Piotr Bozek

Components of the elliptic flow in Pb–Pb collisions at s=2.76 TeV

Abstract We calculate the elliptic flow of charged particles in Pb–Pb collisions at s = 2.76 TeV in relativistic viscous hydrodynamics. The recent data of the ALICE Collaboration on the elliptic flow as function of the centrality can be very well described using the hydrodynamic expansion of a fluid with a small shear viscosity η / s = 0.08 . The elliptic flow as function of the transverse momentum shows systematic deviations from a hydrodynamic behavior in the small momenta region p ⊥ 800 MeV . It indicates that a non-negligible contribution of non-thermalized particles from jet fragmentation is present.

Balance functions in a thermal model with resonances

The π + π − balance function in rapidity is computed in a thermal model with resonances. It is found that the correlations from the neutral-resonance decays are important, yielding about a half of the total contribution, which in general consist of resonance and non-resonance parts. The model yields the pionic balance function a few per cent wider that what follows from the recent data for the Au+Au collisions at \(\sqrt {s_{NN} } = 130\).

Event-by-event pT fluctuations and multiparticle clusters in relativistic heavy-ion collisions

We explore the centrality dependence of the pT correlations in the event-by-event analysis of relativistic heavy-ion collisions at RHIC made recently by the PHENIX and STAR Collaborations. We point out that the observed scaling of strength of dynamical fluctuations with the inverse number of the produced particles can be naturally explained by the formation of clusters. We argue that the large magnitude of the measured covariance implies that the clusters contain at least several particles. We also discuss whether the clusters may originate from jets. In addition, we provide numerical estimates of correlations coming from resonance decays and thermal clusters.

The balance function in azimuthal angle is a measure of the transverse flow

Abstract The charge or barion number balance function in the relative azimuthal angle of a pair of particles emitted in ultrarelativistic heavy-ion collisions is studied. The π + π − and p p ¯ balance functions are computed using thermal models with two different set of parameters, corresponding to a large freeze-out temperature and a moderate transverse flow or a small temperature and a large transverse flow. The single-particle spectra including pions from resonance decays are similar for the two scenarios, on the other hand the azimuthal balance function is very different and could serve as an independent measure of the transverse flow at the freeze-out.

Production of resonances in a thermal model: invariant-mass spectra and balance functions

We present a calculation of the π+π− invariant-mass correlations and the pion balance functions in the single-freeze-out model. A satisfactory agreement with the data for Au+Au collisions is found.

Short-range correlations in asymmetric nuclear matter

The spectral function of protons in the asymmetric nuclear matter is calculated in the self-consistent T-matrix approach. The spectral function per proton increases with increasing asymmetry. This effect and the density dependence of the spectral function partially explain the observed increase of the spectral function with the mass number of the target nucleus in electron scattering experiments.

Event-by-event fluctuations of transverse momentum and multiparticle clusters in relativistic heavy-ion collisions

We analyze the event-by-event fluctuations of mean transverse momentum measured recently by the PHENIX and STAR Collaborations at RHIC. We argue that the observed scaling of strength of dynamical fluctuations with the inverse number of particles can be naturally explained by formation of multiparticle clusters.

Balance functions from a thermal model

A calculation of the pion balance functions in a thermal model is presented. The total result consists of resonance and non-resonance parts. A satisfactory agreement with the data on Au+Au collisions at [FORMULA] GeV is found.

Forward-Backward Flow Correlations in Relativistic Heavy-Ion Collisions

We discuss the torque effect in the initial fireball formed in relativistic heavy-ion collisions, manifesting itself, on the event-by-event basis, in a relative angle between the principal axes of the transverse momentum distributions in the forward and backward rapidity regions. The torque follows from two natural features: 1) the sources of particles (e.g. wounded nucleons) emit predominantly in their forward hemispheres, and 2) there exist fluctuations in the transverse distribution of sources from the two colliding nuclei. On the average, the standard event-by-event deviation of the relative torque angle is about 20 ◦ for the central and 10 ◦ for the mid-peripheral collisions. The hydrodynamic expansion of a torqued fireball leads to a torqued collective flow of the fluid, which, in turn, yields torqued principal axes of the transverse-momentum distributions at different rapidities. We discuss experimental measures based on cumulants involving particles in different rapidity regions, which allow for a quantitative extraction of the effect from the experimental data. We estimate the non-flow contributions from resonance decays with the help of THERMINATOR. The forward-backward rapidity correlations reveal important information on the mechanism of particle production in high-energy hadronic and nuclear collisions, uncovering the features of the dynamical system at a very early stage. This talk is based on our recent work, 1) where more details as well as a complete list of references may be found. We discuss an interesting forward-backward effect, concerning the event-by-event fluctuations of the longitudinal shape of the fireball created in relativistic heavy-ion collisions.

Fluctuation of the initial condition from Glauber models

We analyze measures of the azimuthal asymmetry, in particular the participant harmonic moments, epsilon*, in a variety of Glauber-like models for the early stage of collisions at RHIC. Quantitative comparisons indicate substantial model dependence for epsilon*, reflecting different effective number of sources, while the dependence of the scaled standard deviation sigma(epsilon*)/epsilon* on the particular Glauber model is weak. For all the considered models the values of sigma(epsilon*)/epsilon* range from ~0.5 for the central collisions to ~0.3-0.4 for peripheral collisions. These values, dominated by statistics, change only by 10-15% from model to model. For central collisions and in the absence of correlations between the location of sources we obtain through the use of the central limit theorem the simple analytic formula sigma(epsilon*)/epsilon*(b=0) ~ \sqrt{4/pi-1} ~ 0.52