Search Results (44 total)

We report on results of a measurement of ϕ meson production in central Pb-Au collisions at E_lab=158A  GeV. For the first time in the history of high energy heavy-ion collisions, ϕ mesons were reconstructed both in the K⁺K^- and the dilepton decay channels in the same experiment. This measurement yields rapidity densities near midrapidity, from the two decay channels, of 2.05±0.14(stat)±0.25(syst) and 2.04±0.49(stat)±0.32(syst), respectively. The shape of the measured transverse momentum spectrum is also in close agreement in both decay channels. The data rule out a possible enhancement of the ϕ yield in the leptonic over the hadronic decay channel of a factor 1.6 or larger at the 95% C.L. This rules out the discrepancy reported in the literature between measurements of the hadronic and dimuon decay channels by two different experiments.

Momentum spectra of charged pions over nearly full rapidity coverage from target to beam rapidity have been extracted from 0−5% most central Au+Au collisions in the beam energy range from 2A to 8A  GeV by the E895 experiment. Using a thermal parametrization to fit the transverse mass spectra, rapidity density distributions are extracted. The observed spectra are compared with predictions from the RQMD version 2.3 cascade model and also to a thermal model including longitudinal flow. The total 4π yields of the charged pions are used to infer an initial-state entropy produced in the collisions.

The yield for the multistrange Ξ^- hyperon has been measured in 6A  GeV Au+Au collisions via reconstruction of its decay products π^- and Λ, the latter also being reconstructed from its daughter tracks of π^- and p. The measurement is rather close to the threshold for Ξ^- production and therefore provides an important test of model predictions. The measured yield for Ξ^- and Λ are compared for several centralities. In central collisions the Ξ^- yield is found to be in excellent agreement with statistical and transport model predictions, suggesting that multistrange hadron production approaches chemical equilibrium in high baryon density nuclear matter.

We report on first measurements of low-mass electron-positron pairs in Pb-Au collisions at the CERN SPS beam energy of 40  AGeV. The observed pair yield integrated over the range of invariant masses 0.2<m≤1  GeV/c² is enhanced over the expectation from neutral meson decays by a factor of 5.9±1.5(stat)±1.2(syst   data)±1.8(syst  meson   decays), somewhat larger than previously observed at the higher energy of 158  AGeV. The results are discussed with reference to model calculations based on π⁺π^-→e⁺e^- annihilation with a modified ρ propagator. They may be linked to chiral symmetry restoration and support the notion that the in-medium modifications of the ρ are more driven by baryon density than by temperature.

The fragment yields from the multifragmentation of gold, lanthanum, and krypton nuclei obtained by the EOS Collaboration are examined in terms of Fisher’s droplet formalism modified to account for Coulomb energy. The critical exponents σ and τ and the surface energy coefficient c₀ are obtained. Estimates are made of the pressure-temperature and temperature-density coexistence curve of finite neutral nuclear matter as well as the location of the critical point.

Based on an evaluation of data on pion interferometry and on particle yields at midrapidity, we propose a universal condition for thermal freeze-out of pions in heavy-ion collisions. We show that freeze-out occurs when the mean free path of pions λ_f reaches a value of about 1 fm, which is much smaller than the spatial extent of the system at freeze-out. This critical mean free path is independent of the centrality of the collision and beam energy from the Alternating Gradient Synchrotron to the Relativistic Heavy Ion Collider.

Proton elliptic flow is studied as a function of impact parameter b, for two transverse momentum cuts in 2–6 A GeV Au+Au collisions. The elliptic flow shows an essentially linear dependence on b (for 1.5<b<8 fm) with a negative slope at 2A GeV, a positive slope at 6A GeV, and near zero slope at 4A GeV. These selective flow measurements provide better understanding of the interplay of the different factors responsible for the generation of elliptic flow at AGS energies. In addition, extensive comparisons of the measured and calculated flow values indicate that such measurements offer much more stringent constraints for discriminating between various equations of state.

A systematic analysis of multifragmentation (MF) in fully reconstructed events from 1A GeV Au, La, and Kr collisions with C has been performed. These data are used to provide a definitive test of the variable volume version of the statistical multifragmentation model (SMM). A single set of SMM parameters directly determined by the data and the semi-empirical mass formula are used after the adjustable inverse level density parameter ε₀ is determined by the fragment distributions. The results from SMM for second stage multiplicity, size of the biggest fragment, and the intermediate mass fragments are in excellent agreement with the data. Multifragmentation thresholds have been obtained for all three systems using SMM prior to secondary decay. The data indicate that both thermal excitation energy E_th^* and the isotope ratio temperature T_He-DT decrease with increase in system size at the critical point. The breakup temperature obtained from SMM also shows the same trend as seen in the data. The SMM model is used to study the nature of the MF phase transition. The caloric curve for Kr exhibits back-bending (finite latent heat) while the caloric curves for Au and La are consistent with a continuous phase transition (nearly zero latent heat) and the values of the critical exponents τ, β, and γ, both from data and SMM, are close to those for a “liquid-gas” system for Au and La. We conclude that the larger Coulomb expansion energy in Au and La reduces the latent heat required for MF and changes the nature of the phase transition. Thus the Coulomb energy plays a major role in nuclear MF.

Rapidity distributions of protons from central ¹⁹⁷Au+¹⁹⁷Au collisions measured by the E895 Collaboration in the energy range from (2–8)A GeV at the Brookhaven AGS are presented. Longitudinal flow parameters derived using a thermal model including collective longitudinal expansion are extracted from these distributions. The results show an approximately linear increase in the longitudinal flow velocity, 〈βγ〉_L, as a function of the logarithm of beam energy.

Multifragmentation MF results from 1A GeV Au on C have been compared with the Copenhagen statistical multifragmentation model (SMM). The complete charge, mass, and momentum reconstruction of the Au projectile was used to identify high momentum ejectiles leaving an excited remnant of mass A, charge Z, and excitation energy E^* which subsequently multifragments. Measurement of the magnitude and multiplicity (energy) dependence of the initial free volume and the breakup volume determines the variable volume parametrization of SMM. Very good agreement is obtained using SMM with the standard values of the SMM parameters. A large number of observables, including the fragment charge yield distributions, fragment multiplicity distributions, caloric curve, critical exponents, and the critical scaling function are explored in this comparison. The two stage structure of SMM is used to determine the effect of cooling of the primary hot fragments. Average fragment yields with Z>~3 are essentially unaffected when the excitation energy is ≤7 MeV/nucleon. SMM studies suggest that the experimental critical exponents are largely unaffected by cooling and event mixing. The nature of the phase transition in SMM is studied as a function of the remnant mass and charge using the microcanonical equation of state. For light remnants A<~100, backbending is observed indicating negative specific heat, while for A>~170 the effective latent heat approaches zero. Thus for heavier systems this transition can be identified as a continuous thermal phase transition where a large nucleus breaks up into a number of smaller nuclei with only a minimal release of constituent nucleons. Z<~2 particles are primarily emitted in the initial collision and after MF in the fragment deexcitation process.

A systematic analysis of the moments of the fragment size distribution has been carried out for the multifragmentation of 1A GeV Au, La, and Kr on carbon. The breakup of Au and La is consistent with a continuous thermal phase transition. The data indicate that the excitation energy per nucleon and isotopic temperature at the critical point decrease with increasing system size. This trend is attributed primarily to the increasing Coulomb energy with finite size effects playing a smaller role.

We report a particle source imaging analysis based on two-pion correlations in high multiplicity Au+Au collisions at beam energies between 2A and 8A GeV. We apply the imaging technique introduced by Brown and Danielewicz, which allows a model-independent extraction of source functions with useful accuracy out to relative pion separations of about 20 fm. The extracted source functions have Gaussian shapes. Values of source functions at zero separation are almost constant across the energy range under study. Imaging results are found to be consistent with conventional source parameters obtained from a multidimensional Hanburg-Brown–Twiss analysis.

The transverse momenta (p_x,p_y) of projectile fragments produced by 1.0A GeV ¹⁹⁷Au nuclei incident on Au and C targets have been measured. The medium and heavy fragments have p_x and p_y distributions, which are wider than predicted by models. For the Au target the widths of the distributions are significantly larger than those for C, particularly for the heavy fragments. The C distributions show a different gross structure, which may be due to the target-projectile size difference.

Directed flow measurements for Λ hyperons are presented and compared to those for protons produced in the same Au+Au collisions (2A, 4A, and 6A GeV; b<5-6  fm). The measurements indicate that Λ hyperons flow consistently in the same direction but with smaller magnitudes. A strong positive flow [for Λs] has been predicted in calculations which include the influence of the Λ-nucleon potential. The experimental flow ratio Λ/p is in qualitative agreement with expectations ( ∼2/3) from the quark counting rule at 2A GeV but is found to decrease with increasing beam energy.

The cluster distributions of three different systems are examined to search for signatures of a continuous phase transition. In a system known to possess such a phase transition, both sensitive and insensitive signatures are present; while in systems known not to possess such a phase transition, only insensitive signatures are present. It is shown that nuclear multifragmentation results in cluster distributions belonging to the former category, suggesting that the fragments are the result of a continuous phase transition.

It is shown that the Fisher droplet model, percolation, and nuclear multifragmentation share the common features of reducibility (stochasticity in multiplicity distributions) and thermal scaling (one-fragment production probabilities are Boltzmann factors). Barriers obtained, for cluster production on percolation lattices, from the Boltzmann factors show a power-law dependence on cluster size with an exponent of 0.42±0.02. The EOS Collaboration Au multifragmentation data yield barriers with a power-law exponent of 0.68±0.03. Values of the surface energy coefficient of a low density nuclear system are also extracted.

We have measured the sideward flow of neutral strange ( K_s⁰) mesons in 6A GeV Au+Au collisions. A prominent antiflow signal is observed for an impact parameter range ( b≲7 fm) which spans central and midcentral events. Since the K_s⁰ scattering cross section is relatively small in nuclear matter, this observation suggests that the in-medium kaon vector potential plays an important role in high density nuclear matter.

Multifragmentation in fully reconstructed events from 1A GeV Kr and La collisions with C has been studied. Results are compared with similar data for 1A GeV Au+C. The emitted charged particles and fragments are identified with emission from either a prompt first stage or a second stage in which the remnant resulting from the first stage breaks up. The nuclear charge, mass, and excitation energy distributions of the remnant are determined. The total charged multiplicity, as well as those of the first and second stages are obtained. Freeze-out temperatures and thermal excitation energy permit the determination of the caloric curve. The fragment charge distribution as well as the IMF multiplicity distribution and those of individual fragments are obtained. The various results are examined as to the extent of universal behavior when scaled for varying system size. Comparisons are made with intranuclear cascade and statistical multifragmentation model calculations.

Using the large acceptance Time Projection Chamber of experiment E895 at Brookhaven, measurements of collective sideward flow in Au+Au collisions at beam energies of 2A, 4A, 6A, and 8A GeV are presented in the form of in-plane transverse momentum 〈p_x〉 and the first Fourier coefficient of azimuthal anisotropy v₁. These measurements indicate a smooth variation of sideward flow as a function of beam energy. The data are compared with four nuclear transport models which have an orientation towards this energy range. All four exhibit some qualitative trends similar to those found in the data, although none show a consistent pattern of agreement within experimental uncertainties.

We present the first excitation function of π^- intensity interferometry at Alternating Gradient Synchrotron (AGS) energies (2–8 A GeV). The sensitivity of the multidimensional correlation functions to the geometry and dynamics of the pion-emitting system provides a stringent test of transport models of heavy ion collisions. Detailed comparisons with a realistic transport model, both with and without an explicit nuclear mean field, suggest that the beam energy evolution in the reaction dynamics is different in the model than in the data. A significantly increased π^- emission time scale, which has been suggested as a signal of the onset of the transition to quark-gluon plasma, is not observed.

The inclusive light fragment (Z<~7) yield data in Au+Au reactions, measured by the EOS Collaboration at the LBNL Bevalac, are presented as a function of multiplicity. Moving from central to peripheral collisions the measured charge distributions develop progressively according to a power law which can be fitted, within errors, by a single τ exponent independently of the bombarding energy except for the data at 250A MeV. In addition, the location of the maximum in the individual yields of different charged fragments, for a given beam energy, shifts towards lower multiplicity as the fragment charge increases from Z=3 to Z=7. This trend is common to all six measured beam energies. Moments of charge distribution are also reported. The universal features observed in the present Au + Au data are consistent with previous experimental findings in the Au + C multifragmentation reaction at 1A GeV.

The properties of the remnant resulting from the emission of prompt particles in the interaction of 1A GeV ¹⁹⁷Au+C interactions have been compared with intranuclear cascade and Boltzmann-Uehling-Uhlenback transport calculations. The number of first-stage particles and the energy spectra of first-stage protons are also compared. Both models can fit the general but not the detailed features of the data.

We have measured the proton elliptic flow excitation function for the Au+Au system spanning the beam energy range (2–8)A GeV. The excitation function shows a transition from negative to positive elliptic flow at a beam energy, E_tr∼4A GeV. Detailed comparisons with calculations from a relativistic Boltzmann equation are presented. The comparisons suggest a softening of the nuclear equation of state from a stiff form ( K∼380 MeV) at low beam energies ( E_beam≤2A GeV) to a softer form ( K∼210 MeV) at higher energies ( E_beam≥4A GeV) where the calculated baryon density ρ∼4ρ₀.

Net proton and negative hadron spectra for central Pb+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+Pb compared to S+S central collisions.