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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

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

Measurements of the forward and the transverse energy in 158 GeV per nucleon Pb + Pb collisions are presented.A total transverse energy of about 1 TeV is created in central collisions. An energy density of about 3 GeV/fm is estimated for near head-on collisions. Only statistical fluctuations are seen in the ratio of electromagnetic to hadronic transverse energy. PACS numbers: 25.75.+r, 12.38.Mh Collisions of heavy nuclei at high energies serve as a unique tool for studying strongly interacting matter under conditions of extreme density and temperature. The recent acceleration of Pb ions in the CERN SPS to an energy of 158 GeV per projectile nucleon has extended this field of study to a new domain where bulk nuclear matter is excited to such high energy densities that it might undergo a phase transition into a deconfined and/or chirally restored state, as predicted by lattice QCD calculations [1]. If equilibrium is achieved, global observables such as transverse energy production can be related to thermodynamic variables, such as energy and entropy density, commonly used to characterize these collisions. The systematic comparison of such quantities for various collision systems may elucidate the dynamical mechanisms involved and identify observables and conditions under which to search for signals of a possible phase transition. One such observable is the relative production rate of electromagnetic to hadronic energy, which rejects the relative abundance of neutral to charged pions. In a chiral phase transition, transient domains of disoriented chirality may be formed and if sufficiently large would lead to large fluctuations in the ratio of charged to neutral pions [2]. In this Letter we present and discuss the first data on transverse energy production in Pb + Pb collisions, and also make comparison to previously published S + Au and S + S data [3]. We estimate that the energy density in head-on Pb + Pb collisions at this energy reaches about 3 GeV/fm, thus surpassing the critical energy density predicted by lattice QCD [1].However, no unusual physics is observed in the ratio of electromagnetic to hadronic transverse energy.The NA49 experiment is designed to perform inclusive as well as single event measurements of a variety of observables for 2osPb induced reactions at 158 GeV/nucleon 3814 0031-9007/95/75(21)/3814(4)$06. 00