Pion and kaon polarizabilities and radiative transitions

Abstract

CERN COMPASS plans measurements of γπ and γK interactions using 50–280 GeV pion (kaon) beams and a virtual photon target. Pion (kaon) polarizabilities and radiative transitions will be measured via Primakoff effect reactions such as π⁻ γ → π⁻′ γ and π⁻ γ → meson. The former can test a precise prediction of chiral symmetry; the latter for πγ → a ₁ (1260) is important for understanding the polarizability. The radiative transition of a pion to a low mass two-pion system, π⁻ γ → π⁻′ π⁰, can also be studied to measure the chiral anomaly amplitude F_3π (characterizing γ → 3π) arising from the effective Chiral Lagrangian. We review here the motivation for the above physics program. We describe the beam, target, detector, and trigger requirements for these experiments. We also describe FNAL SELEX attempts to study related physics via the interaction of 600 GeV pions with target electrons. Data analysis in progress aims to identify the reactions πe → π′e′π⁰ related to the chiral anomaly, and πe → π′e′γ related to pion polarizabilities.