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Phase coexistence in multifragmentation?

Description: The charge (Z) distributions from intermediate energy heavy-ion reactions depend upon the multiplicity {eta} of intermediate mass fragments through a factor of the form e{sup {minus}cnZ}. Experimentally c starts from zero at low values of the transverse energy E{sub t} and reaches a saturation value at high E{sub t}. In a liquid-gas phase diagram, c = 0 for the saturated vapor, while c {gt} 0 for the unsaturated vapor. It is suggested that in the c {approx} 0 regime the source evaporates down to a sizable remnant, while for c {gt} 0 the source vaporizes completely. Percolation of a finite system portrays a behavior similar to that observed experimentally.
Date: April 1, 1995
Creator: Moretto, L.G.; Phair, L. & Ghetti, R.
Partner: UNT Libraries Government Documents Department

Nucleus-nucleus collisions at ultra-relativistic energies: Status and prospects

Description: This paper is based on three lectures presented at the Prague Seminar on Relativistic Heavy-Ion Physics in September 1994. The first lecture, following a general introduction, focuses on three different aspects of the CERN experiment WA80. The author first presents results on global event characteristics deduced primarily from measured distributions of transverse energy and of forward energy. The purpose is to introduce the main general features of nucleus-nucleus reactions at the highest energies currently available. He highlights the role of projectile-target geometry, discusses the degree of nuclear stopping, and estimates the energy densities attained in these reactions. This discussion is followed by a presentation of one of two topics that are unique to the WA80 experiment and which are not addressed by any of the other CERN collaborations that study nucleus-nucleus reactions: direct measurements of photons. The second topic unique to WA80, measurements of proton-proton correlations in the target-fragmentation region, is covered in the first part of the second lecture. The remainder of the second lecture is devoted to a selective overview of results obtained at the AGS accelerator of Brookhaven National Laboratory (BNL). The third lecture is devoted to a discussion of the two main experiments, STAR and PHENIX, planned for the Relativistic Heavy Ion Collider, RHIC, under construction at BNL.
Date: April 1, 1995
Creator: Plasil, F.
Partner: UNT Libraries Government Documents Department

Fission hindrance in hot nuclei

Description: The role of dynamics in fission has attracted much interest since the discovery of this process over fifty years ago. However, the study of the dynamical aspects of fission was for many years hampered by the lack of suitable experimental observables against which theoretical calculations could be tested. For example, it was found that the total kinetic energy release in fission can be described equally well by very different dissipation mechanisms, namely the wall formula, that is based on the collisions of the nucleons with the moving wall of the system, as well as a bulk viscosity of the nuclear matter. Although early theoretical work suggested that the fission process may be described as a diffusion process over the fission barrier, this was largely forgotten because of the success of a purely statistical model which instead of enumerating the ultimate final states of the process argues that the fission rate is determined at the {open_quote}transition state{close_quote} as the system traverses the fission saddle point. It was therefore significant when Gavron showed that the transition state model was unable to describe the number of neutrons emitted prior to scission at high excitation energy in reactions of {sup 16}O+{sup 142}Nd. Subsequent experimental work using different methods to measure the fission dissipation/viscosity has confirmed these initial observations. It was therefore very surprising when Moretto in recent publications concluded that their analysis of fission excitation functions obtained with a and {alpha} and {sup 3}He induced projectiles was perfectly in accord with the transition state model and left no room for fission viscosity. In this paper we`ll show that Moretto`s analysis is flawed by assuming first chance fission only (in direct contradiction to the experimental observation of pre-scission neutron emission in heavy-ion induced fission), and reveal why the systematics presented by Moretto looked so convincing ...
Date: July 1, 1997
Creator: Back, B.B.; Hofman, D.J. & Nanal, V.
Partner: UNT Libraries Government Documents Department

Stochastic approaches to dynamics of heavy ion collisions, the case of thermal fission

Description: In order to study the influence of fluctuations on various phenomena linked to heavy ion collisions, a Langevin equation has been derived from a microscopic model. Parameters entering this equation are completely determined from microscopic quantities characterizing nuclear matter. This equation has been applied to various phenomena at intermediate energies. This paper focuses on large amplitude motions and especially thermal fission. Fission rate is calculated and compared to experimental results.
Date: March 30, 1994
Creator: Boilley, D.; Abe, Y.; Suraud, E. & Ayik, S.
Partner: UNT Libraries Government Documents Department

Imaging Three Dimensional Two-Particle Correlations for Heavy-Ion Reaction Studies

Description: The authors report an extension of the source imaging method for analyzing three-dimensional sources from three-dimensional correlations. The technique consists of expanding the correlation data and the underlying source function in spherical harmonics and inverting the resulting system of one-dimensional integral equations. With this strategy, they can image the source function quickly, even with the extremely large data sets common in three-dimensional analyses.
Date: June 27, 2005
Creator: Brown, D; Enokizono, A; Heffner, M; Soltz, R; Danielewicz, P & Pratt, S
Partner: UNT Libraries Government Documents Department

University of Maryland annual progress report

Description: The two main areas of research of intermediate energy heavy-ion reactions and ultrarelativistic heavy-ion reactions are presented in this report. Among the intermediate energy research topics were {sup 129}Xe reactions, calibration of the plastic elements in the Maryland Forward Array, and a cluster recognition model for treating BUU-generated distributions. The ultrarelativistic energy research program included the LED system for the NMA (New Multiplicity Array) in E866 at BNL AGS, the E866 collaboration (antiprotons and NMA), and PHOBOS magnet work. {sup 139}La reactions were also studied.
Date: February 1, 1995
Creator: Mignerey, A.C.
Partner: UNT Libraries Government Documents Department

Interpreting multiplicity-gated fragment distributions from heavy-ion collisions

Description: In recent years, multifragmentation of nuclear systems has been extensively studied, and many efforts have been made to clarify the underlying physics. However, no clear consensus exists on the mechanism for multifragmentation. Is the emission of intermediate mass fragments (IMF: 3 {le} Z {le} 20) a dynamical process (brought on by the occurrence of instabilities of one form or another) or a statistical process (i.e. the decay probabilities are proportional to a suitably defined exit channel phase space)? Historically the charge (mass) distribution has played and still plays a very important role in characterizing multifragmentation. Since this subject`s inception, the near power-law shape of the charge and mass distributions was considered an indication of criticality for the hot nuclear fluid produced in light ion and heavy ion collisions. Here, the authors have studied different aspects of the charge distributions. The implications of the experimental evidence presented here are potentially far reaching. On the one hand, the thermal features observed in the n-fragment emission probabilities for the {sup 36}Ar + {sup 197}Au reaction extend consistently to the charge distributions and strengthen the hypothesis of the important role of phase space in describing multifragmentation. On the other hand, they have investigated charge correlation functions of multi-fragment decays to search for the enhanced production of nearly equal-sized fragments predicted in several theoretical works.
Date: February 1, 1997
Creator: Phair, L.; Moretto, L.G.; Rubehn, T.; Wozniak, G.J.; Beaulieu, L.; Tso, K. et al.
Partner: UNT Libraries Government Documents Department

Deuteron and anti-deuteron production in CERN experiment NA44

Description: The abundances of light nuclei probe the later stages of the evolution of a system formed in a relativistic heavy-ion collision. After the system has cooled and expanded, nucleons in close proximity and moving with small relative momenta coalesce to form nuclei. Light nuclei production enables the study of several topics, including the mechanism of composite particle production, freeze-out temperature, size of the interaction region, and entropy of the system. NA44 is the only relativistic heavy-ion experiment to have both deuteron and anti-deuteron results in both pA and AA collisions and the first CERN experiment to study the physics topics addressed by d and {bar d} production.
Date: April 1, 1995
Creator: Simon-Gillo, J.
Partner: UNT Libraries Government Documents Department

Hadronic spectra from collisions of heavy nuclei

Description: Hadronic spectra from collisions of heavy ions at ultrarelativistic energies are discussed, concentrating on recent measurements at the SPS of central Pb+Pb collisions at 158 GeV/nucleon, which are compared to collisions of lighter ions and at lower beam energies. Baryon stopping is seen to be larger for heavier systems and lower energies. Total yields of pions and kaons scale with the number of participants in central collisions at the SPS; in particular, the K/{pi} ratio is constant between central S+S and Pb+Pb at the SPS. Transverse mass spectra indicate significantly larger radial flow for the heavier systems. At midrapidity, an enhancement of <{pi}{sup {minus}}>/<{pi}{sup +}> and <K{sup {minus}}>/<K{sup +}> at low P{sub T} are best explained by final state Coulomb interaction with the residual charge of the fireball.
Date: March 1, 1997
Creator: Jacobs, P.
Partner: UNT Libraries Government Documents Department

Recent results from NA44 and a review of HBT

Description: High energy heavy ion collisions provide the opportunity to create hadronic matter at high energy density and study its properties. In order to do this, one must characterize the collisions, ascertain the size and density of the hot system in the central region of the nucleus-nucleus system, and determine the energy density achieved. Furthermore, one needs to determine whether or not the system approaches equilibrium so thermodynamic descriptions may be used. One of the experimental tools available is the study of two-particle correlations to map the space-time extent of the system when the hadrons decouple. Other observables include the flow of energy and charged particles transverse to the beam and the rapidity distribution of protons to indicate the amount of stopping and randomization of the incoming energy. The transverse mass distributions of hadrons reflect the temperature of the system at freezeout and effects of radial expansion. The production ratios of different particles are related to the extent of chemical equilibrium reached in the collision and subsequent evolution of the hadron gas. The NA44 Experiment at CERN can address all of these observables, though here the author focus mainly on correlation measurements. Kaons and pions are emitted rather late in the evolution of a heavy ion collision, at the time of {open_quotes}freezeout{close_quotes} when the hadrons cease to interact. Their correlations reflect the space-time evolution of the later part of the collision. In addition to characterizing the collision, correlations can signal a phase transition as they measure the duration of hadronization and particle emission, which should be long in both a first- or second-order phase transition. Furthermore, correlation measurements offer an important tool to help disentangle effects of expansion from the freezeout temperature reflected in the single particle spectra.
Date: April 1, 1995
Creator: Jacak, B.V.
Partner: UNT Libraries Government Documents Department

Hadron distributions - recent results from the CERN experiment NA44

Description: Proton distributions at midrapidity have been measured for 158A{circ}GeV/c Pb + Pb collisions in the focusing spectrometer experiment NA44 at CERN. A high degree of nuclear stopping is found in the truly heavy ion collisions. Systematic results of single particle transverse momentum distributions of pions, kaons, and protons, of 200A-GeV/c S+S and 158A{circ}GeV/c Pb+Pb central collisions will be addressed within the context of thermalization. By comparing these data with thermal and transport models, freeze-out parameters such as the temperature parameter T{sub fo} and mean collective flow velocity ({Beta}) are extracted. Preliminary results of the particle ratios of K{sup -}/K{sup +} and p/p are discussed in the context of cascade models of RQMD and VENUS.
Date: September 1, 1996
Creator: Xu, N.
Partner: UNT Libraries Government Documents Department

Proposal for capital equipment funds for experiment E-864, an experiment to measure rare composite objects and to carry out high sensitivity searches for novel forms of matter produced in high energy heavy ion collisions. Final report, June 1, 1993 - November 14, 1996

Description: The capital grant for the construction of E-864, the high energy heavy ion experiment at the AGS searching for strangelets and measuring the production of rare composite objects, expired on November 14, 1996. The grant, originally for three years, had been extended twice, each time for six months, to allow a prudent use of the funds. All of the equipment proposed was constructed on schedule and precisely within the budget. The equipment performed at the expected level or in a superior fashion. This equipment, together with other devices supported by funds provided by Brookhaven National Laboratory (BNL) allowed the construction of the E-864 spectrometer and its use in experiment 864. An article is under preparation for Nuclear Instruments and Methods which describes the spectrometer. The report describes work done with this equipment as well as performance of the beam counter system, the hodoscope, the straw tube system, the calorimeter, and the data acquisition system. 6 refs.
Date: March 1, 1997
Creator: Sandweiss, J. & Majka, R.
Partner: UNT Libraries Government Documents Department

Strangeness production in relativistic nuclear collisions

Description: A review of strange particle production in heavy ion collisions from AGS to SPS energies is presented. Implications of the newest developments in understanding the collision dynamics and the role of strange particle production in the search for a new phase of matter, in both experimental and theoretical sectors, are discussed.
Date: January 1, 1998
Creator: Odyniec, G.J.
Partner: UNT Libraries Government Documents Department

Scaling between K{sup +} and proton production in nucleus-nucleus collisions

Description: An empirical scaling involving the production of strange particle K{sup +} and protons has been observed at each rapidity over a large range covered by the E-802 spectrometer in reactions from minimum biased p+Be to central Si+Au at 14.6 A-GeV/c. This scaling has implications for the temporal evolution in nucleus-nucleus collisions.
Date: March 1, 1995
Creator: Chen, Ziping & Chasman, C.
Partner: UNT Libraries Government Documents Department

Heavy ion physics at the Brookhaven alternating gradient synchrotron

Description: The collision of large nuclei at relativisitic energies is the only known mechanism for creating and studying the properties (equation-of-state, EOS) of extremely dense nuclear matter. At sufficiently high matter densities, one of the most exciting possibilities is the formation of a Quark Gluon Plasma (QGP). However, it is an extremely difficult task to determine the ultimate density achieved during these collisions due to the fleeting nature of the high density state (the lifetime of these states is typically a few times 10-24 ns). We must rely on detailed comparisons between experimental measurements and complex Monte Carlo simulations of the colliding nuclei in order to extract meaningful estimates of the nuclear matter EOS. Our approach has been to study the behavior of the spectator matter (those protons and neutrons which do not directly interact during the collision) to determine the dynamics of the high density state which must necessarily influence the spectator matter as it decays. This report summarizes some of the key results of our study.
Date: January 30, 1995
Creator: Sangster, T.C.
Partner: UNT Libraries Government Documents Department

Aspects of strangeness

Description: We review various aspects of strangeness production in relativistic heavy ion collisions from AGS to CERN energies. The experimental data are briefly summarized and various possible theoretical interpretations of these data are evaluated, such as quark-gluon- plasma (QGP), hadron gas (HG) thermal models, or event generators (cascade models). Some comments on the production of strange clusters are offered.
Date: March 1, 1995
Creator: Dover, C.B.
Partner: UNT Libraries Government Documents Department

Progress in collective flow studies from the onset to Bevalac/SIS

Description: Collective flow in heavy ion collisions was first observed experimentally more than a decade ago at the Bevalac by the Plastic Ball collaboration. Although early calculations had suggested that measurement of the flow would place tight constraints on the nuclear equation of state, uncertainties in other input parameters of microscopic models, which also affect the flow, led to large ambiguities in the equation of state. This talk will discuss recent flow studies that attempt to overcome these difficulties. The EOS and FOPI experiments at the Bevalac and SIS accelerators have measured flow in the 200--2000 A-MeV bombarding energy range with better acceptance, particle identification, and systematics than was previously available. Meanwhile, programs at MSU and GANIL are studying the disappearance of flow around 50 A-MeV. Systematic comparison of these data with predictions of microscopic models is beginning to reduce the ambiguities in the extraction of physics quantities. Also, new directions in flow studies, such as the flow of produced particles and radial flow, offer the possibility of further information from flow studies. Recent accomplishments and new directions in flow studies are discussed, and areas where further study is needed are pointed out.
Date: March 1, 1995
Creator: Lisa, M.A.
Partner: UNT Libraries Government Documents Department

Multifragmentation at intermediate energy: Dynamics or statistics

Description: In this report the authors consider two contradictory claims that have been advanced recently: (1) the claim for a predominantly dynamical fragment production mechanism; and (2) the claim for a dominant statistical and thermal process. They present a new analysis in terms of Poissonian reducibility and thermal scaling, which addresses some of the criticisms of the binomial analysis.
Date: January 1, 1998
Creator: Beaulieu, L.; Phair, L.; Moretto, L.G. & Wozniak, G.J.
Partner: UNT Libraries Government Documents Department