A. Rybicki

Evidence for an Exotic S = -2, Q = -2 Baryon Resonance in Proton-Proton Collisions at the CERN SPS

Results of resonance searches in the Xi- pi-, Xi- pi+, antiXi+ pi- and antiXi+ pi+ invariant mass spectra in proton-proton collisions at sqrt{s}=17.2 GeV are presented. Evidence is shown for the existence of a narrow Xi- pi- baryon resonance with mass of 1.862+/-0.002 GeV/c^2 and width below the detector resolution of about 0.018 GeV/c^2. The significance is estimated to be 4.0 sigma. This state is a candidate for the hypothetical exotic Xi_(3/2)^-- baryon with S = -2, I = 3/2 and a quark content of (d s d s ubar). At the same mass a peak is observed in the Xi- pi+ spectrum which is a candidate for the Xi_(3/2)^0 member of this isospin quartet with a quark content of (d s u s dbar). The corresponding antibaryon spectra also show enhancements at the same invariant mass.

Transverse momentum fluctuations in nuclear collisions at 158A GeV

Results are presented on event-by-event fluctuations in transverse momentum of charged particles, produced at forward rapidities in p+p, C+C, Si+Si and Pb+Pb collisions at 158 AGeV. Three different characteristics are discussed: the average transverse momentum of the event, the Phi_pT fluctuation measure and two-particle transverse momentum correlations. In the kinematic region explored, the dynamical fluctuations are found to be small. However, a significant system size dependence of Phi_pT is observed, with the largest value measured in peripheral Pb+Pb interactions. The data are compared with predictions of several models.

Bose-Einstein correlations of charged kaons in central Pb + Pb collisions at Ebeam = 158 GeV per nucleon

Abstract Bose–Einstein correlations of charged kaons were measured near mid-rapidity in central Pb+Pb collisions at 158xa0 A xa0GeV by the NA49 experiment at the CERN SPS. Source radii were extracted using the Yano–Koonin–Podgoretsky and Bertsch–Pratt parameterizations. The results are compared to published pion and kaon data. The measured m ⊥ dependence for kaons and pions is consistent with collective transverse expansion of the source and a freeze-out time of aboutxa09.5xa0fm.

Strangeness from 20 A GeV to 158 A GeV

New results from the energy scan programme of NA49, in particular kaon production at 30 A GeV and Φ production at 40 and 80 A GeV are presented. The K + /π + ratio shows a pronounced maximum at 30 A GeV; the kaon slope parameters are constant at SPS energies. Both findings support the scenario of a phase transition at about 30 A GeV beam energy. The Φ/π ratio increases smoothly with beam energy, showing an energy dependence similar to K - /π - . The measured particle yields can be reproduced by a hadron gas model, with chemical freeze-out parameters on a smooth curve in the T-μ B plane. The transverse spectra can be understood as resulting from a rapidly expanding, locally equilibrated source. No evidence for an earlier kinetic decoupling of heavy hyperons is found.

Ω- and Ω̄+ production in central Pb+Pb collisions at 40 and 158AGeV

Results are presented on Omega production in central Pb+Pb collisions at 40 and 158A GeV beam energy. For the first time in heavy ion reactions, rapidity distributions and total yields were measured for the sum Omega(-) + Omega(+) at 40A GeV and for Omega(-) and Omega(+) separately at 158A GeV. The yields are strongly underpredicted by the string-hadronic UrQMD model but agree better with predictions from hadron gas models.