Thermal multifragmentation in p + Au interactions at 2.16, 3.6 and 8.1 GeV incident energies

Multiple emission of intermediate-mass fragments has been studied for the collisions p + Au at 2.16, 3.6 and 8.1 GeV with the FASA setup. The mean IMF multiplicities for events with at least one IMF are equal to 1.7, 1.9 and 2.1 (±0.2) respectively. The multiplicity, charge distributions and kinetic energy spectra of IMF are described in the framework of a intranuclear cascade model followed by the statistical multifragmentation model. However, between the two parts of the calculation the excitation energies and the residual masses and charges are modified to take into account the losses during expansion. The results support a scenario of true thermal multifragmentation of a hot and expanded target spectator. Received: 15 December 1997 / Revised version: 24 April 1998     

Thermal multifragmentation of hot nuclei and liquid-fog phase transition

Multiple emission of intermediate-mass fragments (IMF) in the collisions of protons (up to 8.1 GeV), ⁴He (4 and 14.6 GeV), and ¹²C (22.4 GeV) on Au has been studied with the 4π setup FASA. In all the cases, thermal multifragmentation of the hot and diluted target spectator takes place. The fragment multiplicity and charge distributions are well described by the combined model including the modified intranuclear cascade followed by the statistical multibody decay of the hot system. IMF-IMF-correlation study supports this picture, giving a very short time scale of the process (≤70 fm/c). This decay process can be interpreted as the first-order nuclear “liquid-fog” phase transition inside the spinodal region. The evolution of the mechanism of thermal multifragmentation with increasing projectile mass was investigated. The onset of the radial collective flow was observed for heavier projectiles. The analysis reveals information on the fragment space distribution inside the breakup volume: heavier IMFs are formed predominantly in the interior of the fragmenting nucleus possibly due to the density gradient.     

Characteristics of charged particle production in relativistic heavy-ion collisions

Inclusive spectra of charged particles at midrapidity in Au+Au collisions at $\sqrt {s_{NN} } = 130$ GeV and 200 GeV were measured with the STAR detector at RHIC. The measured mean transverse momentum 〈p _T 〉 shows a characteristic dependence on charged particle multiplicity and beam energy in Au+Au collisions that is distinctly different from pp, $p\bar p$ and e⁺e^? collisions. A 32%±3%(syst) increase in 〈p _T 〉 from pp to Au+Au collisions was observed at 200 GeV. While the charged multiplicity was found to increase by 19%±5%(syst) from $\sqrt {s_{NN} } = 130$ GeV to 200 GeV, no significant difference in 〈p _T 〉 was found between the two energies. A comparison with model predictions is discussed.     

Collective effects in the interactions of small systems on the RHIC

It has long been believed that small colliding systems (p+Au, d+Au, ³He + Au) are can only be used to study the collective effects of cold nuclear matter. However, recent studies on the RHIC and LHC accelerators indicate there are flowlike collective effects characterized by the high multiplicity of charged particles produced in these collisions. Whether these effects result from the hydrodynamic expansion of a dense and hot thermalized medium or are caused by the initial state remains an open question. This work reports the results from measuring flow characteristics in d + Au and ³He + Au collisions at an energy of 200 GeV in the PHENIX experiment on the RHIC collider. Attempts to describe the results theoretically are discussed.     

Centrality dependence of freeze-out parameters from Au+Au collisions at \sqrt {s_{NN} } = 7.7, 11.5 and 39 GeV

The RHIC beam energy scan program in its first phase collected data for Au+Au collisions at beam energies of 7.7, 11.5 and 39 GeV. The event statistics collected at these lower energies allow us to study the centrality dependence of various observables in detail, and compare to fixed-target experiments at SPS for similar beam energies. The chemical and kinetic freeze-out parameters can be extracted from the experimentally measured yields of identified hadrons within the framework of thermodynamical models. These then provide information about the system at the stages of the expansion where inelastic and elastic collisions of the constituents cease. We present the centrality dependence of freeze-out parameters for Au+Au collisions at midrapidity for $\sqrt {s_{NN} } $ = 7.7, 11.5, and 39 GeV from the STAR experiment. The chemical freeze-out conditions are obtained by comparing the measured particle ratios (involving ??, K, p, and p) to those from the statistical thermal model calculations. The kinetic freeze-out conditions are extracted at these energies by simultaneously fitting the invariant yields of identified hadrons (??, K, and p) using Blast Wave model calculations.     

Measurement of Neutral pions in √NN=200 GeV and 62.4 GeV Au+Au collisions at RHIC-PHENIX

Neutral pions (π⁰) in √_NN=200 GeV and 62.4 GeV Au+Au collisions were measured at RHIC-PHENIX Year-4 Run. In √_NN=200 GeV, π⁰ spectra are measured up to p _T=20 GeV/c. A strong suppression by a factor of ～5 is observed and stays almost constant up to 20 GeV/c.     

Initial and Final State Temperatures of Antiproton Emission Sources in High Energy Collisions

The momentum or transverse momentum spectra of antiprotons produced at mid-rapidity in proton-helium (p+He), gold-gold (Au+Au), deuton-gold (d+Au), and lead-lead (Pb+Pb) collisions over an energy range from a few GeV to a few TeV are analyzed by the Erlang distribution, the inverse power-law (the Hagedorn function), and the blast-wave fit, or the superposition of two-component step function. The excitation functions of parameters such as the mean transverse momentum, initial state temperature, kinetic freeze-out temperature, and transverse flow velocity increase (slightly) from a few GeV to a few TeV and from peripheral to central collisions. At high energy and in central collisions, large collision energy is deposited in the system, which results in high degrees of excitation and expansion.

     

Centrality dependence of rapidity spectra of negative pions in 12 C+12 C and 12 C+181 Ta collisions at 4.2 GeV/c per nucleon

The centrality dependences of the experimental rapidity as well as transversemomentumversus rapidity spectra of negative pions were analyzed quantitatively in ¹²C+¹²C and ¹²C+¹⁸¹Ta collisions at 4.2 GeV/c per nucleon using fitting the pion spectra by Gaussian distribution function. The experimental results were compared systematically with the predictions of the Quark-Gluon-String Model (QGSM) adapted to intermediate energies.     

Source velocity at relativistic beams of <Superscript>4</Superscript>He

The source velocities (β = ν/c) extracted from rapidity plots of the fragment invariant probability distribution in terms of the longitudinal versus transversal velocity components has been studied for ⁴He + Au collisions at 4 and 14.6 GeV. It was found transition from broad range source velocities distribution in case of ⁴He(4 GeV) + Au to fixed source velocity in case of ⁴He(14.6 GeV) + Au.     

High p T identified particles in PHENIX: Data vs. theory

Two of the most interesting experimental results of heavy-ion reactions at RHIC collider energies are in the hard scattering sector where central Au+Au data show a very different behaviour compared to p+p and peripheral Au+Au collisions. The so-called “high p _T π ⁰ suppression” and the “anomalous” baryon/meson ratio observed by PHENIX in central Au+Au collisions at $\sqrt {s_{NN} } = 200$ GeV are reviewed and compared to various theoretical calculations based on different strongly interacting medium scenarios.     

Production of charged pions, kaons and antikaons in relativistic C + C and C + Au collisions

Production cross-sections of charged pions, kaons and antikaons have been measured in C+C and C+Au collisions at beam energies of 1.0 and 1.8 AGeV for different polar emission angles. The kaon and antikaon energy spectra can be described by Boltzmann distributions whereas the pion spectra exhibit an additional enhancement at low energies. The pion multiplicity per participating nucleon M(π⁺)/<A _part> is a factor of about 3 smaller in C+Au than in C+C collisions at 1.0 AGeV whereas it differs only little for the C and the Au target at a beam energy of 1.8 AGeV. The K⁺ multiplicities per participating nucleon M(K⁺)/ <A _part> are independent of the target size at 1 AGeV and at 1.8 AGeV. The K^- multiplicity per participating nucleon M(K^-)/ <A _part> is reduced by a factor of about 2 in C+Au as compared to C+C collisions at 1.8 AGeV. This effect might be caused by the absorption of antikaons in the heavy target nucleus. Transport model calculations underestimate the K^-/K⁺ ratio for C+C collisions at 1.8 AGeV by a factor of about 4 if in-medium modifications of K-mesons are neglected. Received: 10 December 1999 / Accepted: 14 November 2000     

Thermal hard-photons probing multifragmentation in nuclear collisions around the fermi energy

Hard-photon (E _γ>30 MeV) emission originating from photon-neutron bremsstrahlung collisions is investigated in four different heavy-ion reactions at intermediate bombarding energies (³⁶Ar+¹⁹⁷Au, ¹⁰⁷Ag, ⁵⁸Ni, ¹²C at 60 A MeV) coupling the TAPS photon spectrometer with two charged-particle multidetectors covering more than 80% of the solid angle. The hard-photon spectra of the three heavier targets result from the combination of two distinct exponential distributions with different slope parameters, a results which deviates from the behaviour expected for hard-photon production just in first-chance proton-neutron collisions. The thermal origin of the steeper bremsstrahlung component is confirmed by the characteristics of its slope and angular distribution. Such thermal hard-photons convey undisturbed information of the thermodynamical state of hot and excited nuclear systems undergoing multifragmentation.     

Energy and system size dependence of charged hadron transverse momentum spectra from Cu+Cu and Au+Au collisions at √S NN =62.4 and 200 GeV

The PHOBOS Collaboration has measured transverse momentum distributions of charged hadrons produced in Cu+Cu collisions at √s _NN =200 and 62.4 GeV. The nuclear modification factor R _AA ^N _part is calculated relative to p+p data at both collision energies as a function of collision centrality. For the same number of participating nucleons, R _AA ^N _part is essentially the same in both systems over the full range of p Tthat is measured. In addition, we observe that within experimental uncertainties, the ratio of 200 GeV to 62.4 GeV Cu+Cu yields has only a moderate centrality dependence and is consistent with the value previously measured in Au+Au collisions for a broad range of p _T.     

Open charm measurements at STAR

We report on the measurements of D⁰ meson production via direct reconstruction through the hadronic decay channel D⁰→Kπ in minimum bias d+Au and Au+Au collisions at with p_T up to ∼3 GeV/c. We derive the charm production total cross-section per nucleon–nucleon collision from a combination of three measurements: the D⁰ meson spectra, the non-photonic electron spectra from charm semi-leptonic decays obtained in p+p, d+Au, and Au+Au collisions, and the charm-decayed single muon (prompt muon) spectra at low p_T in Au+Au collisions. The cross-section is found to follow binary scaling, which is a signature of charm production exclusively at the initial impact. The implications of charm quark energy-loss and thermalization in the strongly interacting matter are discussed. PACS 25.75.Dw; 13.20.Fc; 13.25.Ft; 24.85.+p     

4He fragmentation induced by 46–400 GeV Protons

Inclusivep, d and³He cross sections have been obtained using experimental data on p⁴He collisions over an incident proton energy range of 46-400 GeV. Considering the slopes of spectra, the effective nuclear temperature has been found to be T₀=11 MeV, which is in good agreement with experimental results for lower energies and heavier targets. The shape of the³He inclusive spectrum suggests the existence of two different³He production mechanisms. The experimental³He cross section is compared with theoretical predictions assuming fragment formation as a result of the spectator and cluster knockout mechanisms. The⁴He→³He +n vertex constant has been found to be G²=10.9±0.2 fm.     

Correlation effects in the production of <Superscript>3</Superscript>He nuclei in <Superscript>16</Superscript>O<Emphasis Type="Italic">p</Emphasis> collisions at a momentum of 3.25 GeV/<Emphasis Type="Italic">c</Emphasis> per nucleon

New experimental data on the dependence of the mean multiplicities and kinematical features of ³He nuclei produced in ¹⁶Op collisions at a momentum of 3.25 GeV/c per nucleon on the degree of excitation of the fragmenting nucleus are presented. Strong correlations between the multiplicities of negatively charged pions and ³He nuclei are found.     

Critical temperature for the nuclear liquid-gas phase transition (from multifragmentation and fission)

Critical temperature T _c for the nuclear liquid-gas phase transition is estimated from both the multifragmentation and fission data. In the first case, the critical temperature is obtained by analysis of the intermediate-mass-fragment yields in p(8.1 GeV) + Au collisions within the statistical model of multifragmentation. In the second case, the experimental fission probability for excited ¹⁸⁸Os is compared with the calculated one with T _c as a free parameter. It is concluded for both cases that the critical temperature is higher than 15 MeV. The text was submitted by the authors in English.     

Production of π 0 and η mesons in carbon-induced relativistic heavy-ion collisions

The production of π ⁰ and η mesons has been investigated in the system ¹²C+¹²C at 0.8A GeV, 1.0A GeV, and 2.0A GeV using the TAPS photon detector. The production cross sections and transverse-momentum distributions measured around midrapidity extend the existing systematics of neutral-meson production to light systems and the highest energy available at the GSI accelerator facility. At all beam energies scaling of the differential production cross section with the transverse mass of the emitted meson is observed. In addition π⁰ production has been studied in the mass-asymmetric systems ¹²C+¹⁹⁷Au and ¹⁹⁷Au+¹²C at 0.8A GeV.     

Breakup temperature of target spectators in 197Au + 197Au collisions at E/A = 1000 MeV

Breakup temperatures were deduced from double ratios of isotope yields for target spectators produced in the reaction ¹⁹⁷Au + ¹⁹⁷Au at 1000 MeV per nucleon. Pairs of ^3,4He and ^6,7Li isotopes and pairs of ^3,4He and H isotopes (p, d and d, t) yield consistent temperatures after feeding corrections, based on the quantum statistical model, are applied. The temperatures rise with decreasing impact parameter from 4 MeV for peripheral to about 10 MeV for the most central collisions. The good agreement with the breakup temperatures measured previously for projectile spectators at an incident energy of 600 MeV per nucleon confirms the universality established for the spectator decayat relativistic bombarding energies. The measured temperatures also agree with the breakup temperatures predicted by the statistical multifragmentation model. For these calculations a relation between the initial excitation energy and mass was derived which gives good simultaneous agreement for the fragment charge correlations. The energy spectra of light charged particles, measured at τ_lab = 150°, exhibit Maxwellian shapes with inverse slope parameters much higher than the breakup temperatures. The statistical multifragmentation model, because Coulomb repulsion and sequential decay processes are included, yields light-particle spectra with inverse slope parameters higher than the breakup temperatures but considerably below the measured values. The systematic behavior of the differences suggests that they are caused by light-charged-particle emission prior to the final breakup stage.     

Spectral temperatures of Δ0(1232) resonances produced in p 12C and d 12C collisions at 4.2 GeV/c per nucleon

The reconstructed experimental transverse momentum (p _t ) distributions of Δ⁰(1232) resonances produced in p ¹²C and d ¹²C collisions at 4.2 A GeV/c and the corresponding spectra calculated using Modified FRITIOF model were analyzed in the framework of Hagedorn Thermodynamic Model. The spectral temperatures of Δ⁰(1232) resonances were extracted from fitting their p _t spectra with one-temperature Hagedorn function. The extracted spectral temperatures of Δ⁰(1232) were compared with the corresponding temperatures of π ^? mesons in p ¹²C and d ¹²C collisions at 4.2 A GeV/c obtained similarly from fitting the p _t spectra of π ^? by one-temperature Hagedorn function. The spectral temperatures of Δ⁰(1232) resonances agreed within uncertainties with the corresponding temperatures of π ^? mesons produced in p ¹²C and d ¹²C collisions at 4.2 A GeV/c.