Eur. Phys. J. A (2013) 49: 154
https://doi.org/10.1140/epja/i2013-13154-0

Regular Article - Experimental Physics

Quasi-free photoproduction of η-mesons off ³He nuclei

¹ Department of Physics, University of Basel, CH-4056, Basel, Switzerland
² Institut für Kernphysik, University of Mainz, D-55099, Mainz, Germany
³ School of Physics and Astronomy, University of Glasgow, G12 8QQ, Glasgow, UK
⁴ Kent State University, 44242, Kent, Ohio, USA
⁵ Helmholtz-Institut für Strahlen- und Kernphysik, University of Bonn, D-53115, Bonn, Germany
⁶ Petersburg Nuclear Physics Institute, RU-188300, Gatchina, Russia
⁷ INFN, I-27100, Pavia, Italy
⁸ School of Physics, University of Edinburgh, EH9 3JZ, Edinburgh, UK
⁹ Institute for Nuclear Studies, The George Washington University, 20052, Washington, DC, USA
¹⁰ University of California Los Angeles, 90095-1547, Los Angeles, California, USA
¹¹ Lebedev Physical Institute, RU-119991, Moscow, Russia
¹² II. Physikalisches Institut, University of Giessen, D-35392, Giessen, Germany
¹³ Laboratory of Mathematical Physics, Tomsk Polytechnic University, Tomsk, Russia
¹⁴ Mount Allison University, E4L 1E6, Sackville, Canada
¹⁵ University of Regina, SK S4S-0A2, Regina, Canada
¹⁶ Institute for Nuclear Research, RU-125047, Moscow, Russia
¹⁷ Rudjer Boskovic Institute, HR-10000, Zagreb, Croatia
¹⁸ The Catholic University of America, 20064, Washington, DC, USA

^* e-mail: Bernd.Krusche@unibas.ch

Received: 21 October 2013
Revised: 23 November 2013
Accepted: 29 November 2013
Published online: 16 December 2013

Abstract

Quasi-free photoproduction of η-mesons has been measured off nucleons bound in ³He nuclei for incident photon energies from the threshold region up to 1.4 GeV. The experiment was performed at the tagged photon facility of the Mainz MAMI accelerator with an almost 4π covering electromagnetic calorimeter, combining the TAPS and Crystal Ball detectors. The η-mesons were detected in coincidence with the recoil nucleons. This allowed a comparison of the production cross section off quasi-free protons and quasi-free neutrons and a full kinematic reconstruction of the final state, eliminating effects from nuclear Fermi motion. In the S₁₁(1535) resonance peak, the data agree with the neutron/proton cross section ratio extracted from measurements with deuteron targets. More importantly, the prominent structure observed in photoproduction off quasi-free neutrons bound in the deuteron is also clearly observed. Its parameters (width, strength) are consistent with the expectations from the deuteron results. On an absolute scale the cross sections for both quasi-free protons and neutrons are suppressed with respect to the deuteron target pointing to significant nuclear final-state interaction effects.