JINR-LHE-1011-3 (CERES/NA45 experiment)
· Date approved: 1994 · Date started: 1995 · Date completed: 2002
Accelerator: SPS CERN
Detector: CERES Setup with 2 RICH detectors, vertex tracker based on silicon drift detectors, TPC


Electron - Pair and Photon Production in pp, p-A and AA Collisions

(1) H.Agakishiev, V.Belaga, N.S. Moroz, G.A. Ososkov, Yu.A. Panebratsev, S.Razin, N. Saveljic, S.Shimanskiy, V.Yurevich
(1) LHE, Joint Institute for Nuclear Research, Dubna

CERES collaboration:

(2) Brookhaven National Laboratory
(3) CERN, Geneva, Switzerland
(4) GSI, Darmstadt, Germany
(5) MPIfK , Heidelberg
(6) Muclear Physics Institute ASCR, Rez
(6) Politecnico di Milano, Italy
(7) Universität Heidelberg, Germany
(8) Weizmann Institute, Rehovot, Israel


Spokesperson: Johanna Stachel

Contactman from JINR: Yu.A.Panebratsev


CERES is an experiment dedicated to the measurement of electron-positron pairs and direct photons produced in hadron and nuclear collisions, at CERN SPS energies. Its main goal is to systematically study the pair continuum in the mass region from 50 MeV/c2 up to ~2 GeV/c2, and the vector mesons r/w and F. Due to the absence of final state interactions, these observables are considered as unique probes of the dynamics of ultrarelativistic heavy-ion collisions, and in particular of the hot early stages where a quark-gluon plasma is expected to be formed.

CERES has started the measurement of low-mass electron pairs in 160 GeV/nucleon Pb-Au collisions at central rapidities in a wide multiplicity range [1,2]. This work is the continuation of our systematic studies on pair production in p-Be, p-Au [3,4] and S-Au interactions [5,6,7]. The motivation for this effort derives from a new source of low-mass dileptons recently observed in S-Au collisions.

In 1997-1998 upgrade of the CERES set-up with the addition of a TPC downstream of the present double RICH spectrometer has been made [8]. The main purpose was to improve the spectrometer mass resolution down to the level of the natural width of the w thereby allowing, in addition to the continuum measurement, precision spectroscopy of the vector meson resonances r, w, F.


I. The results of completed works ( in 1995-1998).

  1. The analysis of measurements of low-mass electron pairs in S-Au collisions with the CERES/NA45 spectrometer has been completed [6]. The results have been compared with those from p-Be and p-Au collisions obtained in the same experiment. The shape and the yield of the invariant mass spectra observed in p-Be and p-Au collisions are well described by the known hadronic decay sources, and there is no need to invoke any unconventional source. In the S-Au data sample, however, a significant enhancement is found which starts to rise at pair masses above m 250 MeV/c2.

  2. The main task in 1995 was the completion of the upgrade of the CERES spectrometer for the experiments with the lead beam. The JINR contribution was the design and manufacturing in Dubna of detectors and electronic modules for a new first level trigger system [9, 10, 11].

  3. Participation in data taking and data analysis of e+e- pair production in 160 GeV Pb-Au collisions. It was shown that the pair yield integrated over the range of invariant masses 0.2 m 2.0 GeV/c2 is enhanced by a factor of 2.7 0.3 (stat.) 0.5 (syst.) over the expectation from neutral meson decays for the top third most central collisions. As observed previously in S-Au collisions, the enhancement is most pronounced in the invariant mass region 300 - 700 MeV/c2. The pair yield grows stronger than linearly with charged-particle multiplicity [1, 2].

  4. The main goal for 1997 was the upgrade of the CERES set-up with the addition of a TPC downstream of the present double RICH spectrometer. The main purpose was to improve its mass resolution down to the level of 1%. A new magnet system for the CERES/NA45 experiment was designed and manufactured at JINR.

  5. A new first level trigger system has been designed in 1997-1998. The trigger will be modified from the previous multiplicity trigger - derived from a scintillator hodoscope at the location now occupied by the TPC - to triggering with the silicon drift detectors.

  6. A new robust cherenkov ring fitting techniques for the CERES RHIC detectors has been developed [12].

  7. A new robust fitting algorithm for vertex reconstruction was developed. The suggested iterative robust procedure is about an order of magnitude faster than approaches using standard minimization packages at a comparable or slightly better accuracy [13].

  8. A new algorithm resolving overlapping hits has been developed taking into account both such factors as SiDD signal asymmetry and noise correlations. This enables us to cope with the high charged-multiplicity environment of Pb-nucleus collisions [14].


II. LHE group plans for 1999 - 2002

  1. To run with fully upgraded CERES apparatus we plan the following beam time:

    • 1999 - 30 days of production running with Pb beams at 40 GeV/nucleon.

    • 2000 - Further running with Pb beams at full energy for a high statistic w, F production measurement to achieve the goal as stated in Table.

    Beam energy 160 GeV 40 GeV
    Pair sample 23000 21500
    r - mesons 1700 1250
    w - mesons 2600 2000
    F - mesons 575 450
  2. Data analysis in 1999 - 2002.





"First results from CERES/NA45 on low mass electron pair production in Pb + Au collisions at CERN SPS", Nucl. Phys., A610 (1996) 317c-330c.


"Low-mass e+e- pair production in 158A GeV Pb-Au collisions at the CERN SPS, its dependence on multiplicity and transverse momentum", Phys. Lett., B422 (1998) 405-412.


"Systematic study of low-mass electron pair production in p-Be and p-Au collisions at 450 GeV/c", European Phys. Journal, C4 (1998) 231-247.


"Neutral meson production in p-Be and p-Au collision at 450 GeV beam energy", European Phys. Journal, C4 (1998) 249-257.


"New Results from NA45/CERES", Nucl. Phys. A590 (1995) 103-117.


"Enhanced Production of Low-Mass Electron Pairs in 200 GeV/Nucleon S-Au Collisions at the CERN Super Proton Synchrotron", Phys.Rev.Lett., v.75 (1995) 1272-1275.


"Search for direct photons from S-Au collisions at 200 GeV/u", Z. Phys. C71 (1996) 571-577.


"Study of electron pair and photon production in Pb-Au collisions at the CERN SPS", Addendum, CERN SPSLC/96-35/P280/Add 1.


"Fast detector for triggering on charged particle multiplicity for relativistic nucleus-nucleus collision ", JINR Rapid Communications No 2[82]-97, Dubna, 1997, 95-110.


Cherenkov Beam counter system of the CERES/NA45 spectrometer for investigations with 160 GeV/n lead ions", JINR Rapid Communications No 3[83]-97, Dubna, 1997, 17-34.


"Performance of the CERES Electron Spectrometer in the CERN SPS Lead Beam", Nucl. Instr. Methods Phys. Res. A371 (1996) 16-21.


"Cherenkov Ring Fitting Techniques for the CERES RICH Detectors", Nucl. Instr. Methods Phys. Res. A371 (1996) 243.


"A new robust fitting algorithm for vertex reconstruction in the CERES experiment", Nucl. Instr. Methods Phys. Res. A394 (1997) 225-231.


"Effective pulse resolution algorithms for detectors with Gaussian-like signal shape", JINR Communications E-10-97-105, Dubna, 1997


E-mail: panebrat@sunhe.jinr.ru (Yu.A. Panebratsev)

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