Study of antiproton nucleon annihilations at rest into five pions

Abstract

The Crystal Barrel Experiment at LEAR allows the detection of charged particles and photons with almost 4 π angular coverage, respectively. Here we present a study of pp annihilation and pd annihilation at rest into five-pion final states ( pp →5 π 0 , pp → π + π − 3 π 0 , pd → π − 4 π 0 p and pd → π + 2 π − 2 π 0 p). These reactions are well suited to look for four pion decays of scalar and vector mesons. Previous experiments on pn →2 π + 3 π − [3], np →3 π + 2 π − [4], and pp → + π - 3 π 0 [5] established the dominance of the production of a scalar isoscalar resonance with a mass of about 1370 MeV and a width of about 350 MeV. The ƒ 0 (1370) was found to have a sizeable decay branching ratio into σσ and ρρ . We show, in an analysis of the reaction pp →5 π 0 , that there is indeed a scalar resonance decaying into 4 π 0 , but this resonance can be identified with the ƒ 0 (1500), a new resonance, observed by the Crystal Barrel Collaboration in the decay modes π 0 π 0 , ηη and ηη ′. In the 5 π 0 final state we found evidence for the ƒ 0 (1500) decay to σσ and π (1300) π . A preliminary result of a combined analysis of the four five-pion data sets mentioned above confirms the existence of the ƒ 0 (1500) in the data and shows that the σσ decay of the ƒ 0 (1500) is more important than its decay to ρρ .