Systematics of fragment observables

Multifragmentation is observed in many reaction types: light-ion-induced reactions at large incident energies (in the GeV region), central heavy-ion collisions from 30 to 100MeV/u, and peripheral heavy-ion collisions between 30 and 1000MeV/u or above. When nucleus-nucleus collisions are considered, another entrance channel parameter is the corresponding mass asymmetry. The first question which is addressed in this contribution is: do we observe similar reactions in each case? Multifragmentation may be related to a phase transition of nuclear matter. Some others features indicate that dynamical features are dominant. It is a priori possible that the underlying mechanisms are different in proton- and nucleus-induced reactions, in central and in peripheral collisions, at limited and at large bombarding energies. In order to see to what extent they can reflect similar behaviour, it is useful to compare the results of various reactions. The observables can be the fragment multiplicity, the mass distributions or the kinematical properties. In this contribution, we are looking for such general features. We will limit the discussion to the observations themselves, rather than the interpretation, which is the subject of numerous entries in this volume. The experimental results indicate that multifragmentation exhibits at the same time universal and entrance-channel-dependent properties.     

Hadron-quark phase coexistence in a hybrid MIT-Bag model

The phase transition of hadronic to quark matter and the boundaries of the hadron-quark coexistence phase are studied within the two Equation of State (EoS) models. The relativistic effective mean-field approach with constant and density-dependent meson-nucleon couplings is used to describe hadronic matter, and the MIT-Bag model is adopted to describe quark matter. The boundaries of the mixed phase for different Bag constants are obtained solving the Gibbs equations. We notice that the dependence on the Bag parameter of the critical temperatures (at zero chemical potential) can be well reproduced by a fermion ultrarelativistic quark gas model, without contribution from the hadron part. At variance, the critical chemical potentials (at zero temperature) are very sensitive to the EoS of the hadron sector. Hence, the contribution of the hadronic interaction is much more relevant for the determination of the transition to the quark-gluon plasma at finite baryon density and low T . Moreover, in the low-temperature and finite chemical potential region no solutions of the Gibbs conditions are existing for small Bag-constant values, B < (135 MeV)⁴ . Isospin effects in asymmetric matter appear important in the high chemical-potential regions at lower temperatures, of interest for the inner-core properties of neutron stars and for heavy-ion collisions at intermediate energies.     

Thermal dileptons from quark and hadron phases of an expanding fireball

A fireball model with time evolution based on transport calculations is used to examine the dilepton emission rate of an ultra-relativistic heavy-ion collision. A transition from hadronic matter to a quark-gluon plasma at a critical temperature T _C between 130-170 MeV is assumed. We also consider a possible mixed phase scenario. We include thermal corrections to the hadronic spectra below T _C and use perturbation theory above T _C. The sensitivity of the spectra with respect to the freeze-out temperature, the initial fireball temperature and the critical temperature is investigated. Received: 4 August 2000 / Accepted: 14 November 2000     

Quark matter and non-extensive thermodynamics

We investigate different ways of describing the thermodynamics of prehadronic matter in high-energy heavy-ion collisions. The non-extensive thermodynamics with certain assumptions enables an agreement between two important experimental facts. That cannot be achieved using the conventional Gibbs statistics. A massless quark-gluon plasma with E/N=1 GeV energy per particle and T=175 MeV spectral slope temperature can be assumed.     

Diphoton rates from thermalized matter resulting in ultra-relativistic heavyion collisions

Diphoton radiation off strongly interacting matter resulting from ultrarelativistic heavy-ion collisions is estimated for SPS and RHIC conditions. At SPS energies the thermal diphoton signal competes strongly with the Drell — Yan like background. For RHIC energies we find that radiation from initially undersaturated (but gluon-rich) deconfined matter overwhelms the hadron gas at invariant mass M > 1 GeV; the Drell — Yan like diphotons dominate at M > 2.5 GeV. Due to this the so-called M_⊥ scaling is approximately obtained in a narrow window and can serve as additional tool for discriminating radiation from quark-gluon matter.     

PARTICLE DENSITIES AT THE CENTER REGION IN HIGH ENERGY HEAVY-ION COLLISIONS

Based on the two-chain model by A.Capella et al. the rapidity distributions of the multiplicity in high energy heavy-ion collisions, especially in head-on collisions, are discussed. The energy density at the center region in head-on collisions is estimated for different incident energies and different heavy ions. It is shown that the condition of the phase transition to the quark-gluon plasma can be realized in heavy-ion collisions at several tens of GeV.     

On the origin of the radial flow in low energy heavy-ion reactions

The average transverse energy of nucleons and intermediate mass fragments observed in the heavy-ion reaction Xe(50A MeV) + Sn shows the same linear increase as a function of their mass as observed in heavy-ion collisions up to the highest energies available today and fits well into the systematics. At higher energies this observation has been interpreted as a sign of a strong radial flow in an otherwise thermalized system. Inverstigating the reaction with quantum molecular dynamics simulations we find in between 50A MeV and 200A MeV a change in the reaction mechanism. At 50A MeV the apparent radial flow is merely caused by in-plane flow and Coulomb repulsion. The average transverse fragment energy does not change in the course of the reaction and is equal to the initial fragment energy due to the Fermi motion. At 200A MeV, there are two kinds of fragments: those formed from spectator matter and those from the center of the reaction. There the transverse energy is caused by the pressure from the compressed nuclear matter. In both cases we observe a binary event structure, even in central collisions. This demonstrates as well the non-thermal character of the reaction. The actual process which leads to multifragmentation is rather complex and is discussed in detail.     

On the photon production in nucleus-nucleus collisions at high energy

A modified Landau hydrodynamical model is applied to study hard thermal photon production in central heavy-ion collisions at LHC, RHIC and SPS energies. It is shown that the phase transition from quark-gluon plasma into hadrons in consequence of the thermodynamical expansion is close to the second order phase transition if a resonance production is taken into account. Hard direct photon emission is also investigated with consideration of nuclear shadowing effect on structure function of quarks and gluons. Also π^π photon background is investigated. It is demonstrated that at the LHC energy photon yield from the quark-gluon plasma in the photon transversal momentumk _⊥ range from 5 to 25 GeV/c exceeds both the background and the direct photon yield. This conclusion may be important for the quark-gluon plasma diagnostic aims. It is also shown that for the LHC energy the thermal photon yield in the present model essentially exceeds this yield obtained in the Bjorken scaling model.     

Electromagnetic and hadron signatures of quark-gluon plasma in heavy-ion collisions at 62.4 GeV in the PHOENIX experiment on the RHIC

The motivation for quark-gluon plasma studies at beam energies of 20–100 GeV are the detailed exploration of the phase diagram of QCD matter and the search for the critical point of deconfinement transition. The PHOENIX experiment on the RHIC measures nuclear modification factors for protons, π⁰ and φ mesons in heavy-ion collisions at $\sqrt s$ 6 = 2.4 GeV. The possibility of obtaining the electromagnetic signatures of quark-gluon plasma measurements with an innovative HBD detector is discussed.     

Determination of azimuthal features of jets without measuring the angle of the reaction plane

A new method is proposed for determining the coefficient of elliptic anisotropy of jets in ultrarelativistic heavy-ion collisions. This method, which does not involve directly reconstructing the reaction plane, is a generalization of the method used in current experiments to measure the coefficient of elliptic anisotropy of particle fluxes. It is shown that, for the spectrum of hadronic jets, the method makes it possible to explore the azimuthal asymmetry caused by the rescattering of hard partons and by their energy losses in dense quark-gluon matter formed in the region of the initial overlap of nuclei in collisions at a nonzero value of the impact parameter—in particular, under the conditions of the CMS experiment at LHC.     

Azimuthal asymmetry of jet production as a signal of parton energy losses in semicentral heavy-ion collisions

The question is investigated of whether an azimuthal asymmetry in the hadron-jet spectra can arise because of rescattering and energy losses of partons produced via hard processes in a dense quark-gluon matter formed in the region of the initial nuclear overlap in collisions characterized by a nonzero value of the impact parameter. Methods are discussed for determining the reaction-plane angle in ultrarelativistic heavy-ion collisions with the aid of the flux of semihard particles.     

The quark-nucleon phase diagram and quantum chromodynamics

The temperature dependence of a conjectured first-order phase transition between nuclear matter and quark-gluon matter is calculated for temperatures below T = 200 MeV. On the nuclear side a rather successful meson-nucleon mean field theory is applied while quark-gluon matter at large densities and finite temperatures is described perturbatively by quantum chromodynamics. Outside the finite volume of hot and dense quark-gluon matter the physical vacuum is characterized, by the newly determined bag parameter Λ_B = 235 MeV. We observe a dramatic drop in the density of nuclear matter at the phase transition point as the temperature increases, if the scale parameter Λ of QCD is chosen as Λ = 100 MeV. For larger values of Λ the effect is less pronounced. Further work is required to settle this problem.     

Re-hardening of hadron transverse mass spectra in relativistic heavy-ion collisions

We analyze the spectra of pions and protons in heavy-ion collisions at relativistic energies from 2 A GeV to 65+65 A GeV by using a jet-implemented hadron-string cascade model. In this energy region, hadron transverse mass spectra first show softening until SPS energies, and re-hardening may emerge at RHIC energies. Since hadronic matter is expected to show only softening at higher energy densities, this re-hardening of spectra can be interpreted as a good signature of the quark-gluon plasma formation     

Transverse-momentum spectra of hadrons produced in central heavy-ion collisions

In-medium effects of transverse-mass distributions of quarks and gluons are considered assuming a possible local equilibrium for colorless quark objects like mesons and baryons created in central AA collisions. It is shown that the average square of the transverse momentum for these partons grows and then saturates when the initial energy increases. Within the quark-gluon string model, it leads to an energy dependence of hadron transverse-mass spectra which is similar to that observed in heavy-ion collisions. A comparison with other scenarios is given. The text was submitted by the authors in English.     

Evidence for the production of thermal muon pairs with masses above 1 GeV/c 2 in 158 A GeV indium-indium collisions

The yield of muon pairs in the invariant mass region 1<M<2.5 GeV/c ² produced in heavy-ion collisions significantly exceeds the sum of the two expected contributions, Drell-Yan dimuons and muon pairs from the decays of D meson pairs. These sources properly account for the dimuons produced in proton-nucleus collisions. In this paper, we show that dimuons are also produced in excess in 158 A GeV In-In collisions. We furthermore observe, by tagging the dimuon vertices, that this excess is not due to enhanced D meson production, but made of prompt muon pairs, as expected from a source of thermal dimuons specific to high-energy nucleus-nucleus collisions. The yield of this excess increases significantly from peripheral to central collisions, both with respect to the Drell-Yan yield and to the number of nucleons participating in the collisions. Furthermore, the transverse mass distributions of the excess dimuons are well described by an exponential function, with inverse slope values around 190 MeV. The values are independent of mass and significantly lower than those found at masses below 1 GeV/c ², rising there up to 250 MeV due to radial flow. This suggests the emission source of thermal dimuons above 1 GeV/c ² to be of largely partonic origin, when radial flow has not yet built up.     

Heavy-ion physics with the ATLAS detector

The CERN LHC collider will operate with lead ions at √s of 5.5 TeV/nucleon. The ATLAS detector, designed to study high-pT physics in the pp mode of the LHC, has the potential to study ultrarelativistic heavy-ion collisions in a full range of observables characterized extremely dense matter and the formation of a quark-gluon plasma. The ATLAS physics program includes global event measurements (particle multiplicities, transverse momentum), suppression of heavy-quarkonia production, jet quenching, and a study of ultraperipheral collisions. (on behalf of the ATLAS Collaboration) The text was submitted by the author in English.     

Direct hadron production in ultrarelativistic heavy-ion collisions

Hadrons emitted by the pre-surface layer of a quark-gluon plasma (QGP) before the phase transition into a hadronic gas are considered as possible sources of direct information about QGP. It is shown that if QGP is created in ultrarelativistic heavy-ion collisions, then these hadrons strongly contribute at soft p_t at SpS energy and dominate up to an order of magnitude at LHC energy.     

Unified approach to theJ/Ψ suppression and strangeness enhancement in high-energy nucleus-nucleus collisions

A picture of nuclear interactions at high energies (p _Lab100 GeV/c) is proposed, where the only essential difference between nucleon-neucleon and central nucleus-nucleus collisions is the size of the interaction zone. In both cases intermediate multi-parton systems are created, which achieve thermodynamical equilibrium with respect to characteristics of light quarks and gluons. The characteristics of heavier quarks are far from equilibrium ones in nucleon-nucleon collisions, but the degree of equilibration is substantially increased in central heavy-ion collisions. Therefore, the multi-parton systems produced in central nucleus-nucleus interactions can manifest themseleves as the quark-gluon plasma. A simple model based on this picture is formulated and confronted with the experimental data concerningJ/ suppression and strangeness yield enhancement, both revealed recently in central nucleus-nucleus collisions.This work was supported in part by the Deutsche Forschungsgemeinschaft under grant He 1283/3-1     

Physics of collisions between ultrarelativistic nuclei: Manifestation of collective effects

Experimental results concerning heavy-ion collisions studied by means of the Relativistic Heavy-Ion Collider (RHIC) at an energy of 100 GeV per nucleon and showing manifestations of collective effects are surveyed. These effects are interpreted as a consequence of the formation of a dense thermalized medium referred to as quark-gluon plasma (strongly interacting quark-gluon plasma, quark-gluon matter, parton medium). An azimuthal anisotropy of particles, the suppression of the particle yield at high transverse momenta in relation to proton-proton collisions, and a change in the shape of the peak from a hadron jet in nucleus-nucleus collisions are spectacular manifestations of collective effects.     

Probing the strange quark condensate by di-electrons from φ-meson decays in heavy-ion collisions at SIS energies

QCD sum rules predict that the change of the strange quark condensate 〈ˉss〉 in hadron matter at finite baryon density causes a shift of the peak position of the di-electron spectra from φ-meson decays. Due to the expansion of hadron matter in heavy-ion collisions, the φ peak suffers a smearing governed by the interval of density in the expanding fireball, which appears as an effective broadening of the di-electron spectrum in the φ region. The emerging broadening is sensitive to the in-medium change of 〈ˉss〉. This allows to probe directly in-medium modifications of 〈ˉss〉 via di-electron spectra in heavy-ion collisions at SIS energies with HADES. Received: 22 November 2002 / Accepted: 30 January 2003 / Published online: 29 April 2003