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Several hadronic observables have been studied in central 158 A GeV Pb+Pb collisions using data measured by the WA98 experiment at CERN: single negative pion and kaon production, as well as two- and three-pion interferometry. The Wiedemann-Heinz hydrodynamical model has been fitted to the pion spectrum, giving an estimate of the temperature and transverse flow velocity. Bose-Einstein correlations between two identified negative pions have been analysed as a function of kT, using two different parameterizations. The results indicate that the source does not have a strictly boost invariant expansion or spend time in a long-lived intermediate phase. A comparison between data and a hydrodynamical based simulation shows very good agreement for the radii parameters as a function of kT. The pion phase-space density at freeze-out has been meas...
Neutral pion transverse momentum spectra were measured in p+C and p+Pb collisions at sqrt{s_NN} = 17.4 GeV at mid-rapidity (2.3
The effect of the final state Coulomb interaction on particles produced in Pb+Pb collisions at 158 A GeV/c has been investigated in the WA98 experiment through the study of the pi-/pi+ and K-/K+ ratios measured as a function of transverse mass. While the ratio for kaons shows no significant transverse mass dependence, the pi-/pi+ ratio is enhanced at small transverse mass values with an enhancement that increases with centrality. A silicon pad detector located near the target is used to estimate the contribution of hyperon decays to the pi-/pi+ ratio. The comparison of results with predictions of the RQMD model in which the Coulomb interaction has been incorporated allows to place constraints on the time of the pion freeze-out.
Two-particle correlations of direct photons were measured in central 208Pb+208Pb collisions at 158A GeV. The invariant interferometric radii were extracted for 100
Directed and elliptic flow of inclusive photons near mid-rapidity in $158 $A GeV Pb+Pb collisions has been studied. The data have been obtained with the photon spectrometer LEDA of the WA98 experiment at the CERN SPS. The flow strength has been measured for various centralities as a function of $p_T$ and rapidity over $0.18 < p_T < 1.5 \mathrm{GeV}/c$ and $2.3 < y < 2.9$. The angular anisotropy has been studied relative to an event plane obtained in the target fragmentation region that shows the elliptic flow to be in-plane. The elliptic flow has also been studied using two-particle correlations and shown to give similar results. A small directed flow component is observed. Both the directed and elliptic flow strengths increase with $p_T$. The photon flow results are used to estimate the corresponding neutral pion flow.
The azimuthal distributions of photons and charged particles with respect to the event plane are investigated as a function of centrality in Pb + Pb collisions at 158 A GeV/c in the WA98 experiment at the CERN SPS. The anisotropy of the azimuthal distributions is characterized using a Fourier analysis. For both the photon and charged particle distributions the first two Fourier coefficients are observed to decrease with increasing centrality. The observed anisotropies of the photon distributions compare well with the expectations from the charged particle measurements for all centralities.
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