0 Three - Pion Interferometry Results from Central Pb + Pb Collisions at 158 A GeV / c

Three-particle correlations have been measured for identified π from central 158 A GeV Pb+Pb collisions by the WA98 experiment at CERN. A substantial contribution of the genuine three-body correlation has been found as expected for a mainly chaotic and symmetric source. In nuclear and particle physics, the study of Bose-Einstein correlations between identical particles is widely used. It is an essential tool to obtain information on the size and time evolution of expanding systems created in heavy ion collisions [1]. In particular, two-particle Bose-Einstein interferometry has been used for a sophisticated analysis of the dynamical evolution of the freeze out volume via selection on the transverse momentum and rapidity of the correlated particles. Three-particle interferometry measurements can provide additional information on the space-time emission which is not accessible by two-particle interferometry[2, 3, 4, 5]. In particular, if the emission is fully chaotic, the three-particle interference study gives access to the phase of the source function’s Fourier transform, which is affected by the emission asymmetry. These asymmetries may be induced by source geometry, flow or resonance decays. If the source is not completely chaotic, as is likely to be the case, the interpretation is more difficult. Nevertheless, a comparison of the three-particle to the two-particle result allows to extract the relative strength of the true three-body correlation and can in principle remove obscuring effects, such as background from misidentified tracks or long-lived decay products, and therefore provide information more directly about the degree of coherence of the emission source[4]. In this letter we present first results from three-particle interferometry in central Pb+Pb collisions at the CERN SPS. The fixed target experiment WA98 [6] combined large acceptance photon detectors with a two arm charged particle tracking spectrometer. The incident 158 A GeV Pb beam impinged on a Pb target near the entrance of a large dipole magnet. The results presented here have been obtained from an analysis of the 1995 data set. These data were taken with the most central triggers corresponding to about 10% of the minimum bias cross section of 6190 mb. The π measurements were obtained with data from the negative particle tracking arm of the spectrometer. This tracking arm consisted of six multistep avalanche chambers with optical