Central Pb+Pb Collisions at 158 A GeV/c Studied by Pion-Pion Interferometry

Two-particle correlations have been measured for identified negative pions from central 158 AGeV 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 Rout-long 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 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 different radii depending on the acceptance of the experimental setup used for the measurement.Comment: 13 pages including 10 figure

Search for Disoriented Chiral Condensates in 158 AGeV Pb+Pb Collisions

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 CERN 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.Comment: 20 Pages, LaTeX, uses elsart.cls, 8 eps figures included, submitted to Physics Letters

Production of Direct Photons in Lead-Lead Collisions

This thesis discusses one of the probes of a Quark Gluon Plasma (QGP): direct photon emission. The QGP is a state of matter that is hypothesized to exist at high baryon densities and high temperature. These circumstances are only available for experiments in heavy-ion collisions, and even there the presence of the QGP cannot be measured directly. Several indications of a QGP have already been detected in experiments at the SPS collider at CERN, but the evidence is still inconclusive whether the QGP has been seen. The direct photon signal consists of the photons emitted in the early phases of a collision, partly in thermal processes. The spectrum of these photons is highly dependent on the thermal evolution of the medium, and a phase transition from the QGP to hadronic matter will have a detectable effect on this thermal spectrum. Observation of the direct signal is complicated by the presence of a high number of other photon sources during the collision, mainly the decay of neutral mesons, in the later phases of the collision. One way that this background can be estimated is by an invariant-mass analysis, in which the invariant mass is calculated of all pairs of detected photons. in this thesis, an alternative method is proposed to eliminate the decay photons from the detected photon signal. The method depends on the measurement of the photon spectrum for several centrality classes. By subtracting a scaled peripheral photon spectrum from the central photon spectrum, the decay photon spectrum can be eliminated, and the remaining signal consists of direct photons only. Because this analysis uses the ratio of measured spectra at different centralities, it is less sensitive to a number of systematic effects, compared to the invariant mass analysis. Our inclusive photon analysis has been performed on the photon data of Pb+Pb collisions in the WA98 experiment at a beam energy of 158 GeV per nucleon.. Using our method, it was possible to produce a direct photon spectrum for transverse photon momenta between 0.5 GeV/c and 2.0 GeV/c. For the lower part of this interval, this is the first time that a direct photon signal has been extracted. At higher momenta, the results show a good correspondence with earlier results of the WA98 invariant mass analysis. The results are compared with the outcome of a simple hydrodynamical model first proposed by Bjorken. This shows that the direct photon signal that we found is compatible with an initial temperature of about 300 MeV, and a transition temperature of 180 MeV. With these parameters, the model shows that most of the thermal photons originate in the QGP/hadron gas mix during the phase transition, or in the following hadron gas phase

Production of Direct Photons in Lead-Lead Collisions

This thesis discusses one of the probes of a Quark Gluon Plasma (QGP): direct photon emission. The QGP is a state of matter that is hypothesized to exist at high baryon densities and high temperature. These circumstances are only available for experiments in heavy-ion collisions, and even there the presence of the QGP cannot be measured directly. Several indications of a QGP have already been detected in experiments at the SPS collider at CERN, but the evidence is still inconclusive whether the QGP has been seen. The direct photon signal consists of the photons emitted in the early phases of a collision, partly in thermal processes. The spectrum of these photons is highly dependent on the thermal evolution of the medium, and a phase transition from the QGP to hadronic matter will have a detectable effect on this thermal spectrum. Observation of the direct signal is complicated by the presence of a high number of other photon sources during the collision, mainly the decay of neutral mesons, in the later phases of the collision. One way that this background can be estimated is by an invariant-mass analysis, in which the invariant mass is calculated of all pairs of detected photons. in this thesis, an alternative method is proposed to eliminate the decay photons from the detected photon signal. The method depends on the measurement of the photon spectrum for several centrality classes. By subtracting a scaled peripheral photon spectrum from the central photon spectrum, the decay photon spectrum can be eliminated, and the remaining signal consists of direct photons only. Because this analysis uses the ratio of measured spectra at different centralities, it is less sensitive to a number of systematic effects, compared to the invariant mass analysis. Our inclusive photon analysis has been performed on the photon data of Pb+Pb collisions in the WA98 experiment at a beam energy of 158 GeV per nucleon.. Using our method, it was possible to produce a direct photon spectrum for transverse photon momenta between 0.5 GeV/c and 2.0 GeV/c. For the lower part of this interval, this is the first time that a direct photon signal has been extracted. At higher momenta, the results show a good correspondence with earlier results of the WA98 invariant mass analysis. The results are compared with the outcome of a simple hydrodynamical model first proposed by Bjorken. This shows that the direct photon signal that we found is compatible with an initial temperature of about 300 MeV, and a transition temperature of 180 MeV. With these parameters, the model shows that most of the thermal photons originate in the QGP/hadron gas mix during the phase transition, or in the following hadron gas phase

Source radii at target rapidity from two-proton and two-deuteron correlations in central Pb + Pb collisions at 158-A-GeV

Two-proton and two-deuteron correlations have been studied in the target fragmentation region of central Pb+Pb collisions at 158 A GeV. Protons and deuterons were measured with the Plastic Ball spectrometer of the WA98 experiment at the CERN SPS. The results of one-dimensional and multi-dimensional analyses using both the Bertsch-Pratt and Yano-Koonin-Podgoretsky parameterizations of the two-particle correlation functions are presented. The proton source exhibits a volume emission, while the deuteron source, with small outward radius, appears opaque

Observation of Direct Photons in Central 158A GeV 208Pb+208Pb^{208}Pb+^{208}Pb Collisions

A measurement of direct photon production in ²⁰⁸Pb + ²⁰⁸Pb collisions at 158A GeV has been carried out in the CERN WA98 experiment. The invariant yield of direct photons in central collisions is extracted as a function of transverse momentum in the interval 0.5 < p_T < 4 GeV/c. A significant direct photon signal, compared to statistical and systematical errors, is seen at p_T > 1.5 GeV/c. The result constitutes the first observation of direct photons in ultrarelativistic heavy-ion collisions. It could be significant for diagnosis of quark-gluon-plasma formatio