J.P. Wurm

Low-mass e+e− pair production in 158 A GeV Pb-Au collisions at the CERN SPS, its dependence on multiplicity and transverse momentum

We report a measurement of low-mass electron pairs observed in 158 GeV/nucleon Pb-Au collisions. The pair yield integrated over the range of invariant masses 0.2<m<2.0 GeV is enhanced by a factor of 3.5 +/- 0.4 (stat) +/- 0.9 (syst) over the expectation from neutral meson decays. As observed previously in S-Au collisions, the enhancement is most pronounced in the invariant-mass region 300-700 MeV. For Pb-Au we find evidence for a strong increase of the enhancement with centrality. In addition, we show that the enhancement covers a wide range in transverse momentum, but is largest at the lowest observed pt.We report a measurement of low-mass electron pairs observed in 158 GeV/nucleon Pb-Au collisions. The pair yield integrated over the range of invariant masses 0.2 < m < 2.0 GeV is enhanced by a factor of 3.5 +/- 0.4 (stat) +/- 0.9 (syst) over the expectation from neutral meson decays. As observed previously in S-Au collisions, the enhancement is most pronounced in the invariant-mass region 300-700 MeV. For Pb-Au we find evidence for a strong increase of the enhancement with centrality. In addition, we show that the enhancement covers a wide range in transverse momentum, but is largest at the lowest observed pt.Abstract We report a measurement of low-mass electron pairs observed in 158 GeV/nucleon Pb-Au collisions. The pair yield integrated over the range of invariant masses 0.2 ≤ m ≤ 2.0 GeV/c 2 is enhanced by a factor of 3.5 ± 0.4 (stat) ± 0.9 (syst) over the expectation from neutral meson decays. As observed previously in S-Au collisions, the enhancement is most pronounced in the invariant-mass region 300-700 MeV/c 2 . For Pb-Au we find evidence for a strong increase of the enhancement with centrality. In addition, we show that the enhancement covers a wide range in transverse momentum, but is largest at the lowest observed p ⊥ .

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.

Pre-equilibrium alpha emission accompanying deep-inelastic16O+58Ni collisions

Abstractα particles were measured in coincidence with projectile-like reaction products (oxygen and carbon) produced in deep-inelastic16O+58Ni collisions at about 6 MeV/N bombarding energy. The kinematic analysis of the HI andα energies measured as a function ofΘα gives strong evidence for a sequential process: the target-like fragments are excited by the deep-inelastic collision and undergo subsequentα decay. In contrast, the angular correlations show a pronounced forward peak, indicative of direct or pre-equilibriumα emission. The emission time for the latter is estimated to be of the order of 2×10−21 s. To resolve this conflict of co-existing statistical and direct features of the pre-equilibrium emission, the concept of a hot spot is proposed. From the angular correlation and from theα multiplicities, a local temperature ofT−3.5 MeV is deduced which agrees well with the temperature derived from the shape of theα spectra. The spot size is estimated to be 1/5 of the sphere.

Optical-model parameters and the analysis of (d, p) stripping experiments on 138Ba, 140Ce and 142, 144, 146Nd

Abstract Measurements have been made of the (d, p) reactions from 138Ba, 140Ce and 142, 144, 146Nd to several states in the residual nuclei at a deuteron bombarding energy of 12 MeV. A method is described to extract from (d, d0), (p, p0) and (p, n) data, optical-model parameters with errors reflecting the experimental uncertainties. Proton parameters are obtained from optical-model fits of the (p, p0) excitation functions for 138Ba, 140Ce, 144Sm and 142, 144, 146Nd in the energy range from 8.6 to 12 MeV at various scattering angles. Smoothly energy and mass dependent proton potentials were obtained with a real potential depth V = (50+0.4 Z/A 1 3 +32.4(N–Z)/ A−0.4 E p )±3.1[ MeV ] . The (d, p) data are well reproduced and the spectroscopic factors with uncertainties are extracted using the DWBA.

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.

Universal pion freeze-out in heavy-ion collisions

Based on an evaluation of data on pion interferometry and on particle yields at mid-rapidity, we propose a universal condition for thermal freeze-out of pions in heavy-ion collisions. We show that freeze-out occurs when the mean free path of pions lambda_f reaches a value of about 1 fm, which is much smaller than the spatial extent of the system at freeze-out. This critical mean free path is independent of the centrality of the collision and beam energy from AGS to RHIC.

Semihard Scattering Unraveled from Collective Dynamics by Two-Pion Azimuthal Correlations in 158A GeV/c Pb + Au Collisions

Elliptic flow and two-particle azimuthal correlations of charged hadrons and high-p(T) pions (p(T)>1 GeV/c) have been measured close to midrapidity in 158A GeV/c Pb+Au collisions by the CERES experiment. Elliptic flow (v(2)) rises linearly with p(T) to a value of about 10% at 2 GeV/c. Beyond p(T) approximately 1.5 GeV/c, the slope decreases considerably, possibly indicating a saturation of v(2) at high p(T). Two-pion azimuthal anisotropies for p(T)>1.2 GeV/c exceed the elliptic flow values by about 60% in midcentral collisions. These nonflow contributions are attributed to nearside and back-to-back jetlike correlations, the latter exhibiting centrality dependent broadening.

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.

Systematic study of low-mass electron pair production in p-Be and p-Au collisions at 450 GeV/c

Abstract. In a joint effort the CERES/NA45 and TAPS collaborations have measured low-mass electron pairs in p–Be and p–Au collisions at 450 GeV/

Leptonic and charged kaon decay modes of the phi meson measured in heavy-ion collisions at the CERN super proton synchrotron.

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