B. Shiva Kumar

Strange quark matter

Is strongly interacting matter in its true ground state made of nucleons bound within a nucleus or quarks bound within a nuclear volume? There is reason to believe that at high density, matter made of three quark flavors (strange quark matter) u, d, and s is more stable than {sup 56}Ni, which is the most stable nucleus. It is possible that strange quark matter was created in the early Universe when it underwent the quark-hadron transition. Could such matter exist today and can it be detected? Is it being produced in the cores of neutron stars? Can it be created in the laboratory? High energy heavy ion collisions offer a unique environment for the production of small strange quark matter droplets (strangelets). The author discusses the status of several experiments looking for strangelets, and the prospects for finding strange quark matter in our Universe.

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.

Electron identification in the PHENIX experiment using a tracking TRD

Abstract Measuring dielectron physics signals at RHIC will require the ability to separate electrons from pions to a level of better than 2 × 10−4 over momenta ranging from 0.1 GeV c to 5.0 GeV c . Particle identification over this momentum range is realized in the PHENIX detector at RHIC by a combination of detector subsystems, one of which is a tracking TRD. The PHENIX tracking TRD allows electron-pion separation utilizing both TR and dE/dx information, while simultaneously tracking charged particles with good position accuracy. The electron-pion separation of this device is better than 3 × 10−3 for particles with momenta ranging from 250 MeV c to over 10 GeV c .

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.

Recent results from experiment 814 at Brookhaven

Abstract Recent results from the E814 collaboration are presented for reactions of 14.6 GeV/nucleon 28 Si projectiles with targets of Al, Cu, and Pb. This includes transverse energy distributions over the full solid angle and the distribution of charged particle multiplicity in the forward hemisphere. Furthermore we present recent results on transverse momentum spectra and rapidity distributions for protons and discuss them in terms of stopping and/or transparency. A fraction of nucleons emerges at beam rapidity, even for the most central collisions. These ‘punch-through’ distributions are shown to yield information on the in-medium nucleon-nucleon cross section. Finally, we discuss antiproton production at 0° as a function of event centrality to shed some light on possible reabsorption.

Antiproton production in 28Si-Nucleus interactions

Abstract We have used the E814 apparatus to make a systematic study of antiproton production in collisions of 28 Si ions at 14.6 GeV/ c per nucleon with targets of Pb, Cu, and Al. We have measured the antiproton yield per interaction as a function of transverse energy and have found the yield to increase by a factor of ≈ 2 in going from the least to the most central collisions on the Pb target. A simple first-collision picture of antiproton production predicts the yield to increase by a factor of ≈ 11. We suggest that the discrepancy may result from the annihilation of antiprotons within the nuclear medium.

Nuclear and quark matter

Have we created unusual forms of nuclear and quark matter using heavy ion collisions? I will review the present status of the experimental program, and highlight some of the physics we have learned thus far.

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.

Momentum distributions of light mass fragments in Si-nucleus collisions at 14.6 GeV/nucleon

Abstract Transverse momentum distributions of light mass beam rapidity fragments 1 H, 3 He, and 4 He in reactions of 28 Si + Al , Cu and Pb are presented. The widths of the distributions are found to increase with increasing projectile-target overlap. This dependence is not consistent with the observed distributions being associated with the Fermi momentum of the nucleons in the projectile nucleus.