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Net proton and negative hadron spectra for central Pb 1 Pb collisions at 158 GeV per nucleon at the CERN Super Proton Synchrotron were measured and compared to spectra from lighter systems. Net baryon distributions were derived from those of net protons. Stopping (rapidity shift with respect to the beam) and mean transverse momentum ͗ p T ͘ of net baryons increase with system size. The rapidity density of negative hadrons scales with the number of participant nucleons for nuclear collisions, whereas their ͗ p T ͘ is independent of system size. The ͗ p T ͘ dependence upon particle mass and system size is consistent with larger transverse flow velocity at midrapidity for Pb 1 Pb compared to S 1 S central collisions. Lattice QCD predicts that strongly interacting matter at an energy density greater than 1 2 GeV͞fm 3 attains a deconfined and approximately chirally restored state known as the quark-gluon plasma (for an overview, see [1]). This state of matter existed in the early Universe, and it may influence the dynamics of rotating neutron stars [2]. The collision of nuclei at ultrarelativistic energies offers the possibility in the laboratory of creating strongly interacting matter at sufficiently high energy density to form a quark-gluon plasma [3]. Hadronic spectra from these reactions reflect the dynamics of the hot and dense zone formed in the collision. The baryon density, established 0031-9007͞99͞82(12)͞2471(5)$15.00

We present first data on event-by-event fluctuations in the average transverse momentum of charged particles produced in Pb+Pb collisions at the CERN SPS. This measurement provides previously unavailable information allowing sensitive tests of microscopic and thermodynamic collision models and to search for fluctuations expected to occur in the vicinity of the predicted QCD phase transition. We find that the observed variance of the event-by-event average transverse momentum is consistent with independent particle production modified by the known two-particle correlations due to quantum statistics and final state interactions and folded with the resolution of the NA49 apparatus. For two specific models of non-statistical fluctuations in transverse momentum limits are derived in terms of fluctuation amplitude. We show that a significant part of the parameter space for a model of isospin fluctuations predicted as a consequence of chiral symmetry restoration in a non-equilibrium scenario is excluded by our measurement.

The directed and elliptic flow of protons and charged pions has been observed from the semicentral collisions of a 158 GeV͞nucleon Pb beam with a Pb target. The rapidity and transverse momentum dependence of the flow has been measured. The directed flow of the pions is opposite to that of the protons but both exhibit negative flow at low p t . The elliptic flow of both is fairly independent of rapidity but rises with p t . The study of the early stages of relativistic nuclear collisions is of crucial importance for understanding the possibility of producing new phases of nuclear matter. It is thought that angular correlations generated by collective flow in noncentral collisions retain some signature of the effective pressure achieved at maximum compression in the interaction [1,2]. Such studies have proven to be valuable at lower beam energies for the study of the equation of state of nuclear matter. To address these questions, the azimuthal anisotropy of charged particle emission from the interaction of a 158 GeV͞nucleon Pb beam with a Pb target has been studied in the two main Time Projection Chambers (TPCs) of CERN SPS experiment NA49 [3]. The large phase-space acceptance of these TPCs allows event-by-event study of the angular correlations of the particles from the interaction, essential for the study of collective flow. This is the first study of directed and elliptic flow as a function of rapidity and transverse momentum for collisions of the heaviest nuclei at the highest bombarding energy presently available.Usually three kinds of flow in the plane transverse to the beam are considered: radial transverse flow, directed 4136 0031-9007͞98͞80(19)͞4136(5)$15.00

Influence of plasma-generated negative oxygen ion impingement on magnetron sputtered amorphous SiO 2 thin films during growth at low temperatures