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Piotr Bozek

Publications and source records attributed to Piotr Bozek.

At least 91 records · Page 5Linked to original sources
arXiv2011

Modeling global event properties using hydrodynamics from RHIC to LHC

The relativistic hydrodynamic model is applied to describe the expansion of the dense matter formed in relativistic heavy-ion collisions. The hydrodynamic expansion of the fluid, supplemented with the statistical emission of hadrons at freeze-out gives a satisfactory description of the observables for particles emitted with soft momenta. Experimental data for the transverse momentum spectra, elliptic flow and interferometry radii give constraints on the properties of the fluid, its equation of state and viscosity coefficients. The role of the fluctuations of the initial profile of the energy density is discussed.

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arXiv2011

Spectra, flow and HBT in Pb-Pb collisions at the LHC

The transverse momentum spectra, elliptic flow and interferometry radii for Pb-Pb collisions at the LHC are calculated in relativistic viscous hydrodynamics. For Glauber model initial conditions, we find that the data can be described using a small value of shear viscosity eta/s=0.08. The viscosities and the equation of state are the same as used for RHIC energies.

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arXiv2011

Components of the elliptic flow in Pb-Pb collisions at 2.76 TeV

We calculate the elliptic flow of charged particles in Pb-Pb collisions at 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 eta/s=0.08. The elliptic flow as function of the transverse momentum shows systematic deviations from a hydrodynamic behavior in the small momenta region pt<800MeV. It indicates that a non-negligible contribution of non-thermalized particles from jet fragmentation is present.

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arXiv2011

Interferometry radii in heavy-ion collisions at 200GeV and 2.76TeV

The expansion of the fireball created in Au-Au collisions at 200GeV and Pb-Pb collisions at 2.76TeV is modelled using the relativistic viscous hydrodynamics. The experimentally observed interferometry radii are well reproduced. Additional pre-equilibrium flow improves slightly the results for the lower energies studied.

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arXiv2011

Indications of early thermalization in relativistic heavy-ion collisions

The directed flow of particles emitted from the fireball created in a heavy-ion collision is shown to be a very sensitive measure of the pressure equilibration in the first 1 fm/c of the evolution. Performing a 3+1 dimensional relativistic hydrodynamic calculation with nonequilibrated longitudinal and transverse pressures, we show that the directed flow is strongly reduced if the pressure imbalance survives for even a short time. Transverse momentum spectra, elliptic flow and interferometry correlation radii are not very sensitive to this early pressure anisotropy. Comparison with the data points toward a short equilibration time of the order of $0.25$fm/c or less.

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arXiv2010

Measuring the thermalization time

A new method of measuring the pressure anisotropy in the early stage of heavy-ion collisions is discussed. It is shown that the transverse momentum spectra, elliptic flow and interferometry radii are not sensitive to the early anisotropy. We propose the directed flow as a measure of the early transverse and longitudinal pressures. Calculations indicate that the transverse and longitudinal pressures equilibrate in less than 0.25 fm/c.

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arXiv2010

Elliptic flow in proton-proton collisions at 7 TeV

The angular correlations measured in proton-proton collisions at 7 TeV are decomposed into contributions from back to back emission and elliptic flow. Modeling the dominant term in the correlation functions as a momentum conservation effect or as an effect of the initial transverse velocity of the source, the remaining elliptic flow component can be estimated. The elliptic flow coefficient extracted from the CMS Collaboration data is 0.04-0.08. No additional small-angle, ridge-like correlations are needed to explain the experimental data.

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arXiv2010

Hydrodynamic predictions for Pb+Pb collisions at 2.76 TeV

Using the newest data for pp scattering at the CERN Large Hadron Collider (LHC) combined with the Glauber model, we make hydrodynamic predictions for the soft hadronic observables planned to be measured in the forthcoming Pb+Pb collisions at 2.76 TeV.

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arXiv2010

Directed flow in ultrarelativistic heavy-ion collisions

We study the generation of directed flow in the hydrodynamic expansion of the hot matter formed in ultrarelativistic heavy-ion collisions at 200GeV. The experimentally observed negative directed flow in a wide range of central pseudorapidities isreproduced assuming that the fireball is tilted away from the collision axis. The tilt of the source is consistent with a preferential emission in the forward/backward hemisphere from forward/backward participating nucleons. The model reproduces the experimentally observed scaling of the directed flow when going from Au-Au to Cu-Cu systems.

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arXiv2010

Bulk and shear viscosities of matter created in relativistic heavy-ion collisions

We study the effects of the shear and bulk viscosities in the hadronic phase on the expansion of the fireball and on the particle production in relativistic heavy ion collisions. Comparing simulation with or without viscosity in the hadronic matter we find that elliptic flow observables strongly dependent on dissipative effects in the late stage. On the other hand, interferometry radii are sensitive, through the early transverse flow, on the value of the viscosity at high temperatures. We present first calculations including the effects of bulk viscosity in the hadronic phase and in the hadron emission. We find them important in obtaining a small freeze-out temperature consistent with the measured transverse momentum spectra and elliptic flow of identified particles.

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arXiv2010

Observation of the collective flow in proton-proton collisions

The scenario of a collective expansion of matter created in proton-proton (p-p) collisions at the CERN Large Hadron Collider (LHC) is discussed. Assuming a small transverse size and a formation time of 0.1fm/c of the source we observe the build up of a substantial transverse flow in relativistic hydrodynamic simulations. In order to demonstrate the collectivity in p-p collisions we propose to look at the multiplicity dependence of the elliptic flow coefficient. If high multiplicity events originate from azimuthally asymmetric events containing two flux tubes, an observable signal above the statistical fluctuations in the measured elliptic flow could appear.

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arXiv2009

Flow, spectra and HBT radii in heavy-ion collisions

The expansion of the fireball created in relativistic heavy ion collisions is described using the 3+1D hydrodynamical model. Experimentally observed transverse momentum spectra at different rapdities, elliptic flow and HBT correlations of produced particles can be reproduced. We give estimates of shear viscosity corrections at freeze-out, which we find important only for the elliptic flow coefficient.

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arXiv2009

Rapid hydrodynamic expansion in relativistic heavy-ion collisions

Hydrodynamic expansion of the hot fireball created in relativistic Au-Au collisions at 200GeV in 3+1-dimensions is studied. We obtain a simultaneous, satisfactory description of the transverse momentum spectra, elliptic flow and pion correlation radii for different collision centralities and different rapidities. Early initial time of the evolution is required to reproduce the interferometry data, which provides a strong indication of the early onset of collectivity. We can also constraint the shape of the initial energy density in the beam direction, with a relatively high initial energy density at the center of the fireball.

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arXiv2008

Interplay of the emission from thermal and direct sources in relativistic heavy ion collisions

The separation of the source created in ultrarelativistic heavy-ion collisions into a thermalized dense core and an outer mantle consisting of independent nucleon-nucleon collisions is discussed. Evidence for such a two component picture is found in transverse mass spectra of kaon, protons and antiprotons produced in Au-Au collisions at 200GeV. Estimates of the sizes of the thermal and direct sources are compared to models separating the interaction zone into a core and a corona, according to the density of participants or to the number of collisions. Consequences for the modeling of the dynamics of the small size, thermalized core are described. New initial conditions corresponding to the dense core lead to a stronger azimuthal asymmetry of the hydrodynamically expanding fireball, pressure gradients also increase. 2+1-dimensional hydrodynamic simulations are presented starting from all the matter in the interaction region or from the dense, thermal part of the source. We find faster transverse expansion and stronger elliptic flow for dense core initial conditions. For different impact parameters we find very similar spectra of the thermal part of the source and only adding particles emitted directly from nucleon-nucleon collisions in the corona the experimentally observed softening of the spectra with increasing impact parameter is reproduced. The elliptic flow is stronger for particles emitted from a source separated into a core and a corona.

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arXiv2008

GLISSANDO: GLauber Initial-State Simulation AND mOre

GLISSANDO is a Glauber Monte-Carlo generator for early-stages of relativistic heavy-ion collisions, written in c++ and interfaced to Root. Several models are implemented: the wounded-nucleon model, the binary collisions model, the mixed model, and the model with hot-spots. Subtleties of the distribution of nucleon in the nucleus are discussed. The original geometric distribution of sources in the transverse plane can be superimposed with a statistical distribution simulating the dispersion in the generated transverse energy in each individual collision. The program generates inter alia the fixed axes (standard) and variable-axes (participant) two-dimensional profiles of the density of sources in the transverse plane and their Fourier components. These profiles can be used in further analyses of physical phenomena, such as the the jet quenching, event-by-event hydrodynamics, or analysis of the elliptic flow and its fluctuations. Characteristics of the event (multiplicities, eccentricities, Fourier coefficients, etc.) are evaluated and stored in a file for further off-line studies. A number of scripts is provided for that purpose. Supplied variants of the code can also be used for the proton-nucleus and deuteron-nucleus collisions.

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arXiv2008

Early dissipation and viscosity

We consider dissipative phenomena due to the relaxation of an initial anisotropic local pressure in the fireball created in relativistic heavy-ion collisions, both for the Bjorken boost-invariant case and for the azimuthally symmetric radial expansion with boost-invariance. The resulting increase of the entropy can be counterbalanced by a suitable retuning of the initial temperature. An increase of the transverse collective flow is observed. The influence of the shear viscosity on the longitudinal expansion is also studied. Viscosity reduces the cooling rate from the longitudinal work and counteracts the pressure gradients that accelerate the longitudinal flow.

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arXiv2008

Viscosity and boost invariance at RHIC and LHC

We consider the longitudinal hydrodynamic evolution of the fireball created in a relativistic heavy-ion collision. Nonzero shear viscosity reduces the colling rate of the system and hinders the acceleration of the longitudinal flow. As a consequence, the initial energy density needed to reproduce the experimental data at RHIC energies is significantly reduced. At LHC energies, we expect that shear viscosity helps to conserve a Bjorken plateau in the rapidity distributions during the expansion.

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