Kaon and pion spectra in relativistic and ultrarelativistic nucleus-nucleus collisions

The formation of kaon and pion spectra is considered within the relativistic hydrodynamic model of interaction of heavy nuclei. The terms describing the formation of quarks and gluons are included in the state equation of the hydrodynamic model. This approach allowed consideration of nucleus-nucleus collisions in the range from 4 to 160 GeV/nucleon and reproduce the increase in temperature of the kaons formed, which is not reproduced within the ultrarelativistic quantum molecular-dynamics model and other microscopic approaches.     

Novel subjet observables for jet quenching in heavy-ion collisions

Using a novel observable that relies on the momentum difference of the two most energetic subjets within a jet \(\varDelta S_{12}\) we study the internal structure of high-energy jets simulated by several Monte Carlo event generators that implement the partonic energy-loss in a dense partonic medium. Based on inclusive jet and dijet production we demonstrate that \(\varDelta S_{12}\) is an effective tool to discriminate between different models of jet modifications over a broad kinematic range. The new quantity, while preserving the collinear and infrared safety of modern jet algorithms, it is experimentally attractive because of its inherent resilience against backgrounds of heavy-ion collisions.     

Monte Carlo simulations of jet quenching in heavy ion collisions

Jet quenching is one of the major discoveries of the heavy-ion program at Rhic. While there is a wealth of data from Rhic that will soon be supplemented with measurements at the Lhc, on the theoretical side the situation is less clear. A thorough understanding of jet quenching is, however, beneficial, as it is expected that medium-induced modifications of jets allow one to characterise properties of the QCD matter produced in heavy ion collisions. This talk aims at summarising the main ideas and concepts of the currently available Monte Carlo models for jet quenching.     

Anisotropic flow and jet quenching in ultrarelativistic U+U collisions

Full-overlap U+U collisions provide significantly larger initial energy densities at comparable spatial deformation, and significantly larger deformation and volume at comparable energy density, than semi-central Au+Au collisions. We show quantitatively that this provides a long lever arm for studying the hydrodynamic behavior of elliptic flow in much larger and denser collision systems and the predicted nonlinear path-length dependence of radiative parton energy loss.     

Radiative parton energy loss and jet quenching in high-energy heavy-ion collisions

We study within the light-cone path integral approach [3] the effect of the induced gluon radiation on high-p _T hadrons in high-energy heavy-ion collisions. The induced gluon spectrum is represented in a new form which is convenient for numerical simulations. For the first time, computations are performed with a realistic parameterization of the dipole cross section. The results are in reasonable agreement with the suppression of high-p _T hadrons in Au + Au collisions at \(\sqrt s = 200\)GeV observed at RHIC.     

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.     

Kaon flow in heavy ion collisions

Using transport models that include explicitly the kaon degree of freedom, we have studied kaon flow, i.e., the average kaon transverse momentum as a function of rapidity, and the associated flow parameter in heavy ion collisions at both SIS and AGS energies. It is found that the pattern of kaon flow in heavy ion collisions is sensitive to the kaon potential used in the model and thus provides a useful means for studying the kaon properties in nuclear medium.     

Novel jet quenching observables in heavy ion collisions at the LHC

We discuss such novel jet quenching observables at the LHC as direct measurements of medium-modified jet fragmentation function, nuclear modification factor and azimuthal anisotropy for jets with finite cone size and p _T -imbalance for dimuon tagged jets. The corresponding predictions obtained with PYQUEN partonic energy loss model are presented.     

Electromagnetic probes of nuclear collisions at intermediate energy

We show that emission of high-energy electrons and photons in nuclear collisions at intermediate energies is sensitive to the space-time evolution of the reaction. The electron and photon spectra measure related but complementary quantities connected with the nuclear charge distribution. We show that at 60 MeV/u beam energy production of 50 MeV electrons is predicted to measurable probability.     

The influence of flow on the jet quenching power in heavy-ion collisions

The flow pattern and evolution of the medium created in ultrarelativistic heavy ion collisions can have significant influence on the energy loss of hard partons traversing the medium. We demonstrate that within a range of assumptions for longitudinal and transverse flow which are all compatible with the measured hadronic single particle distributions, the quenching power of the medium can vary within a factor five. Thus, the choice of the medium evolution is one of the biggest uncertainties in jet quenching calculations and needs to be addressed with some care.     

Baryon-to-entropy ratio in very high energy nuclear collisions

We compute as a function of rapidity y the baryon number carried by quarks and antiquarks with p _T > p ₀ ≈ 2 GeV produced in Pb+Pb collisions at TeV energies. The computation is carried out in lowest order QCD perturbation theory using structure functions compatible with HERA results. At p ₀ = 2 GeV the initial gluon density is both transversally saturated and thermalised in the sense that the energy/gluon equals to that of an ideal gas with the same energy density. Even at these high energies the initial net baryon number density at y = 0 at τ = 0.1 fin will be more than the normal nuclear matter density but the baryon-toentropy ratio is only (B-B?)/S ～ 1/5000. Further evolution of the system is discussed and the final baryon-to-entropy ratio is estimated.     

Finite particle number effects in high-energy nuclear collisions: Implications on pion spectra

We study pion production from high-energy nuclear collisions by means of a simple statistical model. The shapes of the observed spectra exclude that all pions result from freely decaying delta resonances. Rather, they have to participate in kinetic equilibration processes. Finite particle number effects are found to be very important: equilibration does not occur globally but rather in groups of only a few particles. The pion production rates cannot be explained in terms of a chemical equilibrium.     

Effects of the formation time of parton shower on jet quenching in heavy-ion collisions

Jet quenching has successfully served as a hard probe to study the properties of Quark-Gluon Plasma (QGP). As a multi-particle system, jets require time to develop from a highly virtual parton to a group of partons close to mass shells. In this study, we present a systematical analysis on the effects of this formation time on jet quenching in relativistic nuclear collisions. Jets from initial hard scatterings were simulated with Pythia, and their interactions with QGP were described using a Linear Boltzmann Transport (LBT) model that incorporates both elastic and inelastic scatterings between jet partons and the thermal medium. Three different estimations of the jet formation time were implemented and compared, including instantaneous formation, formation from single splitting, and formation from sequential splittings, before which no jet-medium interaction was assumed. We found that deferring the jet-medium interaction with a longer formation time not only affects the overall magnitude of the nuclear modification factor of jets but also its dependence on the jet transverse momentum.     

Universal pion freeze-out in heavy-ion collisions

Based on an evaluation of data on pion interferometry and on particle yields at midrapidity, we propose a universal condition for thermal freeze-out of pions in heavy-ion collisions. We show that freeze-out occurs when the mean free path of pions lambda(f) reaches a value of about 1 fm, which is much smaller than the spatial extent of the system at freeze-out. This critical mean free path is independent of the centrality of the collision and beam energy from the Alternating Gradient Synchrotron to the Relativistic Heavy Ion Collider.     

Kaon interferometry in heavy-ion collisions at the CERN SPS

K⁺K⁺ and K^–K^– correlations from S+Pb collisions at 200 GeV/c per nucleon and K⁺K⁺ correlations from p+Pb collisions at 450 GeV/c per nucleon, are presented as measured by the focusing spectrometer of the NA44 experiment at CERN. Multidimensional fits are performed in order to characterize the kaon-emission volume, which is found to be smaller than the pion-emission volume.