An experimental study of anti-proton He-4 anihilation at rest

This paper reports new experimental results on the annihilation of antiprotons stopped in a 4He gas target. The measurement was carried out with the Obelii spectrometer installed at the LEAR machine of CERN. Main results can be summarized as follows. (i) The invariant mass distribution of 2rr+2& produced at rest is similar to those from hydrogen and deuterium targets: the peak seen in recent experiments around 1150 MeV/c’ is absent. (ii) 2rr angular correlations are very close to those found for hydrogen. The nuclear structure does not modify remarkably the pion production and emission. (iii) gp and pp angular correlations are strongly peaked close to 180 O, in qualitative agreement with INC model predictions. (iv) The pp invariant mass distribution reveals events which may be due to q(w) absorption; the estimated upper limit for probabili~ of q and o absorption is about 3 x 10m3. (v) Events having features ~mpatible with do-nucleon annihiIations with production of heavy mesonic systems are detected both in the 2rr+ 2~r- and the 27r- T+ channels. The production rate of the 2nC 2?rchannel is of the order of 5 X 10W3.