V. Kushpil

Enhanced Production of Low-Mass Electron-Positron Pairs in 40-AGeV Pb-Au Collisions at the CERN SPS

We report on first measurements of low-mass electron-positron pairs in Pb-Au collisions at the CERN SPS beam energy of 40 AGeV. The observed pair yield integrated over the range of invariant masses 0.2<m</=1 GeV/c(2) is enhanced over the expectation from neutral meson decays by a factor of 5.9+/-1.5(stat)+/-1.2(syst data)+/-1.8(syst meson decays), somewhat larger than previously observed at the higher energy of 158 AGeV. The results are discussed with reference to model calculations based on pi(+)pi(-)-->e(+)e(-) annihilation with a modified rho propagator. They may be linked to chiral symmetry restoration and support the notion that the in-medium modifications of the rho are more driven by baryon density than by temperature.

Modification of the rho meson detected by low-mass electron-positron pairs in central Pb-Au collisions at 158A GeV/c

Abstract We present a measurement of e + e − pair production in central Pb Au collisions at 158 A GeV / c . As reported earlier, a significant excess of the e + e − pair yield over the expectation from hadron decays is observed. The improved mass resolution of the present data set, recorded with the upgraded CERES experiment at the CERN-SPS, allows for a comparison of the data with different theoretical approaches. The data clearly favor a substantial in-medium broadening of the ρ spectral function over a density-dependent shift of the ρ pole mass. The in-medium broadening model implies that baryon induced interactions are the key mechanism to the observed modifications of the ρ meson at SPS energy.

Event-by-event fluctuations of the mean transverse momentum in 40, 80, and 158 A GeV/c Pb-Au collisions

Measurements of event-by-event fluctuations of the mean transverse momentum in Pb-Au collisions at 40, 80, and 158 A GeV/c are presented. A significant excess of mean p_T fluctuations at mid-rapidity is observed over the expectation from statistically independent particle emission. The results are somewhat smaller than recent measurements at RHIC. A possible non-monotonic behaviour of the mean p_T fluctuations as function of collision energy, which may have indicated that the system has passed the critical point of the QCD phase diagram in the range of mu_B under investigation, has not been observed. The centrality dependence of mean p_T fluctuations in Pb-Au is consistent with an extrapolation from pp collisions assuming that the non-statistical fluctuations scale with multiplicity. The results are compared to calculations by the RQMD and UrQMD event generators.

Beam energy and centrality dependence of two pion Bose-Einstein correlations at SPS energies

Abstract Results are presented of a two-pion interferometry (HBT) analysis in Pb+Au collisions at 40, 80, and 158 Axa0GeV. A detailed study of the Bertsch–Pratt HBT radius parameters has been performed as function of the mean pair transverse momentum kt and in bins of the centrality of the collision. From these results we extract model-dependent information about the space–time evolution of the reaction. An investigation of the effective volume of the pion emitting system provides an important tool to study the properties of thermal pion freeze-out.

Latest results from CERES/NA45

D. Adamová, G. Agakichiev, H. Appelshäuser, V. Belaga, P. Braun-Munzinger, R. Campagnolo, A. Castillo, A. Cherlin, S. Damjanović, T. Dietel, L. Dietrich, A. Drees , S. I. Esumi, K. Filimonov, K. Fomenko, Z. Fraenkel, C. Garabatos, P. Glässel, G. Hering, J. Holeczek, V. Kushpil, B. Lenkeit, W. Ludolphs, A. Maas, A. Maŕın, F. Messer , J. Milošević, A. Milov, D. Mískowiec, L. Musa, Yu. Panebrattsev, O. Petchenova, V. Petráček, A. Pfeiffer, J. Rak, I. Ravinovich, P. Rehak, M. Richter, H. Sako, W. Schmitz, J. Schukraft, S. Sedykh, W. Seipp, A. Sharma, S. Shimansky, J. Sĺıvová, H. J. Specht, J. Stachel, M. Šumbera, H. Tilsner, I. Tserruya, J. P. Wessels, T. Wienold, B. Windelband, J. P. Wurm, W. Xie, S. Yurevich, V. Yurevich

The CERES/NA45 radial drift Time Projection Chamber

The design, calibration, and performance of the first radial drift Time Projection Chamber (TPC) are presented. The TPC was built and installed at the CERES/NA45 experiment at the CERN SPS in the late nineties, with the objective to improve the momentum resolution of the spectrometer. The upgraded experiment took data twice, in 1999 and in 2000. After a detailed study of residual distortions a spatial resolution of 340μm in the azimuthal and 640μm in the radial direction was achieved, corresponding to a momentum resolution of ∆p/p = √ (1% · p/GeV) + (2%).

First results from the CERES radial TPC

The CERES/NA45 experiment at the CERN SPS was upgraded in 1998 by the addition of a cylindrical Time Projection Chamber. The aim is to improve the mass resolution of e+e− pairs in the ρ/ω/φ region to Δm/m < 2%. A progress report is given on the TPC performance during the 1998 commissioning runs (laser rays, beam halo muons, and particles produced by proton/π and lead beams hitting Pb and Au targets).

Flow and non-flow event anisotropies at the SPS

Abstract A study of differential elliptic event anisotropies ( ν 2 ) of charge particles and high- p T pions in 158 AGeV/c Pb+Au collisions is presented. Results from correlations with respect to the event plane and from two-particle azimuthal correlations are compared. The latter give systematically higher ν 2 values at p T >1.2 GeV/c providing possibly an evidence of a non-flow semihard component.

Modification of jet-like correlations in Pb-Au collisions at 158A GeV/c

Abstract Results of a two-particle correlation analysis of high- p t charged particles in Pb–Au collisions at 158 A GeV / c are presented. The data have been recorded by the CERES experiment at the CERN-SPS. The correlations are studied as function of transverse momentum, particle charge and collision centrality. We observe a jet-like structure in the vicinity of a high- p t trigger particle and a broad back-to-back distribution. The yields of associated particles per trigger show a strong dependence on the trigger/associate charge combination. A comparison to PYTHIA confirms the jet-like pattern at the near-side but suggests a strong modification at the away-side, implying significant energy transfer of the hard-scattered parton to the medium.

Event-by-event fluctuations at SPS

Results on event-by-event fluctuations of the mean transverse momentum and net charge in Pb-Au collisions, measured by the CERES Collaboration at CERN-SPS, are presented. We discuss the centrality and beam energy dependence and compare our data to cascade calculations.