AN EXPERIMENTAL-STUDY OF ANTIPROTON - HE-4 ANNIHILATION AT REST

This paper reports new experimental results on the annihilation of ant iprotons stopped in a He-4 gas target. The measurement was carried out with the Obelix spectrometer installed at the LEAR machine of CERN. M ain results can be-summarized as follows. (i) The invariant mass distr ibution of 2pi+2pi- produced at rest is similar to those from hydrogen and deuterium targets: the peak seen in recent experiments around 115 0 MeV/c2 is absent. (ii) 2pi angular correlations are very close to th ose found for hydrogen. The nuclear structure does not modify remarkab ly the pion production and emission. (iii) pip and pp angular correlat ions are strongly peaked close to 180-degrees, in qualitative agreemen t with INC model predictions. (iv) The pp invariant mass distribution reveals events which may be due to eta(omega) absorption; the estimate d upper limit for probability of eta and omega absorption is about 3 x 10(-3). (v) Events having features compatible with two-nucleon annihi lations with production of heavy mesonic systems are detected both in the 2pi+2pi- and the 2pi-pi+ channels. The production rate of the 2pi2pi-channel is of the order of 5 x 10(-3).