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Piotr Bozek

Publications and source records attributed to Piotr Bozek.

At least 37 records · Page 2Linked to original sources

New measures of longitudinal decorrelation of harmonic flow

Harmonic flow in relativistic heavy-ion collisions is observed in a broad range of rapidities, and the flow at different rapidities is correlated. However, fluctuations lead to a small decorrelation of the harmonic flow magnitudes and flow angles at different rapidities. Using a hydrodynamic model with Glauber Monte Carlo initial conditions we show that the flow angle decorrelation strongly depends on the flow magnitude in the event. We propose observables to measure this effect in experiment.

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Principal component analysis of the nonlinear coupling of harmonic modes in heavy-ion collisions

The principal component analysis of flow correlations in heavy-ion collisions is studied. The correlation matrix of harmonic flow is generalized to correlations involving several different flow vectors. The method can be applied to study the nonlinear coupling between different harmonic modes in a double differential way in transverse momentum or pseudorapidity. The procedure is illustrated with results from the hydrodynamic model applied to Pb+Pb collisions at $\sqrt{s}=2760$GeV. Three examples of generalized correlations matrices in transverse momentum are constructed corresponding to the coupling of $v_2^2$ and $v_4$, of $v_2v_3$ and $v_5$, or of $v_2^3$, $v_3^3$, and $v_6$. The principal component decomposition is applied to the correlation matrices and the dominant modes are calculated.

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Longitudinal decorrelation measures of flow magnitude and event-plane angles in ultra-relativistic nuclear collisions

We discuss the forward-backward correlations of harmonic flow in Pb+Pb collisions at the CERN Large Hadron Collider (LHC), applying standard multibin measures, as well as proposed here new measures. We illustrate the methods with hydrodynamic model simulations based on event-by-event initial conditions from the wounded quark model with asymmetric rapidity emission profiles. Within the model we examine independently the event-plane angle and the flow magnitude decorrelations. We find specific hierarchy between various flow decorrelation measures and confirm certain factorization relations. We find qualitative agreement of the model and the data from the ATLAS and CMS Collaborations.

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Azimuthal angle dependence of the charge imbalance from charge conservation effects

The experimental search for the chiral magnetic effect in heavy-ion collisions is based on charge dependent correlations between emitted particles. Recently, a sensitive observable comparing event-by-event distributions of the charge splitting projected on the directions along and perpendicular to the direction of the elliptic flow has been proposed. The results of a 3+1-dimensional hydrodynamic model show that the preliminary experimental data of the STAR Collaboration can be explained as due to background effects, such as resonance decays and local charge conservation in the particle production. A related observable based on the third order harmonic flow is proposed to further investigate such background effects in charge dependent correlations.

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Interferometry correlations in central p+Pb collisions

We present results on interferometry correlations for pions emitted in central p+Pb collisions at 5.02TeV in a 3+1 dimensional viscous hydrodynamic model with initial conditions from the Glauber Monte Carlo model. The correlation function is calculated as a function of the pion pair rapidity. The extracted interferometry radii show a weak rapidity dependence, reflecting the lack of boost invariance of the pion distribution. A cross-term between the $out$ and $long$ directions is found to be nonzero. The results obtained in the hydrodynamic model are in fair agreement with recent data of the ATLAS Collaboration.

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Transverse Momentum Fluctuations and Correlations

We study fluctuations and correlations of the average transverse momentum of particles emitted in heavy-ion collisions. Fluctuations of the average transverse momentum are related to event-by-event fluctuations of the size and entropy of the initial source. Hydrodynamic calculations using a Glauber model with quark degrees of freedom reproduce the data. We study correlations of the average transverse momentum in different rapidity bins. We propose a definition of the observable that can be directly related to correlations of the collective flow variables.

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Pseudorapidity profile of transverse momentum fluctuations in heavy ion collisions

We investigate pseudorapidity correlations of the average transverse flow of particles emitted in relativistic heavy-ion collisions. We employ 3+1 dimensional viscous relativistic hydrodynamics with initial conditions from the quark Glauber Monte Carlo model to confront the recent measurements on the pseudorapidity correlations of the transverse momentum fluctuations in Pb+Pb collisions at $\sqrt{s_{NN}}=2760$GeV. We find good agreement between the model predictions and data. Further, we study two other observables build with the covariance of the average transverse momentum in different rapidity bins. These observables have better stability under various systematics, thus allowing for a robust comparison between data and model. The transverse flow-transverse flow correlation coefficient is directly related to correlations of the underlying collective flow at different pseudorapidities. The 3-bin measure of $p_T$ factorization breaking in pseudorapidity gives an estimate of possible decorrelation of the average transverse flow in the longitudinal direction.

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Effect of bulk viscosity on interferometry correlations in ultrarelativistic heavy-ion collisions

A temperature dependent bulk viscosity coefficient is used in 3+1D hydrodynamic simulations. I study the effect of the increase of bulk viscosity around the critical temperature on the system dynamics in central Pb+Pb collisions at $\sqrt{s_{NN}}=2760$~GeV. With increasing bulk viscosity the life-time of the system increases slightly. Also the shape of the freeze-out hypersurface changes, the outer layers of the fireball live longer. This effect causes a small reduction of the ratio of two interferometry radii $R_{out}/R_{side}$, improving the agreement with experimental data.

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Transverse momentum fluctuations in ultrarelativistic Pb+Pb and p+Pb collisions with wounded quarks

We analyze the phenomenon of size-flow transmutation in ultrarelativistic nuclear collisions in a model where the initial size fluctuations are driven by the wounded quarks and the collectivity is provided by viscous hydrodynamics. It is found that the model properly reproduces the data for the transverse momentum fluctuations measured for Pb+Pb collisions at 2.76TeV by the ALICE Collaboration. The agreement holds for a remarkably wide range of centralities, from 0-5% up to 70-80%, and displays a departure from a simple scaling with $(dN_{\rm ch}/dη)^{1/2}$ in the form seen in the data. The overall agreement in the model with wounded quarks is significantly better than with nucleon participants. This feature joins the previously-found wounded quark multiplicity scaling in the argumentation in favor of subnucleonic degrees of freedom in the early dynamics. We also examine in detail the correlations between measures of the initial size and final average transverse momentum of hadrons. Predictions are made for the transverse momentum fluctuations in p+Pb collisions at 5.02TeV.

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Wounded quarks at the LHC

We review the results of the wounded quark model, with a stress on eccentricity observables in small systems. A new element is a presentation of symmetric cumulants for the elliptic and triangular flow correlations, obtained in the wounded-quark approach.

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Longitudinal correlations in the initial stages of ultra-relativistic nuclear collisions

In this talk we review some of our results for the longitudinal correlations in connection with recent experimental data, in particular for the {\em torque} effect and for the two-particle correlations in pseudorapidity, $C(η_1,η_2)$. The model framework involves event-by-event fluctuations of the initial conditions, followed with 3D viscous hydrodynamic evolution and statistical hadronization.

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On wounded constituents in nuclear collisions

In this talk we summarize the main results of our recent paper, Phys. Rev. C94, 014902, where we explore predictions of the wounded quark model for particle production and the properties of the initial state formed in ultra-relativistic collisions of atomic nuclei.

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Wounded quarks in A+A, p+A, and p+p collisions

We explore predictions of the wounded quark model for particle production and properties of the initial state formed in ultrarelativistic heavy-ion collisions. The approach is applied uniformly to A+A collisions in a wide collision energy range, as well as for p+A and p+p collisions at the CERN Large Hadron Collider (LHC). We find that generically the predictions from wounded quarks for such features as eccentricities or initial sizes are close (within 15\%) to predictions of the wounded nucleon model with an amended binary component. A larger difference is found for the size in p+Pb system, where the wounded quark model yields a smaller (more compact) initial fireball than the standard wounded nucleon model. The inclusion of subnucleonic degrees of freedom allows us to analyze p+p collisions in an analogous way, with predictions that can be used in further collective evolution. The approximate linear dependence of particle production in A+A collisions on the number of wounded quarks, as found in previous studies, makes the approach based on wounded quarks natural. Importantly, at the LHC energies we find approximate uniformity in particle production from wounded quarks, where at a given collision energy per nucleon pair similar production of initial entropy per source is needed to explain the particle production from p+p collisions up to A+A collisions. We also discuss the sensitivity of the wounded quark model predictions to distribution of quarks in nucleons, distribution of nucleons in nuclei, and to the quark-quark inelasticity profile in the impact parameter. In our procedure, the quark-quark inelasticity profile is chosen in such a way that the experiment-based parametrization of the proton-proton inelasticity profile is properly reproduced. The parameters of the overlaid multiplicity distribution is fixed from p+p and p+Pb data.

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Rapidity fluctuations in the initial state

We analyze two-particle pseudorapidity correlations in a simple model, where strings of fluctuating length are attached to wounded nucleons.The obtained straightforward formulas allow us to understand the anatomy of the correlations, i.e., to identify the component due to the fluctuation of the number of wounded nucleons and the contribution from the string length fluctuations. Our results reproduce qualitatively and semiquantitatively the basic features of the recent correlation measurements at the LHC.

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Transverse momentum-flow correlations in relativistic heavy-ion collisions

The correlation between the transverse momentum and the azimuthal asymmetry the flow is studied. A correlation coefficient is defined between the average transverse momentum of hadrons emitted in an event and the square of the elliptic or triangular flow coefficient. The hydrodynamic model predicts a positive correlation of the transverse momentum with the elliptic flow, and almost no correlation with the triangular flow in Pb-Pb collisions at LHC energies. In p-Pb collisions the new correlation observable is very sensitive to the mechanism of energy deposition in the first stage of the collision.

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A simple model for rapidity fluctuations in the initial state of ultra-relativistic heavy-ion collisions

Two-particle pseudorapidity correlations are analyzed in a simple model, where in the initial stage of the reaction multiple sources, extended in rapidity, are created. We show how the fluctuations of the length of the sources in rapidity generate correlations in the initial entropy deposition, which later contribute to the observed longitudinal correlations in hadron production. Our analysis, which is analytic and leads to straightforward formulas, allows us to understand the structure of the correlations, in particular to identify the component related to the fluctuation of the numbers of sources and the component from the length fluctuations. We also present the results in terms of the expansion in the basis of the Legendre polynomials. A number of further effects is discussed, such as smearing of the pseudorapidity distributions or resonance decays. Our results reproduce qualitatively and semiquantitatively the basic features of the recent measurements at the LHC.

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Two-particle correlations in pseudorapidity in a hydrodynamic model

Two-particle pseudorapidity correlations of hadrons produced in Pb+Pb collisions at 2.76 TeV at the CERN Large Hadron Collider are analyzed in the framework of a model based on viscous 3+1-dimensional hydrodynamics with the Glauber initial condition. Based on our results, we argue that the correlation from resonance decays, formed at a late stage of the evolution, produce significant effects. In particular, their contribution to the event averages of the coefficients of the expansion in the Legendre basis explain 60-70% of the experimental values. We have proposed an accurate way to compute these coefficients, independent of the binning in pseudorapidity, and tested a double expansion of the two-particle correlation function in the azimuth and pseudorapidity, which allows us to investigate the pseudorapidity correlations between harmonics of the collective flow. In our model, these quantities are also dominated by non-flow effects from the resonance decays. Finally, our method can be used to compute higher-order cumulants for the expansion in orthonormal polynomials which offers a suitable way of eliminating the non-flow effects from the correlation analyses.

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Multi-particle long-range rapidity correlations from fluctuation of the fireball longitudinal shape

We calculate the genuine long-range multi-particle rapidity correlation functions, $C_{n}(y_1,...,y_n)$ for $n=3,4,5,6$, originating from fluctuations of the fireball longitudinal shape. In these correlation functions any contribution from the short-range two-particle correlations, and in general up to $(n-1)$-particle in $C_n$, is suppressed. The information about the fluctuating fireball shape in rapidity is encoded in the cumulants of coefficients of the orthogonal polynomial expansion of particle distributions in rapidity.

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