S.G. Steadman

Significance of the fragmentation region in ultrarelativistic heavy-ion collisions.

We present measurements of the pseudorapidity distribution of primary charged particles produced in Au+Au collisions at three energies, sqrt[s(NN)]=19.6, 130, and 200 GeV, for a range of collision centrali-ties. The distribution narrows for more central collisions and excess particles are produced at high pseudorapidity in peripheral collisions. For a given centrality, however, the distributions are found to scale with energy according to the limiting fragmentation hypothesis. The universal fragmentation region described by this scaling grows in pseudorapidity with increasing collision energy, extending well away from the beam rapidity and covering more than half of the pseudorapidity range over which particles are produced. This approach to a universal limiting curve appears to be a dominant feature of the pseudorapidity distribution and therefore of the total particle production in these collisions.

Phobos results on charged particle multiplicity and pseudorapidity distributions in Au+Au, Cu+Cu, d+Au, and p+p collisions at ultra-relativistic energies

Pseudorapidity distributions of charged particles emitted in Au+Au, Cu+Cu, d+Au, and p+p collisions over a wide energy range have been measured using the PHOBOS detector at the BNL Relativistic Heavy-Ion Collider (RHIC). The centrality dependence of both the charged particle distributions and the multiplicity at midrapidity were measured. Pseudorapidity distributions of charged particles emitted with |{eta}|<5.4, which account for between 95% and 99% of the total charged-particle emission associated with collision participants, are presented for different collision centralities. Both the midrapidity density dN{sub ch}/d{eta} and the total charged-particle multiplicity N{sub ch} are found to factorize into a product of independent functions of collision energy, {radical}(s{sub N{sub N}}), and centrality given in terms of the number of nucleons participating in the collision, N{sub part}. The total charged particle multiplicity, observed in these experiments and those at lower energies, assumes a linear dependence of (lns{sub N{sub N}}){sup 2} over the full range of collision energy of {radical}(s{sub N{sub N}})=2.7-200 GeV.

An excitation function of K− and K+ production in Au+Au reactions at the AGS

Abstract Mid-rapidity spectra and yields of K− and K+ have been measured for Au+Au collisions at 4, 6, 8, and 10.7 AGeV. The K− yield increases faster with beam energy than for K+ and hence the K−/K+ ratio increases with beam energy. This ratio is studied as a function of both s and s − s th which allows the direct comparison of the kaon yields with respect to the production threshold in p+p reactions. For equal s − s th the measured ratio K−/K+=0.2 at energies above threshold in contrast to the K−/K+ ratio of near unity observed at energies below threshold. The use of the K−/K+ ratio to test the predicted changes of kaon properties in dense nuclear matter is discussed.

Flow and bose-einstein correlations in Au-Au collisions at RHIC

Abstract Argonne flow and Bose-Einstein correlations have been measured in Au-Au collisions at S N N = 130 and 200 GeV using the PHOBOS detector at RHIC. The systematic dependencies of the flow signal on the transverse momentum, pseudorapidity, and centrality of the collision, as well as the beam energy are shown. In addition, results of a 3-dimensional analysis of two-pion correlations in the 200 GeV data are presented.

The characteristics of electric dipole strength built on highly-excited continuum states

Abstract High-energy γ-ray spectra from heavy-ion fusion reactions have been measured with low background and high resolution over a broad mass range. The data have been fitted with statistical-evaporation calculations in which the γ-ray strength associated with each level follows a lorentzian. The extracted resonance widths are generally much larger than the width of the giant dipole resonance built on the ground state of the compound system (gs-GDR). The centroids of these excited-state resonances are shifted from those of the gs-GDRs, but no simple A −1 3 dependence is evident. The most surprising result is that the extracted sum rule strengths of these resonances track those of the gs-GDRs observed in photoabsorption.

Heavy-ion Coulomb excitation of 72Ge

Abstract The nucleus 72Ge has been Coulomb excited using 16O, 58Ni and 208Pb. Seventeen E2 matrix elements coupling the low-lying states have been determined. Based on these data, an interpretation of the structure of 72Ge has been proposed that includes an “intruder” 0+ state perturbing what the present work shows to be a triaxial quadrupole-deformed collective rotational structure.

Particle production at very low transverse momenta in Au + Au collisions at s(NN)**(1/2) = 200-GeV

We present results on charged particle production at very low transverse momenta in the 15% most central Au+Au collisions at {radical}(s{sub NN})=200 GeV obtained with the PHOBOS detector at the Relativistic Heavy Ion Collider. The invariant yields were measured at midrapidity in the transverse momentum ranges from 30 to 50 MeV/c for charged pions, 90 to 130 MeV/c for charged kaons and 140 to 210 MeV/c for protons and antiprotons. No significant enhancement in low transverse momentum particle production is observed as compared to extrapolations of identified particle spectra measured at an intermediate p{sub T} range. The spectra tend to flatten at low p{sub T}, consistent with the expectations of transverse expansion of the system.

Information-theoretic analysis of energy disposal in heavy-ion transfer reactions

Heavy-ion transfer reactions to highly excited states are examined in terms of surprisal analysis--a constrained statistical approach motivated by information-theoretic considerations. The practical use of the procedure is discussed and illustrated by application to the available data from a variety of reactions. The experimentally measured energy spectra for multinucleon transfer are found to be well described by a distribution of maximal entropy subject to constraints. These constraints are shown to imply that the distribution of single nucleon occupation numbers in the heavy residual nucleus is fully relaxed but that the two-particle (and higher) correlation functions are not. For few-nucleon transfers the energy spectra contain a second, smaller, component which is more strongly damped. The dynamical origin of the constraints is discussed in terms of sum rules derived from models for the transfer process. A simple model for grazing collisions which includes the effects of tangential friction (with the same damping constant for all exit channels) provides a qualitative and a quantitative account of the variation of the optimal Q value with the number of transferred nucleons.

Antiproton production in 14.6 AGeVc Si+A collisions

Abstract Antiproton production cross sections have been measured for minimum bias and central Si+Al and Si+Au collisions at 14.6 A GeV c . The data presented cover the range of transverse momentum from 0.3 to 1.2 GeV c and lab rapidities from 1.1 to 1.7 units. The relative p π − and p K − yields are found to be the smallest for the heaviest system measured, central Si+Au collisions. For these collisions, the p π − ratio, determined from integrated yields for 1.1⩽y⩽1.7, is (0.84±0.07)×10−3. In the same rapidity interval, the average antiproton inverse m⊥ slope is 141±14 MeV for central Si+Al and central Si+Au collisions.

Global observations from PHOBOS

Particle production in Au+Au collisions has been measured in the PHOBOS experiment at RHIC for a range of collision energies. Three empirical observations have emerged from this dataset which require theoretical examination. First, there is clear evidence of limiting fragmentation. Namely, particle production in central Au+Au collisions, when expressed as