Eur. Phys. J. A (2014) 50: 173
https://doi.org/10.1140/epja/i2014-14173-y

Regular Article - Experimental Physics

Helicity dependence of the γ ³He → πX reactions in the Δ(1232) resonance region

¹ Sezione di Pavia, INFN, I-27100, Pavia, Italy
² Dipartimento di Fisica, Università di Pavia, I-27100, Pavia, Italy
³ Sezione di Catania, INFN, I-95123, Catania, Italy
⁴ Dipartimento di Fisica e Scienze della Terra, Università di Messina, I-98166, Messina, Italy
⁵ Institut für Kernphysik, Universität Mainz, D-55099, Mainz, Germany
⁶ SUPA School of Physics & Astronomy, University of Glasgow, G12 8QQ, Glasgow, UK
⁷ Helmholtz-Institut für Strahlen und Kernphysik, University of Bonn, D-53115, Bonn, Germany
⁸ Petersburg Nuclear Physics Institute, RU-188300, Gatchina, Russia
⁹ II Physikalisches Institut, University of Giessen, D-35392, Giessen, Germany
¹⁰ The George Washington University, 20052-0001, Washington, DC, USA
¹¹ Institute for Nuclear Research, RU-125047, Moscow, Russia
¹² Dalhousie University, B3H 4R2, Halifax, NS, Canada
¹³ Saint Mary’s University, B3H 3C3, Halifax, NS, Canada
¹⁴ Tomsk Polytechnic University, Tomsk, Russia
¹⁵ SUPA School of Physics & Astronomy, University of Edinburgh, EH9 3JZ, Edinburgh, UK
¹⁶ Institut für Physik, Universität Mainz, D-55099, Mainz, Germany
¹⁷ Mount Allison University, E4L-1E6, Sackville, NB, Canada
¹⁸ Institut für Physik, University of Basel, CH-4056, Basel, Switzerland
¹⁹ University of Regina, S4S-0A2, Regina, SK, Canada
²⁰ Lebedev Physical Institute, RU-119991, Moscow, Russia
²¹ Rudjer Boskovic Institute, HR-10000, Zagreb, Croatia
²² Kent State University, 44242-0001, Kent, Ohio, USA
²³ University of California, 90095-1547, Los Angeles, California, USA

^* e-mail: pedroni@pv.infn.it

Received: 5 September 2013
Revised: 24 October 2014
Accepted: 10 November 2014
Published online: 24 November 2014

Abstract

The helicity dependences of the differential cross sections for the semi-inclusive γ ³He → π⁰ X and γ ³He → π^± X reactions have been measured for the first time in the energy region 200 < E _γ 450 MeV. The experiment was performed at the tagged photon beam facility of the MAMI accelerator in Mainz using a longitudinally polarised high-pressure ³He gas target. Hadronic products were measured with the large-acceptance Crystal Ball detector complemented with additional devices for charged-particle tracking and identification. Unpolarised differential cross sections and their helicity dependence are compared with theoretical calculations using the Fix-Arenhövel model. The effect of the intermediate excitation of the Δ(1232) resonance can be clearly seen from this comparison, especially for the polarised case, where nuclear effects are relatively small. The model provides a better theoretical description of the unpolarised charged pion photoproduction data than the neutral pion channel. It does significantly better in describing the helicity-dependent data in both channels. These comparisons provide new information on the mechanisms involved in pion photoproduction on ³He and suggest that a polarised ³He target can provide valuable information on the corresponding polarised quasi-free neutron reactions.