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Boris Tomasik

Publications and source records attributed to Boris Tomasik.

At least 73 records · Page 4Linked to original sources
arXiv2008

The contribution of hard processes to elliptic flow

I study a possible effect of momentum deposition from many hard partons traversing the hot and dense region produced early in nuclear collisions at the LHC. The expected number of such hard partons is large. It is argued that the induced diffusion wakes which carry the momentum deposited to the medium may interact. Due to azimuthally asymmetric geometry in non-central collisions, this may lead to small preference of the collective flow in the direction of the impact parameter. As a result, a small contribution to the azimuthal asymmetry of hadronic spectra is obtained. It may be important to take even such a small contribution into account if quantitative conclusions about early thermalisation and low viscosity are to be made, based on the measurements of the elliptic flow parameter v2.

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arXiv2008

A simulation of hard spheres gas for educational purposes

We describe an educational simulation of some effects within the gas of hard spheres. The focus of the presented simulation is on the comprehension of random character of the velocity of molecules in the gas and of the energy at fixed temperature. It allows to point out the connection between the temperature, the energy, and the velocity of the molecules.

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arXiv2008

Interplay among the azimuthally dependent HBT radii and the elliptic flow

We present a calculation of the elliptic flow and azimuthal dependence of the correlation radii in the ellipsoidally symmetric generalization of the Buda-Lund model. The elliptic flow is shown to depend only on the flow anisotropy while in case of correlation radii both flow and space anisotropy play an important role in determining their azimuthal oscillation. We also outline a simple procedure for determining the parameters of the model from data.

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arXiv2008

Bulk Viscosity driven clusterization of quark-gluon plasma and early freeze-out in relativistic heavy-ion collisions

We introduce a new scenario for heavy ion collisions that could solve the lingering problems associated with the so-called HBT puzzle. We postulate that the system starts expansion as the perfect quark-gluon fluid but close to freeze-out it splits into clusters, due to a sharp rise of bulk viscosity in the vicinity of the hadronization transition. We then argue that the characteristic cluster size is determined by the viscosity coefficient and the expansion rate. Typically it is much smaller and independent of the total system volume. These clusters maintain the pre-existing outward-going flow, as a spray of droplets, but develop no flow of their own, and hadronize by evaporation. We provide an ansatz for converting the hydrodynamic output into clusters.

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arXiv2007

Fragmentation of the fireball and how to observe it

We argue that fragmentation at hadronisation is likely scenario in ultrarelativistic nuclear collisions. In case of crossover phase transition it is driven by a singularity of the bulk viscosity. We claim that such a scenario can explain the ``HBT puzzle'' and can be identified by non-statistical differences between event-wise rapidity distributions and by proton-proton rapidity correlations.

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arXiv2006

The K/pi ratio and the lifespan of the fireball

We construct a hadronic kinetic model for description of excitation functions of multiplicity ratios K+/pi+, K-/pi-, and Lambda/pi. It is shown that the model is able to describe data rather well under the assumption that the total lifespan of the fireball is decreasing function of the collision energy for SPS energies above 30 AGeV. Thus in order to identify the onset of deconfinement the proposed model will have to be checked against other kinds of data until hadronic description can be safely falsified.

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arXiv2006

Ultrarelativistic heavy-ion collisions: exploring the phase diagram of QCD

I present the motivation for studying nuclear collisions at ultrarelativistic energies which is to map the phase diagram of strongly interacting matter under very extreme conditions. The relevant experimental efforts are overviewed and problems in connecting observables with physics of the hot and dense matter are pointed out. I review the current highlights of the experimental programme: excitation functions of various quantities from the SPS which suggest a possibility of the onset of deconfinement, and the evidence of deconfined perfect fluid at RHIC.

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arXiv2006

Strangeness production time and the K+/pi+ horn

We construct a hadronic kinetic model which describes production of strange particles in ultrarelativistic nuclear collisions in the energy domain of SPS. We test this model on description of the sharp peak in the excitation function of multiplicity ratio K+/pi+ and demonstrate that hadronic model reproduces these data rather well. The model thus must be tested on other types of data in order to verify the hypothesis that deconfinement sets in at lowest SPS energies.

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arXiv2005

Strangeness dynamics in heavy-ion collisions: the K/pi ratios and the lifetime of a fireball

We propose a non-equilibrium, hadronic kinetic model for describing the relative abundance of strange particles in ultra-relativistic heavy-ion collisions. The energy dependence of the multiplicity ratios of charged kaons and lambdas to pions are studied in detail. The pronounced peak in positively charged K/pi is conjectured to be due to the decreasing lifetime of a fireball with an increase in beam energy above 30 AGeV. Such a behaviour may be the consequence of a stronger stopping at lower beam energies. We study the dependence of the multiplicity ratios on energy and baryon density of the fireball and its total lifetime.

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arXiv2005

Strangeness in ultrarelativistic nuclear collisions

A model for description of the s_{NN}^{1/2} dependence of K+/pi+ ratio at the CERN SPS and upper AGS energies is proposed. It uses hadronic degrees of freedom and the amount of produced strangeness is mainly controlled by the total lifetime of the fireball. Decreasing lifetime with increasing collision energy is conjectured.

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arXiv2005

Stopping and the K/pi horn

We propose a non-equilibrium hadronic model to interpret the observed excitation function of kaon-to-pion ratios in nuclear collisions at AGS and SPS energies. The crucial assumption of our model is that due to stronger stopping at lower energies the lifetime of the fireball is prolonged because the system has to build up the longitudinal expansion from internal pressure.

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arXiv2005

Azimthally sensitive femtoscopy and v2

I investigate the correlation between spatial and flow anisotropy in determining the elliptic flow and azimuthal dependence of the HBT correlation radii in non-central nuclear collisions. It is shown that the correlation radii are in most cases dominantly sensitive to the anisotropy in space. In case of v2, the correlation depends strongly on particle species. A procedure for disentangling the spatial and the flow anisotropy is proposed.

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arXiv2005

Disentangling spatial and flow anisotropy

Two generalisations of the blast-wave model to non-central nuclear collisions are constructed, and elliptic flow as well as azimuthal dependence of correlation radii are calculated. Particular attention is paid to how different azimuthal dependences of transverse flow direction can cause qualitatively different anisotropic fireballs to give same v_2 as a function of the transverse momentum. The simultaneous dependence of v_2 and the oscillation of correlation radii on both spatial and flow anisotropy is studied in great detail.

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arXiv2004

Collective dynamics of the fireball

I analyse the identified single-particle p_t spectra and two-pion Bose-Einstein correlations from RHIC. They indicate a massive transverse expansion and rather short lifetime of the system. The quantitative analysis in framework of the blast-wave model yields, however, unphysical results and suggests that the model may not be applicable in description of two-particle correlations. I then discuss generalisations of the blast-wave model to non-central collisions and the question how spatial asymmetry can be disentangled from flow asymmetry in measurements of v_2 and azimuthally sensitive HBT radii.

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arXiv2004

Effect of hard processes on momentum correlations

The effect of hard processes to be encountered in HBT studies at the Large Hadron Collider have been studied. A simple simulation has allowed us to generate momentum correlations involving jet particles as well as particles originating from the kinetic freeze out and to compare them to a simple theoretical model which has been developed. The first results on the effect of hard processes on the correlation function for the case of jet quenching are presented.

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arXiv2003

Understanding of the freeze-out in ultra-relativistic heavy-ion collisions

I discuss the quantities and effects important for the freeze-out and outline a formalism for the description of continuous decoupling of particles from the fireball. Then I present a calculation of the scattering rates of pions at various temperatures and argue that it is important to take continuous particle decoupling into account when modelling the collision dynamics.

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arXiv2003

The freeze-out mechanism and phase-space density in ultrarelativistic heavy-ion collisions

We explore the consequences of a freeze-out criterion for heavy-ion collisions, based on pion escape probabilities from the hot and dense but rapidly expanding collision region. The influence of the expansion and the scattering rate on the escape probability is studied. The temperature dependence of this scattering rate favors a low freeze-out temperature of ~100 MeV. In general, our results support freeze-out along finite four-volumes rather than sharp three-dimensional hypersurfaces, with high-pt particles decoupling earlier from smaller volumes. We compare our approach to the proposed universal freeze-out criteria using the pion phase-space density and its mean free path.

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