M.M. Aggarwal, Z. Ahammed, A.L.S. Angelis, V. Antonenko, V. Arefiev, V. Astakhov, V. Avdeitchikov, T.C. Awes, P.V.K.S. Baba, S.K. Badyal, S. Bathe, B. Batiounia, C. Baumann, T. Bernier, K.B. Bhalla, V.S. Bhatia, C. Blume, D. Bucher, H. Büsching, L. Carlén, S. Chattopadhyay, M.P. Decowski, H. Delagrange, P. Donni, M.R. Dutta Majumdar, K. El Chenawi, A.K. Dubey, K. Enosawa, S. Fokin, V. Frolov, M.S. Ganti, S. Garpman, O. Gavrishchuk, F.J.M. Geurts, T.K. Ghosh, R. Glasow, B. Guskov, H. Å.Gustafsson, H. H.Gutbrod, I. Hrivnacova, M. Ippolitov, H. Kalechofsky, R. Kamermans, K. Karadjev, K. Karpio, B. W. Kolb, I. Kosarev, I. Koutcheryaev, A. Kugler, P. Kulinich, M. Kurata, A. Lebedev, H. Löhner, L. Luquin, D.P. Mahapatra, V. Manko, M. Martin, G. Mart́ınez, A. Maximov, Y. Miake, G.C. Mishra, B. Mohanty, M.-J. Mora, D. Morrison, T. Mukhanova, D. S. Mukhopadhyay, H. Naef, B. K. Nandi, S. K. Nayak, T. K. Nayak, A. Nianine, V. Nikitine, S. Nikolaev, P. Nilsson, S. Nishimura, P. Nomokonov, J. Nystrand, A. Oskarsson, I. Otterlund, S. Pavliouk, T. Peitzmann, D. Peressounko, V. Petracek, S.C. Phatak, W. Pinganaud, F. Plasil, M.L. Purschke, J. Rak, M. Rammler, R. Raniwala, S. Raniwala, N.K. Rao, F. Retiere, K. Reygers, G. Roland, L. Rosselet, I. Roufanov, C. Roy, J.M. Rubio, S.S. Sambyal, R. Santo, S. Sato, H. Schlagheck, H.-R. Schmidt, Y. Schutz, G. Shabratova, T.H. Shah, I. Sibiriak, T. Siemiarczuk, D. Silvermyr, B.C. Sinha, N. Slavine, K. Söderström, G. Sood, S.P. Sørensen, P. Stankus, G. Stefanek, P. Steinberg, E. Stenlund, M. Sumbera, T. Svensson, A. Tsvetkov, L. Tykarski, E.C.v.d. Pijll, N.v. Eijndhoven, G.J.v. Nieuwenhuizen, A. Vinogradov, Y.P. Viyogi, A. Vodopianov, S. Vörös, B. Wys louch, G.R. Young 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22 (WA98 Collaboration) 21 University of Panjab, Chandigarh 160014, India 32 Variable Energy Cyclotron Centre, Calcutta 700064, India 43 University of Geneva, CH-1211 Geneva 4,Switzerland 54 RRC “Kurchatov Institute”, RU-123182 Moscow 65 Joint Institute for Nuclear Research, RU-141980 Dubna, Russia 76 Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6372, USA 87 University of Jammu, Jammu 180001, India 98 University of Münster, D-48149 Münster, Germany 109 SUBATECH, Ecole des Mines, Nantes, France 1110 University of Rajasthan, Jaipur 302004, Rajasthan, India 1211 University of Lund, SE-221 00 Lund, Sweden 1312 MIT Cambridge, MA 02139 1413 Institute of Physics, Bhubaneswar 751005, India 1514 University of Tsukuba, Ibaraki 305, Japan 1615 Universiteit Utrecht/NIKHEF, NL-3508 TA Utrecht, The Netherlands 1716 KVI, University of Groningen, NL-9747 AA Groningen, The Netherlands 1817 Gesellschaft für Schwerionenforschung (GSI), D-64220 Darmstadt, Germany 1918 Nuclear Physics Institute, CZ-250 68 Rez, Czech Rep. 2019 Institute for Nuclear Studies, 00-681 Warsaw, Poland 2120 University of Tennessee, Knoxville, Tennessee 37966, USA Deceased (Dated: May 17, 2021)

Measurements of direct photon production in p+Pb and p+C collisions at $\sqrt{s_\mathrm{NN}} = 17.4\mathrm{GeV}$ are presented. Upper limits on the direct photon yield as a function of $p_\mathrm{T}$ are derived and compared to the results for Pb+Pb collisions at $\sqrt{s_\mathrm{NN}} = 17.3$ GeV. The production of the $\eta$ meson, which is an important input to the direct photon signal extraction, has been determined in the $\eta \rightarrow 2\gamma$ channel for p+C collisions at $\sqrt{s_\mathrm{NN}} = 17.4\mathrm{GeV}$.

Several hadronic observables have been studied in central 158A GeV Pb+Pb collisions using data measured by the WA98 experiment at CERN: single π and K production, as well as twoand 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 π 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 measured and agrees well with the Tomás̆ik-Heinz model. A large pion chemical potential close to the condensation limit of mπ seems to be excluded. The threepion Bose-Einstein interferometry shows a substantial contribution of the genuine three-pion correlation, but not quite as large as expected for a fully chaotic and symmetric source.

Neutral pion transverse momentum spectra were measured in $p+\mathrm{C}$ and $p+\mathrm{Pb}$ collisions at $\sqrt{{s}_{NN}}=17.4\text{ }\text{ }\mathrm{GeV}$ at midrapidity ($2.3\ensuremath{\lesssim}{\ensuremath{\eta}}_{\mathrm{lab}}\ensuremath{\lesssim}3.0$) over the range $0.7\ensuremath{\lesssim}{p}_{T}\ensuremath{\lesssim}3.5\text{ }\text{ }\mathrm{GeV}/c$. The spectra are compared to ${\ensuremath{\pi}}^{0}$ spectra measured in $\mathrm{Pb}+\mathrm{Pb}$ collisions at $\sqrt{{s}_{NN}}=17.3\text{ }\text{ }\mathrm{GeV}$ in the same experiment. For a wide range of $\mathrm{Pb}+\mathrm{Pb}$ centralities (${N}_{\mathrm{part}}\ensuremath{\lesssim}300$), the yield of ${\ensuremath{\pi}}^{0}$'s with ${p}_{T}\ensuremath{\gtrsim}2\text{ }\text{ }\mathrm{GeV}/c$ is larger than or consistent with the $p+\mathrm{C}$ or $p+\mathrm{Pb}$ yields scaled with the number of nucleon-nucleon collisions (${N}_{\mathrm{coll}}$), while for central $\mathrm{Pb}+\mathrm{Pb}$ collisions with ${N}_{\mathrm{part}}\ensuremath{\gtrsim}350$, the ${\ensuremath{\pi}}^{0}$ yield is suppressed.

Neutral pion transverse momentum spectra were measured in p+C and p+Pb collisions at sqrt[S{NN}]=17.4 GeV at midrapidity (2.3 less than or approximately equal eta{lab} less than or approximately equal 3.0) over the range 0.7 less than or approximately equal p{T} less than or approximately equal 3.5 GeV/c. The spectra are compared to pi{0} spectra measured in Pb+Pb collisions at sqrt[S{NN}]=17.3 GeV in the same experiment. For a wide range of Pb+Pb centralities (N{part} less than or approximately equal 300), the yield of pi{0}'s with p{T} greater than or approximately equal 2 GeV/c is larger than or consistent with the p+C or p+Pb yields scaled with the number of nucleon-nucleon collisions (N{coll}), while for central Pb+Pb collisions with N{part}greater than or approximately equal 350, the pi{0} yield is suppressed.

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.

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.

The azimuthal distributions of photons and charged particles with respect to the event plane are investigated as a function of centrality in Pb-208 + Pb-208 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.

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.

Two-particle correlations of direct photons were measured in central 208Pb+208Pb collisions at 158 AGeV. The invariant interferometric radii were extracted for 100<K_T<300 MeV/c and compared to radii extracted from charged pion correlations. The yield of soft direct photons, K_T<300 MeV/c, was extracted from the correlation strength and compared to theoretical calculations.

Results on the study of localized fluctuations in the multiplicity of charged particles and photons produced in 158A GeV/c Pb+Pb collisions are presented for varying centralities. The charged versus neutral particle multiplicity correlations in common phase space regions of varying azimuthal sizes are analyzed by two different methods. Various types of mixed events are constructed to probe fluctuations arising from different sources. The measured results are compared to those from simulations and from mixed events. The comparison indicates the presence of nonstatistical fluctuations in both the charged particle and photon multiplicities in limited azimuthal regions. However, no correlated charged-neutral fluctuations, a possible signature of formation of disoriented chiral condensates, are observed. An upper limit on the production of disoriented chiral condensates is set.

We present an original design for large area gaseous detectors based on the MICROMEGAS technology. This technology incorporates an insulating grid, sandwiched between the micro-mesh and the anode-pad plane, which provides an uniform 200 $\mu$m amplification gap. The uniformity of the amplification gap thickness has been verified under several experimental conditions. The gain performances of the detector are presented and compared to the values obtained with detectors using cylindrical micro spacers. The new design presents several technical and financial advantages.

Event-by-event fluctuations in the multiplicities of charged particles and photons, and the total transverse energy in 158A 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.

The production of nuclear bremsstrahlung photons (E-gamma > 30 MeV) has been studied in inclusive and exclusive measurements in four heavy-ion reactions at 60A MeV. The measured photon spectra, angular distributions, and multiplicities indicate that a significant fraction of the hard photons is emitted in secondary nucleon-nucleon collisions from a thermally equilibrated system. The observation of the thermal component in multifragment Ar-36 + Au-197 reactions suggests that the breakup of the thermalized source produced in this system occurs on a rather long time scale.

The production of hard photons and neutral pions in 190 MeV proton induced reactions on C, Ca, Ni, and W targets has been for the first time concurrently studied. Angular distributions and energy spectra up to the kinematical limit are discussed and the production cross-sections are presented. From the target mass dependence of the cross-sections the propagation of pions through nuclear matter is analyzed and the production mechanisms of hard photons and primordial pions are derived. It is found that the production of subthreshold particles proceeds mainly through first chance nucleon–nucleon collisions. For the most energetic particles the mass scaling evidences the effect of multiple collisions.

The design of a detector based on MICROMEGAS (MICRO MEsh GAseous Structure) technology is presented. Our detector is characterized by a large active area of 398 x 281 mm(2), a pad read-out with 20 x 22, mm(2) segmentation, and an uniform amplification gap obtained by insulating spacers (100 mum high and 200 mum in diameter). The performances of several prototypes have been evaluated under irradiation with secondary beams of 2 GeV/c momentum charged pions and electrons. We consider such a detector as the basic element for a pre-shower detector to equip the PHOton Spectrometer (PHOS) of the ALICE experiment. Its assets are modularity, small amount of material, robustness and lon cost. (C) 2001 Elsevier Science B.V. All rights reserved.