S. Gilbert

Measurement of pion enhancement at low transverse momentum and of the Δ resonance abundance in Si-nucleus collisions at AGS energy

Abstract We present measurements of pion transverse momentum ( p t ) spectra in central Si-nucleus collisions in the rapidity range 2.0 y p t down to and including p t = 0. The data exhibit an enhanced pion yield at low p t compared to what is expected for a purely thermal spectral shape. This enhancement is used to determine the Δ resonance abundance at freeze-out. The results are consistent with a direct measurement of the Δ resonance yield by reconstruction of proton-pion pairs and imply a temperature of the system at freeze-out close to 140 MeV.We present measurements of the pion transverse momentum (p_t) spectra in central Si-nucleus collisions in the rapidity range 2.0<y<5.0 for p_t down to and including p_t=0. The data exhibit an enhanced pion yield at low p_t compared to what is expected for a purely thermal spectral shape. This enhancement is used to determine the Delta-resonance abundance at freeze-out. The results are consistent with a direct measurement of the Delta-resonance yield by reconstruction of proton-pion pairs and imply a temperature of the system at freeze-out close to 140 MeV.

Two pion correlations in Au + Au collisions at 10.8-GeV/c per nucleon

Two-particle correlation functions for positive and negative pions have been measured in Au+Au collisions at 10.8GeV/{ital c} per nucleon. The data were analyzed using one- and three-dimensional correlation functions. From the results of the three-dimensional fit the phase space density of pions was calculated. It is consistent with local thermal equilibrium. {copyright} {ital 1997} {ital The American Physical Society}

Two-proton correlations from 14.6A GeV/c Si+Pb and 11.5A GeV/c Au+Au central collisions

Two-proton correlation functions have been measured in Si+Pb collisions at 14.6A GeV/c and Au+Au collisions at 11.5A GeV/c by the E814/E877 collaboration. Data are compared with predictions of the transport model RQMD and the source size is inferred from this comparison. Our analysis shows that, for both reactions, the characteristic size of the system at freeze-out exceeds the size of the projectile, suggesting that the fireball created in the collision has expanded. For Au+Au reactions, the observed centrality dependence of the two-proton correlation function implies that more central collisions lead to a larger source sizes.

Particle spectra and correlations from experiment 814

In this article the authors summarize recent results from the E814 forward spectrometer on proton and pion distributions that give insight into the initial baryon (and energy) density achieved in central collisions as well as the temperature of the system at freeze-out. An independent measure for the latter, complementing the slope constants of particle spectra, is the population of nucleon excited states, in particular the {Delta}(1232) resonance. Besides its influence on the pion spectra it has also been identified directly in the experiment and first results are discussed. The first results from E814 on kaon spectra at low transverse momentum p{sub t} are presented; the spectra show an unexpected very steep rise at the lowest p{sub t}. The two pion correlation function has been studied for positive and negative pions and the authors show that it is consistent with a large source size at freeze-out. Finally, they show that the present data give a consistent picture of a system in thermal and chemical equilibrium at freeze-out.

Two-particle correlations in Au+Au collisions at AGS energy

Two-particle correlation functions have been measured in Au+Au collisions at the AGS. The one-dimensional correlations are compared to the RQMD predictions. A multidimensional analysis of two-pion correlations allows to extract the average phase space density of pions at freeze-out and the pion source velocity.

Baryon distributions in ultrarelativistic nucleus-nucleus collisions

We have measured distributions in transverse momentum and rapidity of protons from interactions of 14.6 GeV/nucleon28Si projectiles with targets of Al and Pb. The transverse momentum spectra exhibit a thermal shape with a rapidity dependent temperature parameter. For very central or violent collisions the proton rapidity distributions exhibit the large rapidity shifts and (for Si+Al) a peak at midrapidity as required for full stopping.

A time-of-flight hodoscope for the E877 spectrometer

Abstract The design and performance of the main time-of-flight hodoscope used in the E877 spectrometer at the BNL-AGS are presented. The hodoscope covers an area of 0.70×2.00 m 2 . It is made of 150 scintillator counters viewed by phototubes at each end. The detectors are mounted in an arc and arranged to eliminate gaps between the counters. An average time resolution of 85ps has been measured over the entire hodoscope.

Backward yields of pions, protons, and deuterons in relativistic 28Si+Pb collisions at 14.6 A GeV/c

Abstract The production of pions, protons and deuterons is studied at a laboratory angle of 144° in 28 Si+Pb collisions at 14.6 GeV/ c per nucleon. The centrality dependence of the pion yields is studied over the full impact parameter range using a zero degree calorimeter. The results are compared with the hadronic cascade model RQMD. These calculations are generally in agreement with the experimental results. According to these calculations, the pion yield in our acceptance is completely dominated by Δ-decay at freeze-out. Our measurements thus support the importance of baryon resonance production as one of the central features of relativistic heavy ion collisions at AGS energies. Although the strength of the pion spectrum is adequately described for kinetic energies above 50 MeV, an additional very soft component is observed in the pion spectra which is not predicted by RQMD. This very soft component accounts for a significant fraction of the total pion yield in this rapidity range but remains unexplained.

Flow in Ultrarelativistic Heavy Ion Collisions at the AGS

The azimuthal distributions of produced transverse energy from the reaction Au + Au at 10.8 AGeV/c have been studied with the nearly 4π calorimetric coverage of the E877-setup at the AGS. A Fourier analysis of these distributions has been performed in different regions of pseudorapidity as a function of centrality. The extracted sideward flow is found to be maximal in semi-central collisions. This analysis has also been employed to extract the orientation of the reaction plane. Spectra of particles identified in the forward spectrometer are then analyzed with respect to the reaction plane and their triple differential cross sections are determined. The nucleons are found to be the carriers of the sideward flow identified in the calorimeters. While the pions are also preferentially emitted into the reaction plane they show a slight tendency for emission to the opposite side of where the nucleons are found.