction. 1 together with: F. Balestra, R. Bertini, M.P. Bussa, M. Colantoni, O. Denisov, A. Dolgopolov, M. Faessler, A. Ferrero, L. Ferrero, J. Friedrich, V. Frolov, R. Garfagnini, N. Grasso, V. Kolossov, R. Kuhn, A. Maggiora, M. Maggiora, A. Manara, Y. Mikhailov, V. Obraztsov, A. Olchevski, D. Panzieri, S. Paul, G, Piragino, J. Pochodzalla, V. Poliakov, A. Sadovski, M. Sans, L. Schmitt, H. Siebert, A. Skachkova, T. Walcher, A. Zvyagin