Eur. Phys. J. A (2005) 23: 113-127
https://doi.org/10.1140/epja/i2004-10056-2

Measurement of the πmeson polarizabilities via the γp → γπ⁺n reaction

¹ Institut für Kernphysik der Johannes-Gutenberg-Universität, Mainz, Germany
² P.N. Lebedev Physical Institute, Moscow, Russia
³ Department of Physics and Astronomy, Glasgow University, Glasgow, UK
⁴ School of Physics, University of Edinburgh, Edinburgh, UK
⁵ School of Physics and Astronomy, Tel Aviv University, Tel Aviv, Israel
⁶ Physikalisches Institut, Universität Tübingen, Germany
⁷ Mount Allison University, Sackville, NB, Canada
⁸ Institut für Physik, Basel, Switzerland
⁹ II. Physikalisches Institut, Universität Giessen, Giessen, Germany

Received: 28 May 2004
Accepted: 24 July 2004
Published online: 6 December 2004

Abstract

An experiment on the radiative π^{+}-meson photoproduction from the proton ( γp → γπ^{+} n) was carried out at the Mainz Microtron MAMI in the kinematic region 537MeV < E _γ < 817MeV, 140^°≤≤180^°. The π^{+}-meson polarizabilities have been determined from a comparison of the data with the predictions of two different theoretical models, the first one being based on an effective pole model with pseudoscalar coupling while the second one is based on diagrams describing both resonant and nonresonant contributions. The validity of the models has been verified by comparing the predictions with the present experimental data in the kinematic region where the pion polarizability contribution is negligible ( s ₁ < 5m _π ²) and where the difference between the predictions of the two models does not exceed 3%. In the region, where the pion polarizability contribution is substantial ( 5 < s ₁/m _π ² < 15, -12 < t/m _π ² < - 2), the difference of the electric (α) and the magnetic (β) polarizabilities has been determined. As a result we find . This result is at variance with recent calculations in the framework of chiral perturbation theory.