S.J. Bailey, J.G. Cramer, D.J. Prindle, T.A. Trainor and D. Weerasundara
Analysis of NA49 calorimetry data indicates that of the initial 33 TeV beam energy 1 TeV is converted into transverse energy in a central lead-lead collision, carried mainly by about 2500 hadrons. This transverse energy corresponds to about 60% of the total amount possible within kinematic constraints. Thus, although nuclear stopping is nearly complete at SPS energies, a significant fraction of the incident energy remains in directed longitudinal motion, either from incomplete stopping or from subsequent longitudinal expansion of the hot collision system.
The observed differential transverse energy of 450 GeV per unit pseudorapidity corresponds, by an estimating procedure due to Bjorken, to an energy density of 3 GeV/fm3 in the initial reaction volume. Although there is some model uncertainty to this estimate this number compares favorably with lattice gauge estimates of a Quark-Gluon Plasma (QGP) phase transition at 1 GeV/fm3 in terms of creating conditions for color deconfinement at the SPS.
Looking at the systematics of nuclear stopping in heavy ion collisions over a broad energy range, including the new lead-beam results, one observes that the participant particle rapidity shift is proportional to the total rapidity range available. This implies exponentially increasing stopping over the presently accessible energy range. Whether this trend continues to higher energies or whether a sufficient number of interactions in the nuclear medium finally strips partons of their ability to interact (leaving 'bare' quarks for example) is a very interesting question related to the possibility of a baryon-free region at mid rapidity at RHIC and LHC energies, which would facilitate more direct comparison with lattice gauge calculations.
Looking at the systematics of particle production in nucleus-nucleus collisions including the lead-beam data one observes that the number of produced hadrons per participant baryon does not increase significantly with stopped energy. This implies that the energy per produced hadron must increase. What is observed in the lead data are 'temperatures' or slope parameters for net baryon transverse mass spectra exceeding 250 MeV, and far exceeding the 150 MeV appropriate for a typical thermal or Hagedorn model. This suggests that a substantial part of the transverse energy in heavy nucleus-nucleus collisions is not thermal.
What is increasingly evident from the lead data analysis is that there is a surprisingly large transverse flow component to the momentum distribution of produced particles in heavy nucleus collisions. The large and mass-dependent apparent 'temperatures' for the hadron transverse-mass spectra are better interpreted as the consequence of radial flow. This can be interpreted as evidence that an initially thermalized hot prehadronic phase expands isentropically, cooling to 150 MeV and transferring some of the previously thermalized energy to directed radial flow with a beta of about 0.5. This combination leads to apparent temperatures for net baryons in the neighborhood of 250 MeV as observed.
In addition to radial flow there is now preliminary evidence from analysis of NA49 calorimeter data for directed transverse flow associated with peripheral collisions and correlated with the reaction plane.
In summary, preliminary analysis of NA49 lead data indicate formation of a substantial reaction volume with energy densities at or above that needed for substantial color deconfinement according to lattice gauge calculations. The analysis also indicates a surprising degree of nuclear stopping, and the persistence of a substantial radial flow field with beta as high as 0.5. The presence of flow in turn suggests that the hadronization process preserves considerable memory of earlier stages of the collision process, which may facilitate the detection and study of color deconfinement.