Thermal dileptons in high-energy nuclear collisions

Clear signs of excess dileptons above the known sources have been found at the SPS for a long time. However, a real clarification of these observations was only recently achieved by NA60, measuring dimuons with unprecedented precision in 158A GeV In-In collisions. The excess mass spectrum in the region is consistent with a dominant contribution from π⁺π⁻→ρ→μ⁺μ⁻ annihilation. The associated ρ spectral function shows a strong broadening, but essentially no shift in mass. In the region , the excess is found to be prompt, not due to enhanced charm production. The inverse slope parameter associated with the transverse momentum spectra rises with mass up to the ρ, followed by a sudden decline above. While the initial rise, coupled to a hierarchy in hadron freeze-out, points to radial flow of a hadronic decay source, the decline above signals a transition to a low-flow source, presumably of partonic origin. The mass spectra show a steep rise towards low masses characteristic of Planck-like radiation. The polarization of the excess referred to the Collins Soper frame is found to be isotropic. All observations are consistent with a global interpretation of the excess as thermal radiation. We conclude with a short discussion of a possible link to direct photons.     

Partonic collectivity in high-energy nuclear collisions

We show evidences for partonic flow in Au+Au collisions at RHIC.     

Jet charge in high-energy nuclear collisions

The averaged jet charge characterizes the electric charge of the initiating parton and provides a powerful tool to distinguish quark jets from gluon jets.We predict,for the first time,the medium modification of the averaged jet charge in the heavy-ion collisions at the LHC,where jet productions in p+p collisions are simulated by PYTHIA6,and the parton energy loss in QGP is calculated with two Monte Carlo models of jet quenching:PYQUEN and JEWEL.We found that the distribution of averaged jet charge is significantly suppressed by initial state isospin effects due to the participation of neutrons with zero electric charge during nuclear collisions.The considerable enhancement of the averaged jet charge in central Pb+Pb collisions is observed relative to peripheral collisions,since the jet quenching effect is more pronounced in central collisions.The distinct feature of the averaged jet charge between quark and gluon jets,along with the sensitivity of medium modifications on the jet charge to flavor dependence of the parton energy loss,could be very useful to discriminate the energy loss pattern between quark and gluon jets in heavy-ion collisions.     

Baryon number transport in high-energy nuclear collisions

Recent SPS data on the rapidity distribution of protons inp+S,p+Au and S+S collisions at 200 AGeV and preliminary Pb+Pb collisions at 160 AGeV are compared to HIJING and VENUS calculations as well as to predictions based on the Multi-Chain Model (MCM). The preliminary Pb data suggest that a linear dependence of the proton rapidity shift as a function of the nuclear thicknes, as first observed inp+A reactions, may apply up to Pb+Pb reactions. The observed rapidity dependence of produced hyperons in bothp+A and A+A reactions however cannot be explained in terms of such models without introducing additional non-linear effects.     

Mesonic atom production in high-energy nuclear collisions

Zeitschrift für Physik A Hadrons and nuclei - The production probability ofπ-mesonic atom in high-energy nuclear collisions is estimated by a coalescence model. The production cross...     

Leading baryon spectrum in high-energy nuclear collisions

We compute the leading baryon spectrum in high-energy nuclear collisions in the framework of the Dual parton model. Nucleus-nucleus collisions are treated in full detail. The average rapidity shift of the leading baryon (nuclear stopping power) is studied as a function of the degree of centrality of the collision and the cross-section of the selected events is given. Important differences are found here between proton-nucleus and nucleus-nucleus interactions. The agreement with existing data on proton-nucleus and α?α collisions is quite good.     

Hadronization of excited nucleons in nuclear collisions at ISR energies

Calorimeters downstream of the intersection of pα and αα beams in the ISR have been used to study the hadronization of excited nucleons. These data extend and support the conclusions from previous studies of the A-dependence of particle multiplicities in ultrarelativistic p-nucleus collisions, which provided evidence that the proton hadronizes outside the nuclear volume.     

Probes of phase decomposition in high-energy nuclear collisions

High-energy nuclear collisions produce quark-gluon plasmas that expand and hadronize. If the associated phase transition is of first order then the hadronization should proceed through a spinodal phase separation. We explore here the possibility of identifying the associated clumping by analysis of suitable N-particle momentum correlations.     

Full jet tomography of high-energy nuclear collisions

Parton energy loss in the hot QCD medium will manifest itself not only in the leading hadron spectra but also in the reconstructed jet productions in high-energy nucleus-nucleus collisions. In this paper we report on recent theoretical efforts in studying full jet observables in relativistic heavy-ion collisions by discussing the modifications of jet shapes, inclusive jet cross section and the vector boson accompanied jet production in the presence of the QGP-induced jet quenching.     

Cross sections and barriers for nuclear fission induced by high-energy nucleons

The cross sections for the fission of ²³²Th, ^235,238U, ²³⁷Np, and ²³⁹Pu target nuclei that was induced by 20- to 1000-MeV neutrons and protons were calculated. The respective calculations were based on the multiconfiguration-fission (MCFx) model, which was used to describe three basic stages of the interaction of high-energy nucleons with nuclei: direct processes (intranuclear cascade), equilibration of the emerging compound system, and the decay of the compound nucleus (statistical model). Fission barriers were calculated within the microscopic approach for isotopic chains formed by 15 to 20 nuclei of the required elements. The calculated fission cross sections were compared with available experimental data. It was shown that the input data set and the theoretical model used made it possible to predict satisfactorily cross section for nuclear fission induced by 20- to 1000-MeV nucleons.     

Correlated charged pions from chiral dynamics in high-energy nuclear collisions

It is expected that chiral symmetry is approximately restored in the very excited matter produced in a high-energy nuclear collision and that the chiral order parameter will thereafter quickly relax towards its aligned vacuum value. This non-equilibrium evolution of the environment endows the pionic degrees of freedom with a time-dependent effective mass and this agency, in turn, may create correlated soft pions that could provide specific observable signatures of the chiral dynamics that are based solely on charged pions.     

Non-extensive statistics, fluctuations and correlations in high-energy nuclear collisions

Starting from the experimental evidence that high-energy nucleus–nucleus collisions cannot be described in terms of superpositions of elementary nucleon–nucleon interactions, we analyze the possibility that memory effects and long-range forces imply a non-extensive statistical regime during high-energy heavy-ion collisions. The relevance of these statistical effects and their compatibility with the available experimental data are discussed. In particular, we show that theoretical estimates obtained in the framework of the generalized non-extensive thermostatistics, can reproduce the shape of the pion transverse mass spectrum and explain the different physical origin of the transverse momentum correlation function of the pions emitted during the central Pb + Pb and during the p +p collisions at 158 GeV. Received: 6 June 1999 / Published online: 21 December 1999     

The production of high-energy protons in central relativistic nuclear collisions

We extend our model of transport theory to be applicable to the inclusive production of protons with very high energy. We then consider the angular distribution of such protons, produced in a central collision of Ar on KC1 at 800 MeV per nucleon. The slight anisotropy observed in the data can be explained by a finite value of the friction constant which in turn determines the number of collisions needed for equilibrium to be reached. We also show that these data are quite sensitive to the reaction geometry and cannot be explained by the firestreak model.     

Elementary reference for the analysis of high-energy nuclear collisions

In this contribution we shall discuss some aspects of the role of elementary reference data for the interpretation of the results from nuclear collisions and show in particular that neutron-proton interactions play a significant role. Furthermore we point out that a complete set of elementary data is desirable, because most theoretical concepts for the interpretation of nuclear collisions rely on such data as input for their calculations.     

Non-central effective forces between nucleons in nuclear matter

The non-central components of the Sprung-Banerjee effective interaction are derived from a nuclear matter calculation based on the Reid potential. The strength of the spin-orbit force is about 10% smaller than empirical values used in recent Hartree-Fock calculations using density-dependent interactions.     

Radial profile of bottom quarks in jets in high-energy nuclear collisions

Angular correlations between a heavy quark(HQ) and its tagged jet are potentially new tools to gain insight into the in-medium partonic interactions in relativistic heavy-ion collisions.In this work,we present the first theoretical study on the radial profiles of B mesons in jets in Pb+Pb collisions at the Large Hadron Collider(LHC).The initial production of a bottom quark tagged jet in p+p is computed by SHERPA,which matches the next-toleading order matrix elements with contributions of parton showers,whereas the massive quark traversing the quarkgluon plasma is described by a Monte Carlo model,SHELL,which can simultaneously simulate light and heavy flavor in-medium energy loss within the framework of Langevin evolution.In p+p collisions,we find that at lower p_T~Q the radial profiles of heavy flavors in jets are sensitive to the heavy quark mass.In 0-10% Pb+Pb collisions at S_(NN)~(1/2)=5.02 TeV,we observe an inverse modification pattern of the B meson radial profiles in jets at 4 p_T~Q 20 GeV compared to those of D mesons:the jet quenching effects narrow the jet radial profiles of B mesons in jets while broadening those of D mesons in jets.We find that in A+A collisions,the contribution dissipated from the higher p_T~Q 20 GeV region naturally has a narrower initial distribution and consequently leads to a narrower modification pattern of the radial profile;however the diffusion nature of the heavy flavor in-medium interactions will give rise to a broader modification pattern of the radial profile.These two effects consequently compete and offset with each other,and the b quarks in jets benefit more from the former and suffer less diffusion effect compared to that of c quarks in jets.These findings can be tested in the future experimental measurements at the LHC to gain better understanding of the mass effect of jet quenching.