Search

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, 10 pages, 9 figures, Proceedings of the XIV Int. Conf. on Nucleus-Nucleus Collisions,Quark Matter 99, Torino, Italy, May 10 - 15, 1999

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]).

Hadronic spectra from interactions of Pb on Au at 158 AGeV/c have been measured by the CERES experiment at the CERN SPS. We report on preliminary results on transverse mass spectra of identified pions and negative hadrons as well as spectra of the positive charge excess obtained by subtracting the negative particle distribution from that of positive tracks. We have also performed a study of the event azimuthal anisotropies which give evidence for directed and elliptic flow in non-central collisions.

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 ee 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.

In this paper we report on the application of a robust approach based on a reweighted least squares method for the vertex reconstruction of the CERES/NA45 experiment at the CERN SPS for the case of 160 GeV/u Pb+Au collisions and compare its performance with the presently used algorithm based on the minimization of summed Gaussian weights. In the CERES/NA45 experiment a doublet of cylindrical silicon drift detectors (SDD) is used for vertex reconstruction and tracking. The vertex reconstruction algorithm has to cope with a background-contaminated environment. Furthermore, in the case of ultrarelativistic heavy-ion collisions where high event multiplicities occur (dnchdη ⋍ 500), the probability of confusing hits from close or intersecting tracks is high. The algorithm needs to provide the precise vertex location even at high occupancies, at a reasonable speed. Our findings are that the robust iterative procedure presented here is about an order of magnitude faster than approaches using standard minimization packages, at a comparable or slightly better accuracy.

We report preliminary results of the first measurement of low-mass electron pairs in 160 GeV/nucleon PbAu 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 SAu interactions. The motivation for this effort derives from a new source of lowmass dileptons recently observed in SAu and S-W collisions by CERES and HELIOS/3, respectively. The results obtained in the analysis of the PbAu data confirm our previous finding that pair production in the mass range 0.2 < mee < 1.5 GeV/c2 is enhanced over the contributions from hadron decays while no significant excess could be observed for mee ≲ 2mπ. 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 experiment has measured inclusive photon production in S-Au collisions of 200 GeV/nucleon at the CERN SPS. No evidence for direct emission of photons was found. For the kinematic region 2.1

AZTEC, a large area cylindrical silicon drift detector was designed, produced and tested. AZTEC will be the building block of the NA45 and WA98 micro vertex detectors at CERN. Two AZTEC detectors are placed down stream from the target to measure trajectories of charged particles produced in the forward direction. The active area of AZTEC is practically the full usable surface of a 100 mm diameter wafer. The electrons drift radially from the center towards the outside. The sensing anodes are located at a radius of 42 mm. The center of the wafer is cut out and forms a passage for the non interacting beam. With a minimal radius for this hole the active region of the drift detector starts at an inner radius of 3.1 mm. Any larger radius can be selected if necessary. With this geometry and a typical operating voltage the maximum drift time is less than 4μs. Due to constrains in the mask layout the readout region and field electrodes are designed along the 360 sides of a symmetric polygon. All structures on one surface of the wafer are rotated by 0.5° with respect to the other surface. In the middle plane of the detector, where the electrons are mostly transported, the effective geometry is close to a smoothed polygon with 720 sides, cancelling practically all effects of the non-perfect cylindrical symmetry. The radial position of fast charged particles is measured by the electron drift time within the detector. The drift velocity can be monitored by 48 injection points at three different radii. The azimuthal angle is measured by the 360 readout anodes. Each anode is subdivided into five segments, which are interlaced with the neighbouring anodes. By this methode the azimuthal resolution is improved and corresponds to a 720 channel read out.

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.

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 PbPb 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 PbPb collisions.

We have for the first time identified e + e − -pairs produced by strongly varying electromagnetic fields in distant SPt collisions at 200 GeV/u projectile energy at the CERN-SPS. The differential cross section for the mass range 10 MeV/ c 2 ≤ m ee ≤ 100 MeV/ c 2 and the polar angle range 141 mrad ≤ θ e ≤ 260 mrad agrees well with a lowest-order perturbation QED-calculation.

A silicon drift detector of circular geometry giving unambiguously the radial and azimuthal coordinates of particle's interaction point for events with high multiplicity is part of the experimental set up of the NA45 experiment at CERN SPS. The paper reviews the characteristics of the detector and of its assembly among the other detectors of the experiment. The first experimental results showing the performance of the detector in term of resolution and its effectiveness in reconstructing the position of the event are reported.

Summary form only given. An advanced kind of silicon detector, a large-area cylindrical drift detector, was designed, produced, tested, and installed in an experiment. The active area of the detector is practically the total area of a 3-in-diameter wafer. Signal electrons created in the silicon detector by fast charged particles drift radially outside toward an array of 360 anodes located at the outermost radius of the detector. The drift time measures the radial coordinate of the fast particle; the charge sharing between anodes measures the azimuthal coordinate. The detector provides unambiguous pairs of r, phi coordinates for very high multiplicity events up to several hundred. The precision of the position measurement in each coordinate is about 10 mu m, giving about 10/sup 7/ 2-D elements. There is a small hole in the center of the detector to allow the passage of the noninteracting particle beam. The longest drift distance is about 3 cm. The nominal value of the drift field is 500 V/cm, resulting in a maximum drift time of 4 mu s. >

The CERES/NA45 experiment at CERN uses a silicon pad detector with 64 ring shaped pads for first level triggering on minimum bias proton-nucleus as well as on high multiplicity ion-ion collisions. In addition, the counter serves as a multiplicity detector to estimate the energy density obtained in the collision. The design and performance of the pad counter in the laboratory and under real beam conditions together with the first level trigger electronics are reported.

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/c at the CERN SPS. In the range covered up to approximate to 1.5 GeV/c(2) the mass spectra from p-Be and p-Au collisions are well explained by electron pairs from decays of neutral mesons. For p-Au our result is new. For p-Be, the simultaneously measured electron pairs and photons served as a direct measure of the eta Dalitz decay contribution to the inclusive pair spectrum in which instrumental uncertainties are highly reduced. We confirm the earlier finding of HELIOS-1 with significantly reduced systematic uncertainties of 23% in the mass:range below 450 MeV/c(2), and of 28% in the mass range above 750 MeV/c(2) at 90% confidence limit. Any unconventional source of electron pairs is limited by these error margins as the percentage fraction of the hadronic contribution.

In a joint experiment the TAPS and CERES collaborations have studied the production of the neutral mesons $\pi^\circ,\eta$ and $\omega$ in 450 GeV p-Be and p-Au collisions at the CERN SPS. The mesons were identified by their $\pi^\circ\rightarrow\gamma\gamma$ , $\eta\rightarrow\gamma\gamma$ , and $\omega\rightarrow\pi^\circ\gamma$ decay modes. The cross sections were measured down to very low $p_T$ ( $\geq$ 20 MeV/c) near mid-rapidity ( $3.0 < y < 4.0$ ). The $\eta$ and $\omega$ cross sections are smaller than expected from extrapolations of earlier data to low $p_T$ . Integrated over all $p_T$ , the ratios $\eta/\pi^\circ$ and $\omega$ / $\pi^\circ$ are lower by about 25% compared to previously published values. The reduction may be attributed to the large number of low $p_T$ pions from decays of heavier mesons. In addition, we have studied the dependence of neutral meson production on the mass of the nuclear target and find it in good agreement with previous measurements of charged mesons.

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., Comment: 17 pages, 4 figures, submitted to Phys.Lett.B

We report on measurements of low-mass electron pairs in 450 GeV $p$-Be, $p$-Au, and 200 GeV/nucleon S-Au collisions at central rapidities. For the proton induced interactions, the low-mass spectra are, within the systematic errors, satisfactorily explained by electron pairs from hadron decays, whereas in the S-Au system an enhancement over the hadronic contributions by a factor of $5.0\ifmmode\pm\else\textpm\fi{}0.7(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}2.0(\mathrm{syst})$ in the invariant mass range $0.2lml1.5\phantom{\rule{0ex}{0ex}}\mathrm{GeV}{/c}^{2}$ is observed. The properties of the excess suggest that it arises from two-pion annihilation $\ensuremath{\pi}\ensuremath{\pi}\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}}$.

The CERES experiment (CErenkov Ring Electron Spectrometer) studies the production of low mass e + e − pairs in proton-proton, proton-nucleus and nucleus-nucleus interactions at the CERN SPS. The CERES spectrometer, has a novel design based on two Ring Imaging Cherenkov (RICH) counters, and it operates close to its design specifications. Data were recorded with 200 GeV u sulfur beam and 450 GeV proton beam. The analysis is in progress. We have extracted first e + − -pairs samples for p+Be, p+Au and S+Au collisions. In addition other physics topics were addressed. Inclusive photon spectra were measured in S+Au interactions. No excess over known hadronic sources was found within our present systematic error of 11%. Results on high p i charged pion spectra are presented up to 4 GeV c . We also studied the production of e + e − -pairs m the strong electromagnetic fields of very peripheral S+Pt collisions. The data are well described by a first-order perturbative QED-calculation.

We describe the two RICH detectors of the CERES electron pair spectrometer at the CERN SPS which are used for electron identification and, in conjunction with a novel silicon drift detector, for tracking in pp, pA and AA collisions. The RICH detectors are operated at a high γ th ⋍ 32 (CH4 at 1 atm.) and are thus rather insensitive to hadrons. The UV-detectors are multistep counters with a multiwire chamber as the last stage. They are operated at gains of 2–4 × 105 using a mixture of He + 6% C2H6 (or CH4) + TMAE. The two UV-detectors are equipped with 53 800 (48 400) square pads. The front end electronics consists of modules with 256 (121) channels, based on a 64-channel charge-sensitive preamplifier VLSI chip. The total readout time is 280 (1600) μs per event. Subsets of the pad data are used as input to a fast trigger processor selecting events with at least two separated electron rings. The trigger achieved an enrichment factor of ∼ 100 in proton-induced interactions, and a factor of ∼3 in 32SAu collisions. The RICH detectors perform very close to their design values. We observe clean Cherenkov rings with an average number of 11.2 (12.5) photons/ring. This is consistent with the calculated value N0 = 131 (75) cm−1 within the systematical error of about 10%. The spatial resolutions of 1.0 and 0.8 mrad (rms) for individual photons are dominated by chromatic aberration and in very good agreement with theoretical expectations.