Coherent photoproduction of η-mesons off 3He – search for η-mesic nuclei

F Pheron,L Witthauer,G Rosner,S Schumann,M Ostrick,Z Marinides,B Boillat,D Howdle,M Oberle,T Rostomyan,E Heid,M Unverzagt,A Braghieri,R Kondratiev,D Werthmüller,J Robinson,J Ahrens,A Nikolaev,M Korolija,I Supek,A Thomas,H Berghäuser,A Mushkarenkov,B Oussena,J Mancell,F Zehr,K Livingston,L.V FilʼKov,B Demissie,V Metag,B Krusche,M Kotulla,K Bantawa,S Cherepnya,P Drexler,M Thiel,A Polonski,P Pedroni,D Hornidge,A Starostin,R Novotny,M Dieterle,D Branford,R Beck,V Lisin,V Bekrenev,J Brudvik,Y Maghrbi,I Keshelashvili,O Jahn,V L Kashevarov ,M H Sikora ,E J Downie ,M Martínez ,T C Jude ,D I Glazier ,Saša Mićanović ,H J Arends ,J R M Annand ,S P Kruglov ,B M K Nefkens ,D Watts ,I Jaeglé ,D M Manley ,J C Mcgeorge ,W J Briscoe ,D Mekterović ,A A Kulbardis ,S Prakhov ,D I Sober ,D G Middleton ,I J D Macgregor ,E F Mcnicoll ,P Aguar Bartolomé

doi:10.1016/j.physletb.2012.01.075

Abstract

Coherent photoproduction of η-mesons off 3He, i.e. the reaction γ3He→η3He, has been investigated in the near-threshold region. The experiment was performed at the Glasgow tagged photon facility of the Mainz MAMI accelerator with the combined Crystal Ball – TAPS detector. Angular distributions and the total cross section were measured using the η→γγ and η→3π0→6γ decay channels. The observed extremely sharp rise of the cross section at threshold and the behavior of the angular distributions are evidence for a strong ηHe3 final state interaction, pointing to the existence of a resonant state. The search for further evidence of this state in the excitation function of π0-proton back-to-back emission in the γ3He→π0pX reaction revealed a very complicated structure of the background and could not support previous conclusions.

Full Text