Eur. Phys. J. A (2007) 34: 11-21
https://doi.org/10.1140/epja/i2007-10491-5

Regular Article - Experimental Physics

First measurement of the helicity dependence for the γp → pπreaction

¹ Insitut für Experimentalphysik, Ruhr-Universität Bochum, D-44801, Bochum, Germany
² Physikalisches Institut, Universität Bonn, D-53115, Bonn, Germany
³ Department of Physics, Duke University, NC 27708, Durham, USA
⁴ Subatomaire en Stralingsfysica, Universiteit Gent, B-9000, Gent, Belgium
⁵ CEA Saclay, DSM/DAPNIA/SPhN, F-91191, Gif-sur-Yvette Cedex, France
⁶ Department of Physics and Astronomy, University of Glasgow, G12 8QQ, Glasgow, UK
⁷ II.Physikalisches Institut, Universität Göttingen, D-37077, Göttingen, Germany
⁸ Institut für Kernphysik, Universität Mainz, D-55099, Mainz, Germany
⁹ INR, Academy of Science, Moscow, Russia
¹⁰ Department of Physics, Nagoya University, Chikusa-ku, Nagoya, Japan
¹¹ CIRSE, Nagoya University, Chikusa-ku, Nagoya, Japan
¹² Sezione di Pavia, INFN, I-27100, Pavia, Italy
¹³ Dipartimento di Fisica Nucleare e Teorica, Università di Pavia, I-27100, Pavia, Italy
¹⁴ Laboratory of Mathematical Physics, Tomsk Polytechnic University, 634050, Tomsk, Russia
¹⁵ Physikalisches Institut, Universität Tübingen, D-72076, Tübingen, Germany

^* e-mail: Paolo.Pedroni@pv.infn.it

Received: 12 July 2007
Accepted: 9 October 2007
Published online: 29 October 2007

Abstract

The helicity dependence of the total cross-section and the invariant-mass distributions in the (pπ^±) and (π⁺π^-) final states for the γp → pπ⁺π^- reaction have been measured for the first time at incident photon energies from 400 to 800MeV. The measurement was performed with the large-acceptance detector DAPHNE at the tagged photon beam facility of the MAMI accelerator in Mainz. Although this channel is found to be predominantly excited by the intermediate production of a Δπ state, both the measured unpolarized and the helicity-dependent observables are generally not well described by the existing theoretical models.