Hadron production and phase changes in relativistic heavy-ion collisions

We study soft hadron production in relativistic heavy-ion collisions in a wide range of reaction energy, 4.8 GeV < < 200 GeV, and make predictions about yields of particles using the statistical hadronization model. In fits to experimental data, we obtain both the statistical parameters as well as physical properties of the hadron source. We identify the properties of the fireball at the critical energy threshold, 6.26 GeV < < 7.61 GeV, marking for higher energies the hadronization of an entropy-rich phase. In terms of the chemical composition, one sees a phase which at low energy is chemically under-saturated, and which turns into a chemically over-saturated state persisting up to the maximum accessible energy. Assuming that there is no change in physical mechanisms in the energy range 15 > ≥200 GeV, we use continuity of particle yields and statistical parameters to predict the hadron production at = 62.4 GeV, and obtain total yields of hadrons at = 130 GeV. We consider, in depth, the pattern we uncover within the hadronization condition, and discuss possible mechanisms associated with the identified rapid change in system properties at . We propose that the chemically over-saturated 2 + 1 flavor hadron matter system undergoes a 1st-order phase transition.     

Transverse energy per charged particle at relativistic energies from a statistical model with expansion

Transverse-energy and charged-particle pseudorapidity densities at midrapidity and their ratio, dE_T/d|_mid/dN_ch/d|_mid, are evaluated in a statistical model with longitudinal and transverse flows for the wide range of colliders, from AGS to RHIC at = 200 GeV. Evaluations are done at freeze-out parameters obtained from independent fits to observed particle yields and p_T spectra. Decays of hadron resonances are treated thoroughly and are included in derivations of dE_T/d|_mid and dN_ch/d|_mid. The predictions of the model agree well with the experimental data. However, some (explicable) overestimation of the ratio has been observed.     

Probing meson spectral functions with double differential dilepton spectra in heavy-ion collisions

The double differential dilepton spectrum d/(d ² d _⊥ ²) at fixed transverse mass M _⊥ allows a direct access to the vector meson spectral functions. Within a fireball model the sensitivity of d/(d ² d _⊥ ²) against variations of both the in-medium properties of mesons and the dynamics of the fireball is investigated. In contrast to the integrated invariant-mass spectrum d/d ², in the spectrum d/(d ² d _⊥ ²) with fixed M _⊥ the ω signal is clearly seen as bump riding on the ρ background even in case of strong in-medium modifications.[3mm] Received: 16 November 2000 / Accepted: 16 January 2001     

Correlations of neutral and charged particles in 40Ar- 58Ni reaction at 77 MeV/u

The measurement of the two-particle correlation function for different particle species allows to obtain information about the development of the particle emission process: the space-time properties of emitting sources and the emission time sequence of different particles. The single-particle characteristics and two-particle correlation functions for neutral and charged particles registered in forward direction are used to determine that the heavy fragments (deuterons and tritons) are emitted in the first stage of the reaction (pre-equilibrium source) while the majority of neutrons and protons originates from the long-lived quasi-projectile. The emission time sequence of protons, neutrons and deuterons has been obtained from the analysis of non-identical particle correlation functions.     

Inclusive meson production in peripheral collisions of ultrarelativistic heavy ions

There exist several proposals to use Weizsäcker-Williams photons generated by ultrarelativistic heavy ions to produce exotic particles in γγ fusion reactions. To estimate the background conditions for such reactions we analyze various mechanisms of meson production in very peripheral collisions of ultrarelativistic heavy ions at RHIC and LHC energies. Besides the γγ fusion they include also electromagnetic γA interactions and strong nucleon-nucleon interactions in grazing AA collisions. All these processes are characterised by low multiplicities of produced particles. The γA an d AA events are simulated by corresponding Monte Carlo codes, RELDIS and FRITIOF. In each of these processes a certain fraction of pions is produced close to the mid-rapidity regionthat gives a background for the γγ events. The possibility of selecting the mesons produced in the γγ fusion events via different p _t cut procedures is demonstrated.     

Fluctuations and correlations in STAR

The study of correlations and fluctuations can provide evidence for the production of the quark-gluon plasma (QGP) in relativistic heavy-ion collisions. Various theories predict that the production of a QGP phase in relativistic heavy-ion collisions could produce significant event-by-event correlations and fluctuations in transverse momentum, multiplicity, etc. Some of the recent results using STAR at RHIC will be presented along with results from other experiments at RHIC. The focus is on forward-backward multiplicity correlations, balance function, charge and transverse-momentum fluctuations, and correlations.     

(<Emphasis Type="Italic">p</Emphasis>,π<Superscript>±</Superscript>) correlations in central heavy-ion collisions at 1 ÷ 2 AGeV

The proton–charged pion correlated emission is studied in the reactions Au (1.06 AGeV) + Au, Ni (1.06 and 1.93 AGeV) + Ni and Ni (1.97 AGeV) + Cu within the BUU approach. The associated invariant mass distributions are shifted to smaller energies with respect to the free Δ(1232) mass distribution due to kinematical reasons. We find that the existing and partly conflicting experimental data do not allow to draw definite conclusions on the in-medium modification of the Δ(1232). Received: 18 October 1999 / Revised version: 4 February 2000     

Transverse energy and charged-particle multiplicity at various centralities at RHIC: Statistical model estimates⋆

The transverse energy and the charged-particle multiplicity at midrapidity are evaluated in a single-freeze-out model for different centrality bins at RHIC at = 130 and 200GeV. The predictions of the model are done at the freeze-out parameters determined earlier from measured particle yields and p_T spectra. The results agree qualitatively well with the experimental data.     

Direct comparison of phase-space distributions of K- and K+ mesons in heavy-ion collisions at SIS energies -- evidence for in-medium modifications of kaons?

The ratio of K^- to K⁺-meson yields has been measured in the systems ⁹⁶Ru + ⁹⁶Ru at 1.69 A GeV, ⁹⁶Ru+⁹⁶Zr at 1.69 A GeV, and ⁵⁸Ni+⁵⁸Ni at 1.93 A GeV incident beam kinetic energy. The yield ratio is observed to vary across the measured phase space. Relativistic transport-model calculations indicate that the data are best understood if in-medium modifications of the kaons are taken into account. Received: 30 November 1998 / Accepted: 20 December 2000     

Δ<Subscript>33</Subscript> -medium mass modification and pion spectra

We study the π^± spectra obtained in 2, 4, 6 and 8A GeV Au - Au collisions within the thermal model. We find that the main features of the data can be well described after we include the pions from the decay of the Δ -resonance with medium mass modification.     

On the Absorption of antibaryons in nuclei

Antiproton (ˉp) and antilambda (ˉΛ) production has been measured for minimum bias in p+A collisions and central A₁+ A₂ collisions at the CERN-SPS by the collaborations NA35/49 and NA44. The measurements are extrapolated from rapidity distributions to absolute minimum bias cross sections. It is shown that the ˉp cross sections divided by A₁· A₂ follow an exponential trend as a function of a characteristic length obtained from a Glauber type absorption model, while the ˉΛ cross sections divided by A₁· A₂ are constant. The exponential trend also holds for ˉp production at the lower energies of the Brookhaven AGS. A discussion of the physics interpretation of the established trends in terms of an effective absorption cross section is presented. Received: 15 July 1999 / Revised version: 10 November 1999     

Hadronization of expanding QGP

We discuss how the dynamics of an exploding hot fireball of quark-gluon matter impacts the actual phase transition conditions between the deconfined and confined state of matter. We survey the chemical conditions prevailing at hadronization.     

Relativistic diffusion model

Evidence is presented that diffusion drives colliding many-particle systems at relativistic energies from the initial δ–functions in rapidity towards the equilibrium distribution. Analytical solutions of a linear Fokker-Planck equation represent rapidity spectra for participant protons in central heavy-ion collisions at SPS-energies accurately. Thermal equilibrium in the interaction region is not attained, nonequilibrium features persist and can account for the broad rapidity spectra. Received: 24 November 1998 / Revised version: 20 February 1999     

Multiparticle correlations in resonance-matter formation

The effect of multiparticle correlations on resonance and pion populations, in relativistic nuclear reactions, is calculated in the context of an intranuclear cascade model which includes N-body (N > 2) collisional processes. The resonance-matter population present in the highly-compressed phase of nucleus-nucleus collisions is investigated, in reactions between different intermediate-mass nuclear systems. Received: 29 December 1998 / Revised version: 20 December 1999     

Self-affine multiplicity fluctuation of target residues in relativistic nuclear collisions at a few GeV to a few hundred GeV

Self-affine multiplicity scaling is investigated in the framework of two-dimensional factorial moment methodology using the concept of the Hurst exponent (H). Our investigation on experimental data of the target-evaporated slow particles emitted in ³²S-AgBr interactions at 200 AGeV and ²⁸Si-AgBr interactions at 14.5 AGeV reveals that a better power law behavior is exhibited in self-affine analysis than self-similar analysis. This work shows a clear evidence of self-affine target fragmentation. Received: 22 October 2001 / Accepted: 6 March 2002     

Identification of baryon resonances in central heavy-ion collisions at energies between 1 and 2 AGeV

The mass distributions of baryon resonances populated in near-central collisions of Au on Au and Ni on Ni are deduced by defolding the p_t spectra of charged pions by a method which does not depend on a specific resonance shape. In addition the mass distributions of resonances are obtained from the invariant masses of (p, π^±) pairs. With both methods the deduced mass distributions are shifted by an average value of −60 MeV/c² relative to the mass distribution of the free Δ(1232) resonance, the distributions descent almost exponentially towards mass values of 2000 MeV/c². The observed differences between (p, π⁻) and (p, π⁺) pairs indicate a contribution of isospin I = 1/2 resonances. The attempt to consistently describe the deduced mass distributions and the reconstructed kinetic energy spectra of the resonances leads to new insights about the freeze out conditions, i.e. to rather low temperatures and large expansion velocities. Received: 26 June 1998 / Revised version: 2 September 1998