We present the combined results on electron-pair production in 158 GeV/n Pb-Au (\(\sqrt{s}\) = 17.2 GeV) collisions taken at the CERN SPS in 1995 and 1996, and give a detailed account of the data ...analysis. The enhancement over the reference of neutral meson decays amounts to a factor of 2.31 \(\pm0.19 (stat.)\pm0.55 (syst.)\pm0.69 (decays)\) for semi-central collisions (28\(\%\)\(\sigma/\sigma_{geo}\)) when yields are integrated over m > 200 MeV/c2 in invariant mass. The measured yield, its stronger-than-linear scaling with \(N_{\rm ch}\), and the dominance of low pair pt strongly suggest an interpretation as thermal radiation from pion annihilation in the hadronic fireball. The shape of the excess centring at \(m\approx\) 500 MeV/c2, however, cannot be described without strong medium modifications of the \(\rho\) meson. The results are put into perspective by comparison to predictions from Brown-Rho scaling governed by chiral symmetry restoration, and from the spectral-function many-body treatment in which the approach to the phase boundary is less explicit.
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
⊥.
During the 1996 lead run time, CERES has accumulated 42 million events, corresponding to a factor of 5 more statistics than in 1995 and 2.5 million events of a special photon-run. We report on the ...results of the low-mass e
+e
−-pair analysis. Since the most critical item is the poor signal-to-background ratio we also discuss the understanding of this background, in absolute terms, with the help of a detailed Monte Carlo simulation. We show preliminary results of the photon analysis and summarize the results of the hadron analysis preliminarily reported on already at QM'97 1.
We report preliminary results of the first measurement of low-mass electron pairs in 160 GeV/nucleon PbAu collisions at central rapidities in a wide multiplicity range. This work is the continuation ...of our systematic studies on pair production in p-Be, p-Au and SAu interactions. The motivation for this effort derives from a new source of lowmass dileptons recently observed in SAu and S-W collisions by CERES and HELIOS/3, respectively. The results obtained in the analysis of the PbAu data confirm our previous finding that pair production in the mass range 0.2 < m
ee
< 1.5 GeV/
c
2 is enhanced over the contributions from hadron decays while no significant excess could be observed for m
ee
≲ 2m
π
. A comparison of the results for different multiplicity bins favours a non-linear dependence of the e
+e
−-yield on the accompanying charged particle densities.
The CERES/NA45 experiment at the CERN-SPS is dedicated to study the production of e
+e
−-pairs in ultrarelativistic heavy ion collisions. Compared to extrapolations from results obtained in ...proton-induced reactions, it has previously observed an enhanced production in S-Au 1 and Pb-Au 2 collisions. To verify this enhancement and to study its properties, higher statistics were accumulated during a runtime in 1996. We report here, for the first time, on the results of this analysis (for details see ref. 3).
We report on the performance of a doublet of 3 in. cylindrical silicon drift detectors installed as an upgrade of the CERES/NA45 electron pair spectrometer for the Pb-beam at the CERN SPS. The ...silicon detectors provide external particle tracking and background rejection of conversions and close Dalitz pairs. Results on vertex reconstruction and rejection from a Pb test-run in 1994 are presented.
The CERES/NA45 collaboration measured, at the CERN SPS, low-mass electron pair production in Pb
Au collisions at 158 A GeV/c incident momentum. In this paper we present an analysis of the data taken ...during the SPS Pb-beam period in the fall of 1995. The results confirm previous findings of CERES showing that the pair production in the mass range 0.2 <m
ee < 1.5 GeV/c
2, when scaled to the ion case with the event multiplicity, is enhanced over the expected yield from the known hadronic sources in pp collisions. A comparison of the results for different multiplicity bins indicates that the excess increases faster than linearly with charged-particle multiplicity. We also present the transverse-momentum distributions of the observed pairs.
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.
Performance of the CERES electron spectrometer in the CERN SPS lead beam Agakichiev, G; Baur, R; Ceretto, F ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
03/1996, Letnik:
371, Številka:
1
Journal Article
Recenzirano
CERES is an experiment dedicated to measure e
+ e
−-pairs in nucleus-nucleus collisions at the CERN SPS. Following the experiences gained with proton and sulfur beams the CERES spectrometer was ...upgraded to cope with the much larger charged particle densities produced in PbPb collisions. Major parts of the upgrade were completed in time for a 10 days test with a lead beam in November 1994. The results clearly demonstrate the feasibility of the experiment in the environment of central PbPb collisions.
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