arXiv subjects

Piotr Bozek

Publications and source records attributed to Piotr Bozek.

At least 19 recordsLinked to original sources

arXiv2026

Wounded parton scaling of multiplicities in ultra-relativistic light- and heavy-ion collisions

Multiplicities of charged particles produced in O+O, Ne+Ne, Xe+Xe, and Pb+Pb collisions at $\sqrt{s_{NN}} \sim 5$~TeV are studied in a uniform way within a wounded parton Glauber framework with overlaid negative binomial fluctuations. In this model, the nucleon's inelastic interaction is modeled via its constituent partons, whose number is a parameter, with best description obtained with four partons per nucleon. We fit directly the experimental multiplicity distributions (histograms), using {\it the same model parameters} for each reaction. The fit is performed in the c=1--80 $\%$ centrality range. Avoiding the most peripheral events makes the method insensitive to the normalization issues caused by the difficulty in separating the Coulomb interactions, whereas the most central collisions may involve a different particle production mechanism. We find a proper model description of multiplicity distributions across all the studied systems for $c \lesssim 1\%$.

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arXiv2026

Accessing proton number fluctuations in limited regions of phase space

In heavy ion collisions particle distributions fluctuate from event to event. It is interesting to study local fluctuations of a specific particle specie, e.g. baryons, in the transverse plane. Fluctuations of the harmonic flow provide an integrated measure of such fluctuations. An alternative approach is to study proton number fluctuations measured in a relatively narrow azimuthal angle window. Due to the transverse flow, particles registered in a narrow azimuthal angle window are emitted predominantly from a region of the fireball located at the same azimuthal angle. Similarly, it is informative to measure the covariance of the number of protons emitted in two opposite windows of azimuthal angles. The main difficulty lies in distinguishing effects of local baryon density fluctuations in the transverse plane from global particle density and collective flow fluctuations. These global volume and flow fluctuations can be estimated using some reference particles, e.g. charged mesons. Reliable estimates of such fluctuations can be constructed both for the second factorial cumulant of the proton number in an azimuthal angle window and for the covariance between two such windows. The simultaneous measurement of both quantities provides a sensitive probe of local baryon number fluctuations superimposed on top of a fluctuating fireball background density.

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arXiv2024

Momentum dependent measures of correlations between mean transverse momentum and harmonic flow in heavy ion collisions

The correlation between the mean transverse momentum and the harmonic flow coefficients is an observable which is of great interest; it is sensitive to shape fluctuations in the initial state of a relativistic nuclear collision. The measurement of that correlation coefficient in central collisions allows one to infer about the intrinsic deformation of the colliding nuclei. We propose to study the momentum dependent covariance and correlation coefficient between the mean transverse momentum and the harmonic flow in a given transverse momentum bin. Two possible constructions of such observables are provided and predictions are obtained from a viscous hydrodynamic model. We find that such momentum dependent correlation coefficients between the mean transverse momentum and the harmonic flow show a strong and nontrivial momentum dependence. We also explore the effects of granularity (nucleon width) in the initial state, the nuclear deformation, and the shear viscosity on this momentum dependent correlation coefficient. The shape of the momentum dependence of the correlation coefficient for the triangular flow is found to be sensitive to the size of small scale fluctuations in the initial state. On the other hand, the shape of the momentum dependence of the covariance between the mean transverse momentum and the harmonic flow coefficients is found to be sensitive to the value of the shear viscosity and to the granularity of the initial state.

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arXiv2024

Higher order cumulants of transverse momentum and harmonic flow in relativistic heavy ion collisions

Higher order symmetric cumulants of global collective observables in heavy ion collisions are studied. The symmetric cumulants can be straightforwardly constructed for scalar observables: the average transverse momentum, the multiplicity, and the squares of harmonic flow vectors. Third and fourth order cumulants are calculated in the hydrodynamic model. A linear predictor of the average transverse momentum and harmonic flow coefficients in a collision is used to predict the value of the cumulants from the moments of the initial distribution. The symmetric cumulants divided by the averages (or the standard deviations) of the considered observables can be used as a fine tool to study correlations present in the initial state of the collision.

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arXiv2024

Charm balance function in relativistic heavy-ion collisions

We calculate the balance function for charm in relativistic heavy-ion collisions. The distribution of pairs of charm-anticharm quarks produced in hard processes in the early stages of the nucleus-nucleus collision evolves in the dense fireball formed in the collision. The evolution of the dense matter is described using a relativistic viscous hydrodynamic model and the quark diffusion with a Langevin equation. The evolution of the charm quark balance function from the formation of the charm-anticharm pair up to the freeze-out traces the partial thermalization of the heavy quarks in the dense matter. For the balance function in azimuthal angle we reproduce the collimation effect due to the transverse flow. The evolution in rapidity shows the thermalization of the longitudinal velocity of the quark in the fluid. We provide predictions for the one and two-dimensional balance functions for $D^0$-$\bar{D^0}$ mesons produced in ultarelativistic Pb+Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV. The shape of the charm balance function in relative rapidity is sensitive to the rescattering of heavy quarks in the early stages of the collision, while the shape of the balance function in azimuthal angle is sensitive to the rescattering in the latter stages.

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arXiv2023

Can we observe effects of early nonequilibrium dynamics using collective flow

The rapid expansion of the fireball created in a heavy-ion collision causes strong departures from local equilibrium. Such effects are especially important in the very early phase of the collision, bringing a substantial pressure asymmetry. We investigate effects of this early stage pressure asymmetry in a kinetic model without boost-invariance. In the kinetic evolution the pressure asymmetry results from the competition of the longitudinal expansion and the kinetic relaxation. Unlike the transverse flow, the directed flow is found to be very sensitive to the equilibration rate and the pressure anisotropy.

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arXiv2023

Random model of flow decorrelation

The collective flow generated in relativistic heavy-ion collisions fluctuates from event to event. The fluctuations lead to a decorrelation of flow vectors measured in separate bins in phase space. These effects have been measured in experiments and observed in numerical simulations in hydrodynamic models. We present a simple random model of flow decorrelation in pseudorapidity. Analytical expressions for the flow factorization breaking coefficients for flow vectors, flow vector magnitudes, and flow angles are derived. The model explains the relations between different factorization breaking coefficients found in experimental data and model simulations. In particular, it is found that the flow angle decorrelation constitutes about one half of the total flow vector decorrelation.

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arXiv2023

Momentum dependent flow correlations in deformed nuclei at collision energies available at the BNL Relativistic Heavy Ion Collider

Flow fluctuations in ultra-relativistic heavy-ion collision can be probed by studying the momentum dependent correlations or the factorization-breaking coefficients between flow harmonics in separate kinematic bins (transverse momentum or pseudorapidity). We study such factorization-breaking coefficients for collisions of deformed U+U nuclei to see the effect of the nuclear deformation on momentum dependent coefficients. We also study momentum dependent mixed-flow correlations for the isobar collision system : Ru+Ru and Zr+Zr, which have the same mass number but different nuclear structure, thus providing the ideal scenario to study nuclear deformation effect on such observables. We use the TRENTO + MUSIC model for simulations and event-by-event analysis of those observables. We find that these momentum dependent correlation coefficients are not only excellent candidates to probe the fluctuation in heavy-ion collision, but also show significant sensitivity to the nuclear deformation.

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arXiv2023

Interplay of longitudinal and transverse expansion in the kinetic dynamics of heavy-ion collisions

The early dynamics in heavy-ion collisions involves a rapid, far from equilibrium evolution. This early pre-equilibrium stage of the dynamics can be modeled using kinetic equations. The effect of this pre-equilibrium stage on final observables derived from transverse momenta of emitted particles is small. The kinetic equations in the relaxation time approximation for a non-boost invariant system are solved. The asymmetry of the flow with respect to the reaction plane at different rapidities is found to be very sensitive to the degree of non-equilibrium in the evolution. This suggests that the rapidity odd directed flow could be studied to identify the occurrence of non-equilibrium effects and to estimate the asymmetry of the pressure between the longitudinal and transverse directions in the collision.

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arXiv2022

Probing flow fluctuations through factorization breaking of harmonic flows in heavy-ion collisions

We study factorization-breaking coefficients between the momentum dependent and momentum averaged flow vectors to probe flow fluctuations caused by initial-state fluctuations in heavy-ion collision. The coefficients for the flow vector squared and flow magnitude squared could be used for the extraction of flow angle decorrelations. We compare our model results with preliminary experimental data. We also present the predictions for the momentum dependent correlation between mixed flow harmonics.

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arXiv2022

Splitting of proton-antiproton directed flow in relativistic heavy-ion collisions

The rapidity dependent directed flow of particles produced in a relativistic heavy ion collision can be generated in the hydrodynamic expansion of a tilted source. The asymmetry of the pressure leads to a build up of a directed flow of matter with respect to the collision axis. The experimentally observed ordering of the directed flow of baryons, pions and antibaryons can be described as resulting from the expansion of a baryon inhomogeneous fireball. An uneven distribution of baryons in the transverse plane leads to an asymmetry in the collective push for protons and antiprotons. Precise measurements of the collective flow as a function of rapidity could serve as a strong constraint on mechanism of baryon stopping in the early phase of the collision.

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arXiv2021

Factorization breaking for higher moments of harmonic flow

We study correlations between harmonic flow vectors squared measured at different transverse momenta. One of the flow harmonics squared is taken at a fixed transverse momentum and correlated to the momentum averaged harmonic flow squared of the same order. Such four particle correlators, dependent on transverse momentum, have been recently measured experimentally. Factorization coefficients based on the ratio of such four-particle correlators allow the independent measurement of the flow vector and flow magnitude factorization breaking coefficient. Moreover, the correlation of the angles of flow harmonics as a function of transverse momentum can be extracted. Results are compared to preliminary data of the ALICE Collaboration. We also present predictions for the momentum dependent factorization breaking coefficient between mixed flow harmonics. The correlators with squares of mixed harmonics can serve as a way to independently measure the flow vector, flow magnitude, and flow angle correlations, and could be used to gain additional information on the fluctuating initial state and the dynamics in heavy-ion collisions.

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arXiv2020

Correlation coefficient between harmonic and transverse flow in heavy-ion collisions

The correlation between the harmonic flow and the transverse flow in relativistic heavy ion collisions is calculated in the hydrodynamic model. The partial correlation coefficient, corrected for fluctuations of multiplicity, is compared to experimental data. Estimators of the final transverse and harmonic flow are used to predict the value of the correlation coefficient from the moments of the initial distribution. A good description of the hydrodynamic simulation results is obtained if the estimator for the final transverse flow, besides the most important transverse size and entropy, includes also the eccentricities.

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arXiv2019

Interplay of drag by hot matter and electromagnetic force on the directed flow of heavy quarks

Rapidity-odd directed flow in heavy ion collisions can originate from two very distinct sources in the collision dynamics i. an initial tilt of the fireball in the reaction plane that generates directed flow of the constituents independent of their charges, and ii. the Lorentz force due to the strong primordial electromagnetic field that drives the flow in opposite directions for constituents carrying unlike sign charges. We study the directed flow of open charm mesons $D^0$ and $\overline{D^0}$ in the presence of both these sources of directed flow. The drag from the tilted matter dominates over the Lorentz force resulting in same sign flow for both $D^0$ and $\overline{D^0}$, albeit of different magnitudes. Their average directed flow is about ten times larger than their difference. This charge splitting in the directed flow is a sensitive probe of the electrical conductivity of the produced medium. We further study their beam energy dependence; while the average directed flow shows a decreasing trend, the charge splitting remains flat from $\sqrt{s_{NN}}=60$ GeV to $5$ TeV.

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arXiv2019

GLISSANDO 3: GLauber Initial-State Simulation AND mOre..., ver. 3

We present ver. 3 of GLISSANDO, a versatile Monte-Carlo generator for Glauber-like models of the initial stages of ultra-relativistic heavy-ion collisions. The present version incorporates the wounded quark model, which is phenomenologically successful in reproducing multiplicities of particle production at RHIC and the LHC. Within this model, one can study the nucleon substructure fluctuation effects, recently explored in p-A collisions. In addition, the code includes the possibility of investigating collisions of light nuclei, such as $^{3}$He and $^{3}$H, or the $α$-clustered $^{7,9}$Be, $^{12}$C, and $^{16}$O, where the deformation of the intrinsic wave function influences the transverse shape of the initial state. The current version, being down-compatible, retains the functionality of the previous releases, such as incorporation of various variants of Glauber-like models, a smooth $NN$ inelasticity profile in the impact parameter obtained from a parametrization of experimental data, fluctuating strength of the entropy deposition, or realistic nuclear distributions of heavy nuclei with deformation. The code can provide output in the format containing the event-by-event source location, which may be further used in modeling the intermediate evolution phase, e.g., with hydrodynamics or transport models. The interface is simplified, such that in the control input file the user may supply only the very basic information, such as the collision energy, the mass numbers of the colliding nuclei, and the model type. GLISSANDO 3 is integrated with the CERN ROOT platform. The package includes numerous illustrative and useful ROOT scripts to compute and plot various results.

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arXiv2019

Elliptic flow in ultra-relativistic collisions with light polarized nuclei

Estimates for elliptic flow in collisions of polarized light nuclei with spin $j\ge1$ with a heavy nucleus are presented. In such collisions the azimuthal symmetry is broken via polarization of the wave function of the light nucleus, resulting in nonzero one-body elliptic flow coefficient evaluated relative to the polarization axis. Our estimates involve experimentally well known features of light nuclei, such as their quadrupole moment and the charge radius, yielding the one-body elliptic flow coefficient in the range from 1\% for collisions with the deuteron to 5\% for for collisions with $^{10}$B nucleus. Prospects of addressing the issue in the upcoming fixed-target experiment at the Large Hadron Collider are discussed.

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arXiv2018

Elliptic flow in ultra-relativistic collisions with polarized deuterons

Predictions are made for elliptic flow in collisions of polarized deuterons with a heavy nucleus. It is shown that the eccentricity of the initial fireball, evaluated with respect to the deuteron polarization axis perpendicular to the beam direction, has a substantial magnitude for collisions of highest multiplicity. Within the Glauber approach we obtain $\sim 7\%$ for the deuteron states with spin projection 0, and $\sim -3 \%$ for spin projection $\pm 1$. We propose to measure the elliptic flow coefficient as the second order harmonic coefficient in the azimuthal distribution of produced charged hadrons with respect to the fixed polarization axis. Collective expansion yields a value of the order of $1\%$ for this quantity, as compared to zero in the absence of polarization and/or collectivity. Such a vivid rotational symmetry breaking could be measured with the current experimental accuracy of the relativistic heavy-ion experiments. The effect has a fundamental significance for understanding the nature of dynamics in small systems, as its experimental confirmation would prove the presence of the shape-flow transmutation mechanism, typical of hydrodynamic expansion or rescattering in the later stages of the fireball evolution.

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arXiv2018

Angle and magnitude decorrelation in the factorization breaking of collective flow

The collective harmonic flow in heavy-ion collisions correlates particles at all transverse momenta to be emitted preferably some directions. The factorization breaking coefficient measures the small decorrelation of the flow harmonics at two different transverse momenta. Using the hydrodynamic model I study in details the decorrelation of the harmonic flow due to the flow angle and the flow magnitude decorrelation at two transverse momenta. The effect can be seen in experiment measuring factorization breaking coefficients for the square of the harmonic flow vector at two transverse momenta. The hydrodynamic model predicts that the decorrelation of the flow magnitudes is about one half of the decorrelation of the overall flow (combining flow angle and flow magnitude decorrelations). These results are consistent with the principal component analysis of correlators of flow vectors squared.

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