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.
We present a measurement of e+e− pair production in central PbAu collisions at 158A 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.
A study of elliptic flow and two-particle azimuthal correlations of charged particles (\(0.5 < p_{{\text{T}}} < 2.5\) GeV/c) and high-\(p_{\rm T}\) pions (\(1.2 < p_{{\text{T}}} < 3.5\) GeV/c) in Pb ...+ Au collisions at 158A GeV/c, close to midrapidity, is presented. Elliptic flow (v2) rises linearly with \(p_{\rm T}\) to a value of about 10\(\%\) at 2 GeV/c. Beyond \(p_{\rm T}\approx\) 1.5 GeV/c, the slope decreases and possibly indicates a v2 saturation at high \(p_{\rm T}\). Two-pion azimuthal anisotropies for \(p_{\rm T} > \) 1.2 GeV/c exceed the v2 values by about 60\(\%\) in semicentral collisions. This non-flow component is attributed to near-side and away-side jetlike correlations. While the near-side peak remains constant with centrality 0.23\(\pm\)0.03 rad, as expected for fragmentation, the away-side peak experiences broadening and disappears in central collisions.
Phys.Rev.Lett.92:032301,2004 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 mid-rapidity 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
$p_T\approx$ 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 mid-central
collisions. These non-flow contributions are attributed to near-side and
back-to-back jet-like correlations, the latter exhibiting centrality dependent
broadening.
Measurements of event-by-event fluctuations of the mean transverse momentum in PbAu 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 behavior 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
μ
B
under investigation, has not been observed. The centrality dependence of mean
p
T
fluctuations in PbAu 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.
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