Annihilations of 3.0 and 3.6 GeV/c antiprotons into six or more pions

Abstract

We report a study of the reactions\(\bar pp \to n\pi ^ + + n\pi ^ - + m\pi ^0 \) at 3.0 GeV/c and 3.6 GeV/c (Ec.m.s. is equal to 2.77 GeV and 2.96 GeV respectively) wheren=3, 4 or 5 andm≥0. The sample contains 1988 six-prong events at 3.0 GeV/c, 440 eight-prong events at 3.0 and 3.6 GeV/c and 10 ten-prong events at 3.0 and 3.6 GeV/c. The cross-sections for various processes observed are compared with the cross-sections obtained in other experiments at different incident antiproton momenta. A maximum in the 7π annihilation cross-section is seen at a c.m.s. energy of about 2.7 GeV. Appreciable production of ρ0 in the six-pion events and of\(\rho ^{\begin{array}{*{20}c} \pm \\ 0 \\ \end{array} } \) and ω0 mesons in the seven-pion events is observed. In the six-pion events the 2π+2π− mass distribution and the ρ0π+π− mass distribution show evidence of structure with peaks at (1704±10) MeV and (1834±10) MeV. In the seven-pion events selected to contain a π+π− and π+π−π0 mass combination in the ρ0 and ω0 regions respectively, the ρ0π± mass distribution shows peaks in the mass regions corresponding to the A1 and A2 mesons with masses of (1054±7) MeV and (1269±9) MeV respectively, the width of the peak in the region of the A2 being smaller, (38±19) Mev, than that usually observed in bubble chambers but is compatible with the lower-mass peak of the split A2 as observed by Levratet al. The ρ0ω0 mass distribution shows two peaks at masses of (1689±10) MeV and (1848±11) MeV. The angular distributions of pions are slightly collimated along the direction of the incident particles, this collimation being an increasing function of pion momentum in the c.m.s. The ρ0 and ω0 mesons also have anisotropic distributions in the c.m.s. A strong angular correlation is observed between pairs of pions of opposite charge. This correlation decreases with increasing energy of the c.m.s. or when the number of π0 produced in the annihilation increases.