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Piotr Bozek

Publications and source records attributed to Piotr Bozek.

99 records · Page 6Linked to original sources

Glauber Monte-Carlo predictions for LHC

In the framework of various Glauber-like models we compute several correlation observables in nuclear collisions at the SPS, RHIC, and LHC energies. We analyze fluctuations of the eccentricity of the fireball created in the collision, in particular the {variable-axes} harmonic moments $ε^\ast$, as well as the fluctuations of multiplicity of charged particles. We find moderate model dependence of the scaled standard deviation $σ(ε^\ast)/ε^\ast$ on the choice of the particular Glauber model. For all considered models the values of $σ(ε^\ast)/ε^\ast$ range from $\sim$~0.5 for central collisions to $\sim$~0.3-0.4 for peripheral collisions. The results are confronted to the recent measurement of the elliptic-flow fluctuations at RHIC. We also find that the dependence of multiplicity fluctuations on the centrality of the collision is too weak to explain the measurements at the SPS energies. The magnitude of the Glauber multiplicity fluctuations increases by about 20% from the RHIC to LHC energies.

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Viscous evolution of the rapidity distribution of matter created in relativistic heavy-ion collisions

Longitudinal hydrodynamic expansion of the fluid created in relativistic heavy-collisions is considered taking into account shear viscosity. Both a on-vanishing viscosity and a soft equation of state make particle distributions in rapidity narrower. The presence of viscosity has dramatic consequence on the value of the initial energy density. The reduction of the longitudinal work and dissipative processes due to the shear viscosity, increase the total entropy and the particle multiplicity at central rapidities. The total energy in the collision, dominated by the longitudinal motion, is conserved. Viscous corrections make the longitudinal velocity of the fluid to stay close to the Bjorken scaling v_z = z/t through the evolution. At the freeze-out viscous corrections are the strongest for non-central rapidities.

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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$, independent on the collision energy, mass number, or the number of sources. We investigate the shape-fluctuation effects for jet quenching and find they are important only for very central events. Finally, we list some remarks and predictions from smooth hydrodynamics on higher flow coefficients and their fluctuations, in particular sigma(v_4)/v_4=2 sigma(v_2)/v_2.

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Fluctuating initial conditions in heavy-ion collisions from the Glauber approach

In the framework of the Glauber approach we analyze the shape parameters of the early-formed system and their event-by-event fluctuations. We test a variety of models: the conventional wounded nucleon model, a model admixing binary collisions to the wounded nucleons, a model with hot spots, as well as the hot-spot model where the deposition of energy occurs with a superimposed probability distribution. We look in detail at the so-called participant multipole moments, obtained by an averaging procedure where in each event the system is translated to its center of mass and aligned with the major principal axis of the ellipse of inertia. Quantitative comparisons indicate substantial relative effects for eccentricity in variants of Glauber models. On the other hand, the dependence of the scaled standard deviation of the participant eccentricity on the chosen model is weak. For all models the values range from about 0.5 for the central collisions to about 0.3-0.4 for peripheral collisions, both for the gold-gold and copper-copper collisions. They are dominated by statistics and change only by 10-15% from model to model. We provide an approximate analytic expansion for the multipole moments and their fluctuations given in terms of the fixed-axes moments. For central collisions and in the absence of correlations it gives the simple formula for the scaled standard deviation of the participant eccentricity: sqrt(4/pi-1). Similarly, we obtain expansions for the radial profiles of the multipole distributions. We investigate the relevance of the shape-fluctuation effects for jet quenching and find them important only for very central events. Finally, we argue how smooth hydro leads to the known result v_4 ~ v_2^2, and further to the prediction Delta v_4/v_4 = 2 Delta v_2/v_2.

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pT-fluctuations and multiparticle clusters in heavy-ion collisions

We investigate 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 notice that sigma^2 dynamical scales to a very good accuracy with the inverse number of the produced particles, n. This scaling can be naturally explained by formation of clusters. We discuss the nature of clusters and provide numerical estimates of correlations coming from resonance decays, which are tiny, and a model where particle are emitted from local thermalized sources moving at the same collective velocity. This "lumped cluster" model can explain the data when on the average 6-15 particles are contained in a cluster. We also point out simple relations of the popular correlation measures to the covariance, which arise under the assumptions (well holding at RHIC) that the distributions are sharply peaked in n and that the dynamical compared to statistical fluctuations are small.

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Size of the thermal source in relativistic heavy-ion collisions

The dependence of the size of the thermal source on the centrality in ultrarelativistic heavy-ion collisions is studied. The interaction region consists of a well defined thermalized core, and of an outer mantle where the production scales with the number of participants. The thermal source builds up in the region with the largest density of participants in the transverse plane. Particle production in the thermalized core is enhanced in comparison to the wounded nucleon model. The change of the degree of strangeness saturation with centrality is also discussed. We perform an estimate of high pT jet absorption finding that an increase of the absorption in the thermal core is compatible with the data.

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

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 pi+pi- and ppbar 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.

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In medium T matrix with realistic nuclear interactions

We calculate the self-consistent in-medium T matrix for symmetric nuclear matter using realistic interactions with many partial waves. We find for the interactions used (CDBonn and Nijmegen) very similar results for on-shell quantities. The effective mass and the renormalization factor Z_F at the Fermi momentum are given for a range of densities.

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