A measurement of strange baryon and antibaryon production in Pb–Pb collisions has been carried out by the NA57 experiment at the CERN SPS, with 40 and 158 AGeV/c beam momentum. Results on Λ, Ξ and Ω ...hyperon yields at mid-rapidity in the most central 53% of Pb–Pb collisions at 40 AGeV/c are presented and compared with those obtained at higher energy, in the same collision centrality range.
The Λ and Ξ− yields per unit rapidity stay roughly constant while those of Ω−, Λ̄, Ξ̄+ and Ω̄+ increase when going to the higher SPS energy. Hyperon yields at the SPS are compared with those from the STAR experiment in sNN=130 GeV Au–Au collisions at RHIC.
We propose a simple Glauber-type mechanism for the suppression of jet production up to transverse momenta of about 10 \(\mathrm {GeV}/c\) at RHIC. For processes in this kinematic region, the ...formation time is smaller than the interval between two successive hard partonic collisions and the subsequent collision influences the jet production. The number of jets then roughly scales with the number of participants. Proportionality to the number of binary collisions is recovered for very high transverse momenta. The model predicts suppression of jet production in \(d + {\mathrm {Au}}\) collisions at RHIC.
We present a few estimates of energy densities reached in heavy-ion collisions at the CERN SPS. The estimates are based on data and models of proton-nucleus and nucleus-nucleus interactions. In all ...of these estimates the maximum energy density in central Pb+Pb interactions is larger than the critical energy density \(\varepsilon_c\approx 0.7 \)GeV/fm3 following from lattice gauge theory computations. In estimates which we consider as realistic the maximum energy density is about \(2\varepsilon_c\). In this way our analysis gives some support to claims that deconfined matter has been produced at the CERN SPS. Any definite statement requires a deeper understanding of formation times of partons and hadrons in nuclear collisions. We also compare our results with implicit energy estimates contained in earlier models of anomalous \(J/\psi\) suppression in nuclear collisions.
The data of J/psi suppression at large x(F) in pA collisions are used to infer the existence of gluon depletion as the projectile proton traverses the nucleus. The modification of the gluon ...distribution is studied by use of a convolution equation whose nonperturbative splitting function is determined phenomenologically. The depletion factor at x(1) = 0.8 is found to be about 25% at A = 100.
It is shown first that nuclear geometry is responsible for only a very small part of observed nucleon number dependence of longitudinal radius
R
L
for proton- and light-ion interactions with nuclear ...targets in the CERN-SPS energy region. This dependence can be described by a model based on multiple nucleon-nucleon sub-collisions provided that a sub-collision in proton-nucleus or nucleus-nucleus collisions differs from that of free nucleons. We present arguments, based on the principle of uncertainity, which ascribe this difference to short space- and time-intervals between successive sub-collisions.
On the other hand the PbPb data give
R
L
significantly higher than such a picture could accomodate, presenting an evidence for the onset of a new dynamical regime, probably connected with the expansion of matter, between OAu and PbPb collisions.
Hanbury-Brown and Twiss correlations of bremsstrahlung photons produced in hadronic collisions at very high energies show intermittent features. The phenomenon is due to large logitudinal dimensions ...of the space-time evolution of hadronic collisions and to the fact that the rapidity of a bremsstrahlung photon need not be close to the rapidity of the charged particle radiating the photon.
The dimension of the region emitting particles in nuclear collisions can be determined by Hanbury-Brown and Twiss (HBT) correlations of identical particles. In thiss note we demonstrate that this ...dimension as determined by the HBT correlation of bremsstrahlung photons is larger than the actual one due to the photon formation length. Qualitatively
r
2 →
r
2 +
a/
ω
2 where the last term is due to the formation length being proportional to 1/ω, where ω is the energy of the photon. This phenomenon permits to “see” experimentally the photon formation length.
The charged particle multiplicity distributions measured by two experiments, WA97 and NA57, in Pb+Pb collisions at 158 A GeV/c have been analyzed in the framework of the Wounded Nucleon Model (WNM). ...We obtain a good description of the data within the centrality range of our samples. This allows us to make use of the measured multiplicities to estimate the number of wounded nucleons of the collision.
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