Search Results (88 total)

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.

The NA44 experiment has measured single-particle inclusive spectra for charged pions, kaons, and protons as a function of transverse mass near midrapidity in 158A GeV/c Pb+Pb collisions. From the particle mass dependence of the observed m_T distributions, we are able to deduce a value of about 120 MeV for the temperature at thermal freeze-out. From the observed ratios of the rapidity densities, we find values of the chemical potentials for light and strange quarks and a chemical freeze-out temperature of approximately 140 MeV.

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.

NA44 uses a 512-channel Si pad array covering 1.5<η<3.3 to study charged hadron production in 158A GeV Pb+Pb collisions at the CERN SPS. We apply a multiresolution analysis, based on a discrete wavelet transformation, to probe the texture of particle distributions event by event, allowing a simultaneous localization of features in space and scale. Scanning a broad range of multiplicities, we search for signals of clustering and of critical behavior in the power spectra of local density fluctuations. The data are compared with detailed simulations of detector response, using heavy-ion event generators, and with a reference sample created via event mixing. An upper limit is set on the probability and magnitude of dynamical fluctuations.

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.

Two-particle interferometry of positive kaons is studied in Pb+Pb collisions at mean transverse momenta 〈p_T〉≈0.25 and 0.91 GeV/c. A three-dimensional analysis was applied to the lower p_T data, while a two-dimensional analysis was used for the higher p_T data. We find that the source-size parameters are consistent with the m_T scaling curve observed in pion-correlation measurements in the same collisions, and that the duration time of kaon emission is consistent with zero within the experimental sensitivity.

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.

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.

The invariant cross section as a function of transverse momentum for antideuterons produced in 158A GeV/c per nucleon Pb+Pb central collisions has been measured by the NA44 experiment at CERN. This measurement, together with a measurement of antiprotons, allows for the determination of the antideuteron coalescence parameter. The extracted coalescence radius is found to agree with the deuteron coalescence radius and radii determined from two particle correlations.

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.

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.

We find the analytic expressions and obtain plots of the Wigner distribution function for the bound eigenstates of the Morse potential on quantum phase space.

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.

In 1981, Agarwal proposed a Wigner quasiprobability distribution function on the group SU(2) that serves to analyze two-particle spin states on a sphere. Recent work by our group has included the definition of an apparently distinct Wigner function on generic Lie groups whose natural range has the dimension of the group and serves for all square-integrable representations; for the SO(3) case this entails a three-dimensional “meta-phase” space. Both have the fundamental properties covariance and completeness. Here we show how the former is obtained as a restriction of the latter.

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 propose a spin model which exhibits the main properties of the Kerr medium. The description of the model uses a recently generalized quasiprobability distribution (Wigner function) for the SU(2) group. This function is naturally defined on the sphere, which plays the role of phase space for the spin system. Our model leads to macroscopic quantum superposition states on the sphere.

The NA44 Collaboration has measured charged kaon and pion distributions at midrapidity in sulphur and proton collisions with nuclear targets at 200 and 450 GeV/c per nucleon, respectively. The inverse slopes of kaons, are larger than those of pions. The difference in the inverse slopes of pions, kaons, and protons, all measured in our spectrometer, increases with system size and is consistent with the buildup of collective flow for larger systems. The target dependence of both the yields and inverse slopes is stronger for the sulphur beam, suggesting the increased importance of secondary rescattering for SA reactions. The rapidity density dN/dy of both K⁺ and K^- increases more rapidly with system size than for π⁺ in a similar rapidity region. This trend continues with increasing centrality, and according to RQMD, it is caused by secondary reactions between mesons and baryons. The K^-/K⁺ ratio falls with increasing system size but more slowly than the p̅ /p ratio. The π^-/π⁺ ratio is close to unity for all systems. From pBe to SPb the K⁺/p ratio decreases while K^-/p̅ increases and sqrt[(K⁺⋅K^-)/(p⋅p̅ )] stays constant. These data suggest that as larger nuclei collide, the resulting system has a larger transverse expansion and baryon density and an increasing fraction of strange quarks.

Two-pion correlations from Pb+Pb collisions at 158 GeV/c per nucleon are measured by the NA44 experiment at CERN. Multidimensional fits characterize the emission volume, which is found to be larger than in S-induced collisions. Comparison to the RQMD model is used to relate the fit parameters to the actual emission volume.

Transverse kinetic energies of individual fragments have been measured over a broad range of emitter excitation energies for the reaction 1A GeV Au+C. For excitation energies leading to large intermediate mass fragment multiplicities, these transverse energies require large collective radial expansion of the emitting systems. However, the traditional decomposition of the transverse energy into a thermal component and a Coulomb and collective component proportional to the fragment mass cannot account for this expansion. Expansion velocities show an increase with decreasing fragment Z and thus indicate fractionation of the collective energy for the expanding system. This collective energy increases with emitter excitation up to about 50% of the energy deposited for a nuclear system with total energy ∼12A MeV. The bulk of the collective energy is carried away by ejectiles of Z<~3.

Experiment NA44 has measured proton and antiproton distributions at midrapidity in sulphur and proton collisions with nuclear targets at 200 and 450 GeV/c per nucleon respectively. The inverse slopes of transverse mass distributions increase with system size for both protons and antiprotons but are slightly lower for antiprotons. This could happen if antiprotons are annihilated in the nuclear medium. The antiproton yield increases with system size and centrality and is largest at midrapidity. The proton yield also increases with system size and centrality, but decreases from backward rapidity to midrapidity. The stopping of protons at these energies lies between the full stopping and nuclear transparency scenarios. The data are in reasonable agreement with RQMD predictions except for the antiproton yields from sulphur-nucleus collisions.

Invariant mass analyses of (p,π^±) pairs in ⁵⁸Ni+Cu collisions at 1.97A GeV have been performed and show correlations resulting from the decays of the Δ resonance, the Λ baryon, and possibly the N^*(1440) resonance. A reduction in the Δ mass is observed and the mass reduction increases with collision centrality. Events generated by the relativistic cascade model (ARC) also reveal a mass reduction. The mass reduction is related to the size of the reaction volume and the details of Δ production mechanisms in heavy ion collisions.

Transverse flow has been studied as a function of impact parameter for pions and protons from the reaction 1.15A GeV ¹⁹⁷Au+¹⁹⁷Au. We observe an “antiflow” behavior for both π⁺ and π^- in peripheral collisions.

We study the energy dependence of collective (hydrodynamic-like) nuclear matter flow in (400–1970)A MeV Ni+Au and (1000–1970)A MeV Ni+Cu reactions. The flow increases with energy, appears to reach a maximum, and then to decrease at higher energies. A way of comparing the energy dependence of flow values for different projectile-target mass combinations is introduced, which demonstrates a more-or-less common scaling behavior among flow values from different systems.

Transverse mass spectra of pions, kaons, and protons from the symmetric heavy-ion collisions 200 A GeV S+S and 158 A GeV Pb+Pb, measured in the NA44 focusing spectrometer at CERN, are presented. The mass dependence of the slope parameters provides evidence of collective transverse flow from expansion of the system in heavy-ion induced central collisions.