Charged particle multiplicities in p̄p reactions 540 GeV C.M. energy

Abstract: Antiproton-proton reactions at a centre of mass energy of 540 GeV are studied. The results were obtained using the UA5 detector installed at the CERN SPS Collider.The pseudorapidity (-n) distribution for non single-diffractive events is presented and compared to ISR data. The pseudorapidity density in the central region grows approximately like ln(s). The mean charged multiplicity is compared to lower energy data.The charged particle multiplicity distribution in full phase space at 540 GeV is compared to data at lower c.m. energies, and using the scaling variable z=n/<n> a change of shape is observed. An investigation of multiplicity distributions in limited parts of phase space shows that the widths of the distributions, scaled to their means, increase as the T\-interval is made smaller.The multiplicity distributions for the non single-diffractive component of pp reactions at 540 GeV and of pp reactions above about 10 GeV are shown to be remarkably well described by a negative binomial distribution at each energy. The approximate KNO scaling in the c.m. energy range 10-62 GeV is, in this framework, shown to be accidental. The multiplicity distributions in limited parts of phase space are also shown to fit the negative binomial distribution very well as long as the centre of the pseudorapidity interval is close to zero.Forward-backward multiplicity correlations are observed. A picture in which small clusters are emitted along the rapidity axis is consistent with our data. The average cluster size is about two charged particles, which is the same as at ISR energies.Various phenomenological models are compared to the multiplicity distribution from the UA5 experiment. No model shows a better fit to the data than the negative binomial distribution.