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Photon and Hadron Spectrometer
WA98 - Selected Publications
   Event-by-event fluctuations in particle multiplicities and transverse energy produced in 158A GeV Pb+Pb collisions
Published in: Phys. Rev. C 65 (2002) 054912
   Abstract: Event-by-event fluctuations in the multiplicities of charged and photons, and the transverse energy in 158 A GeV Pb+Pb collisions are studied for a wide range of centralities. For narrow centrality bins the multiplicity and transverse energy distributions are found to be near perfect Gaussians. The effect of detector acceptance on the multiplicity fluctuations has been studied and demonstrated to follow statistical considerations. The centrality dependence of the charged particle multiplicity fluctuations in the measured data has been found to agree reasonably well with those obtained from a participant model. However, for photons the multiplicity fluctuations have been found to be lower compared to those obtained from a participant model. The multiplicity and transverse energy fluctuations have also been compared to those obtained from the VENUS event generator.
   [Publisher archive] [HEP arXiv] [PDF] [PostScript]
   Localized charge-neutral fluctuations in Pb+Pb Collisions at 158 A GeV/c
Published in: Phys. Rev. C 64 (2001) 011901(R)
   Abstract: Localized fluctuations in the multiplicity of charged particles and photons produced in central 158A GeV/c Pb+Pb collisions are studied. The charged versus neutral correlations in common eta-phi phase space regions of varying azimuthal size are analyzed by two different methods. The analysis provides a model-independent demonstration of nonstatistical fluctuations in both charged particle and photon multiplicities in limit azimuthal regions. However, no correlated charge-neutral fluctuations are observed, contrary to expectations for the production of a disoriented chiral condensate. The result is not explained by the widely used VENUS model.
   [Publisher archive] [HEP arXiv] [PDF] [PostScript]
   Three-Pion Interferometry Results from Central Pb+Pb Collisions at 158 A GeV/c
Published in: Phys. Rev. Lett. 85 (2000) 2895
   Abstract: Three-particle correlations have been measured for identified pions from central 158 A GeV Pb+Pb collisions by the WA98 experiment at CERN. A substantial contribution of the genuine three-body correlation has been found a s expected for a mainly chaotic and symmetric source.
   [Publisher archive] [HEP arXiv] [PDF] [PostScript]
   Central Pb+Pb Collisions at 158 A GeV/c Studied by Pion Pion Interferometry
Published in: Eur. Phys. J. C 16 (2000) 3, 445-451
   Abstract: Two-particle correlations have been measured for identified negative pions from central 158 A GeV Pb+Pb collisions and fitted radii of about 7 fm in all dimensions have been obtained. A multi-dimensional study of the radii as a function of kT is presented, including a full correction for the resolution effects of the apparatus. The cross term R(out-long)**2 of the standard fit in the Longitudinally CoMoving System (LCMS) and the vL parameter of the generalised Yano-Koonin fit are compatible with 0, suggesting that the source undergoes a boost invariant expansion. The shapes of the correlation functions in Qinv and Qspace=sqrt(Qx**2+Qy**2+Qz**2) have been analyzed in detail. They are not Gaussian but better represented by exponentials. As a consequence, fitting Gaussians to these correlation functions may produce d ifferent radii depending on the acceptance of the experimental setup used for the measurement.
   [Publisher archive] [HEP arXiv] [PDF] [PostScript]
   Centrality Dependence of Neutral Pion Production in 158 A GeV Pb + Pb Collisions
Published in: Phys.Rev.Lett. 81 (1998) 4087-4091
   Abstract: The production of neutral pions in 158AGeV Pb+Pb collisions has been studied in the WA98 experiment at the CERN SPS. Transverse momentum spectra are studied for the range 0.3 GeV/c < mT-m0 < 4.0 GeV/c. The results for cent ral collisions are compared to various models. The centrality dependence of the neutral pion spectral shape and yield is investigated. An invariance of the spectral shape and a simple scaling of the yield with the number of participating nucleons is obser ved for centralities with greater than about 30 participating nucleons which is most naturally explained by assuming an equilibrated system.
   [Publisher archive] [HEP arXiv] [PDF] [PostScript]
   Erratum: Centrality Dependence of Neutral Pion Production in 158A GeV 208Pb + 208Pb Collisions
Published in: Phys.Rev.Lett. 84 (2000) 578-579
   Abstract: Erratum to [Phys.Rev.Lett. 81 (1998) 4087-4091]
   [Publisher archive] [PDF] [PostScript]
   Freeze-Out Parameters in Central 158AGeV Pb+Pb Collisions
Published in: Phys.Rev.Lett. 83 (1999) 926-930
   Abstract: Neutral pion production in central 158AGeV Pb+Pb collisions has been studied in the WA98 experiment at the CERN SPS. The pi0 transverse mass spectrum has been analyzed in terms of a thermal model with hydrodynamic expansio n. The high accuracy and large kinematic coverage of the measurement allow to limit previously noted ambiguities in the extracted freeze-out parameters. The results are shown to be sensitive to the shape of the velocity distribution at freeze-out.
   [Publisher archive] [HEP arXiv] [PDF] [PostScript]
   Search for Disoriented Chiral Condensates in 158 AGeV Pb+Pb Collisions
Published in: Phys.Lett. B420 (1998) 169-179
   Abstract: The restoration of chiral symmetry and its subsequent breaking through a phase transition has been predicted to create regions of Disoriented Chiral Condensates (DCC). This phenomenon has been predicted to cause anomalous fluctuations in the relative production of charged and neutral pions in high-energy hadronic and nuclear collisions. The WA98 experiment has been used to measure charged and photon multiplicities in the central region of 158 AGeV Pb+Pb collisions at the C ERN SPS. In a sample of 212646 events, no clear DCC signal can be distinguished. Using a simple DCC model, we have set a 90% C.L. upper limit on the maximum DCC production allowed by the data.
   [Publisher archive] [CERN archive] [HEP arXiv] [PDF] [PostScript]
   Elliptic Emission of K+ and pi+ in 158 AGeV Pb+Pb Collisions
Published in: Phys.Lett. B469 (1999) 30-36
   Abstract: An event-by-event analysis of the azimuthal angular correlation with respect to the reaction plane has been carried out for K+ and pi+ emission near mid-rapidity in 158 AGeV Pb + Pb collisions. In semi-central collisions, K+ mesons are found to be preferentially emitted out of the reaction plane, while pi+ mesons are emitted in the reaction plane. The results suggest that the kaon emission is influenced by in-medium potential effects in addition to collective flow effects.
   [PDF] [PostScript]
   Production of Eta-Mesons in 200 AGeV/c S+S and S+Au Reactions
Published in: Phys.Lett. B361 (1995) 14-20
   Abstract: Minimum Bias production cross sections of eta mesons have been measured in 200 AGeV/c S+Au and S+S collisions at the CERN SPS by reconstructing the eta -- gamma gamma decay. The measurements have been made over the rapidity range 2.1 <= y <= 2.9 using the leadglass spectrometer of WA80. Within the statistical and systematical uncertainties the spectral shapes of pi^0 and eta mesons yields are identical when their invariant differential cross section is plotted as a function of the transverse mass. The relative normalization of the eta to pi0 transverse mass spectra is found to be 0.53 +- 0.07 for S+Au and 0.43 +- 0.15 for S+S reactions. Extrapolation to full phase space leads to an integrated cross section ratio of eta to pi 0 mesons of 0.147 +- 0.017 (stat.) +- 0.015 (syst.), and 0.120 +- 0.034 (stat.) +- 0.022 (syst.) for S+Au and S+S collisions, respectively.
   [HEP arXiv] [PostScript]
   Bose-Einstein correlations of soft pions in ultrarelativistic nucleus-nucleus collisions
Published in: Z.Phys.C69 (1995) 67-76
   Abstract: A detailed analysis of pair correlations of positive pions in the target fragmentation region is presented. Data on nuclear collisions at 200 GeV per nucleon were measured with the Plastic Ball in the WA80 experiment at th e CERN SPS. The correlation functions are compared with analytical functions and with simulations incorporatingw Bose-Einstein symmetrization, final-state interactions and detector resolution. Source radii are shown to increase with increasing target size and with centrality. For central collisions the radii are larger than the geometrical sizes of the involved nuclei.
   [Publisher archive] [PDF] [PostScript]
   Limits on the production of direct photons in 200 A GeV$^{32} + Au collisions
Published in: Phys.Rev.Lett. 76 (1996) 3506-3509
   Abstract: A search for the production of direct photons in S+Au collisions at 200AGeV has been carried out in the CERN-WA80 experiment. For central collisions the measured photon excess at each p_T, averaged over the range 0.5GeV/c <= p_T <= 2.5GeV/c, corresponded to 5.0% of the total inclusive photon yield with a statistical error of sigma_stat = 0.8% and a systematic error of sigma_syst = 5.8%. Upper limits on the invariant yield for direct photon production at the 90%C.L. are pre sented. Possible implications for the dynamics of high-energy heavy-ion collisions are discussed.
   [Publisher archive] [CERN archive] [PDF] [PostScript]
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Last modified Tuesday 21 May 2002 by H.Schlagheck [schlagheck@cern.ch]