arXiv subjects
Publications and source records attributed to Piotr Bozek.
We briefly review highlights for ultra-relativistic light-heavy collisions (p-Pb, d-Au, 3He-Au, 12C-Au) which display collective evolution, with the same very characteristic features as in the A-A systems.
The decorrelation of the orientation of the event-plane angles in the initial state of relativistic Pb-Pb and p-Pb collisions, the "torque effect," is studied in a model of entropy deposition in the longitudinal direction involving fluctuations of the longitudinal source profile on large scales. The radiation from a single wounded nucleon is asymmetric in space-time rapidity. It is assumed that the extent in rapidity of the region of deposited entropy is random. Fluctuations in the deposition of entropy from each source increase the event-plane decorrelation: for Pb-Pb collisions they improve the description of the data, while for p-Pb collisions the mechanism is absolutely essential to generate any sizable decorrelation. We also show that the experimental data for rank-four flow may be explained via folding of the elliptic flow. The results suggest the existence of long range fluctuations in the space-time distribution of entropy in the initial stages of relativistic nuclear collisions.
We analyze correlations between the event-plane angles in different intervals of pseudorapidity within the 3+1-dimensional viscous hydrodynamics with the Glauber-model initial conditions. As predicted earlier, the fluctuations in the particle production mechanism in the earliest stage, together with asymmetry of the emission profiles in pseudorapidity from the forward- and backward going wounded nucleons, lead to the torque effect, namely, decorrelation of the event-plane angles in distant pseudorapidity bins. We use two- or three-bin measures of correlation functions to quantify the effect, with the latter compared to the recent data from the CMS collaboration. We find a sizable torque effect, with magnitude larger at RHIC than at the LHC.
The rapidity dependence of elliptic, $v_2$, and triangular, $v_3$, flow coefficients in proton-nucleus (p+A) collisions is predicted in hydrodynamics and in a multi-phase transport model (AMPT). We find that $v_n$ ($n=2,3$) on a nucleus side is significantly larger than on a proton side and the ratio between the two, $v_n^{\rm Pb}/v_n^{\rm p}$, weekly depends on the transverse momentum of produced particles.
Collective flow and femtoscopy in ultrarelativistic 3He-Au collisions are investigated within the 3+1-dimensional (3+1D) viscous event-by-event hydrodynamics. We evaluate elliptic and triangular flow coefficients as functions of the transverse momentum. We find the typical long-range ridge structures in the two-particle correlations in the relative azimuth and pseudorapidity, in the pseudorapidity directions of both Au and 3He. We also make predictions for the pionic interferometric radii, which decrease with the transverse momentum of the pion pair. All features found hint on collectivity of the dynamics of the system formed in 3He-Au collisions, with hydrodynamics leading to quantitative agreement with the up-to-now released data.
We investigate ultrarelativistic collisions of the 12C nucleus with heavy targets and show that the harmonic flow measures based on ratios of cumulant moments are particularly suited to study the intrinsic deformation of the 12C wave function. That way one can probe the expected alpha clusterization in the ground state, which leads to large initial triangularity in the shape of the fireball in the transverse plane. We show that the clusterization effect results in very characteristic behavior of the ratios of the cumulant moments as functions of the number of participant nucleons, both for the elliptic and triangular deformations. Thus the experimental event-by-event studies of harmonic flow in ultrarelativistic light-heavy collisions may offer a new window to look at the ground-state structure of light nuclei.
Femtoscopy is providing information on system size and its dynamics in heavy-ion collisions. At ultra-relativistic energies, such as those obtained at the LHC, significant production of pions, kaons and protons enables femtoscopic measurements for these particles. In particular the dependence of system size on pair momentum and particle type is interpreted as evidence for strong collective flow. Such phenomena are naturally modeled by hydrodynamics. We present calculations within the 3+1D hydrodynamic model coupled to statistical hadronization code THERMINATOR 2, corresponding to Pb--Pb collisions at $\sqrt{s_{\rm NN}}=$2.76 TeV. We obtain femtoscopic radii for pions, kaons, and protons, as a function of pair transverse momentum and collision centrality. We find that an approximate universal scaling of radii with pair transverse mass and final state event multiplicity is observed, and discuss the consequences for the interpretation of experimental measurements.
The triangular flow in ultrarelativistic $^3$He-Au collisions at RHIC energies is enhanced due to the triangular arrangement of the nucleon configurations in $^3$He. We study the fireball eccentricities in the Glauber Monte Carlo model and find that since the configurations of the projectile $^3$He are elongated triangles, the created fireball has a large ellipticity and a smaller triangularity. The dependence of the triangularity on centrality is weak, so it cannot be extracted from the centrality dependence of the triangular flow $v_3$, as it is dominated by the centrality dependence of the hydrodynamic response. We propose to look at the centrality dependence of the ratio $v_n\{4\}/v_n\{2\}$, where the uncertainties from the hydrodynamic response cancel, and show that the basic signature of the geometry-driven collective flow is the raise of the ratio $v_3\{4\}/v_3\{2\}$ with the number of participant nucleons for centralities less than 10%.
The femtoscopy correlation radii for d-Au collisions at $200$GeV are calculated in the hydrodynamic model and compared to PHENIX collaboration data. For asymmetric systems, such as d-Au or p-Pb collisions, the correlation radius $R_{out-long}$ is estimated and predicted to be non-zero. It appears due to different lifetimes of the parts of the fireball at positive and negative rapidities. Azimuthally sensitive Hanbury Brown-Twiss analysis with respect to the second order event plane shows a significant angular dependence of the radii. It reflects the strong azimuthal asymmetry of the d-Au source geometry and flow for central interactions.
The large density of matter in the interaction region of the proton-nucleus or deuteron-nucleus collisions enables the collective expansion of the fireball. Predictions of the hydrodynamic model for the asymmetric transverse flow are presented and compared to experimental data.
We discuss the use of the hydrodynamic model for the description of the evolution of dense matter formed in ultrarelativistic heavy-ion collisions. The collective flow observed in heavy-ion collisions at the BNL Relativistic Heavy Ion Collider and at the CERN Large Hadron Collider is consistent with the assumption that a fireball of strongly interacting matter is formed. Experimental results from p-Pb and d-Au collisions show similar phenomena, which suggests that collective expansion appears in small systems as well. We review the recent application of the hydrodynamic model to small systems and discuss limitations and possible further checks of this scenario.
We analyze the pion femtoscopy correlations in noncentral Au-Au and Pb-Pb collisions. The azimuthally sensitive Hanbury Brown-Twiss (HBT) method is used to extract the interferometry radii depending on the azimuthal angle with respect to the second and third-order event plane. The results are in semiquantitative agreement with the STAR collaboration data on the HBT radii with respect to the second-order reaction plane, with the preliminary PHENIX collaboration data on the HBT radii with respect to the third-order reaction plane in Au-Au collisions at 200GeV, and with the preliminary ALICE collaboration data for the HBT radii with respect to the second-order event plane for Pb-Pb collisions at 2.76TeV.
The formation and collective expansion of the fireball formed in ultrarelativistic p-A and d-A collisions is discussed. Predictions of the hydrodynamic model are compared to recent experimental results. The presence of strong final state interaction effects in the small dense systems is consistent with the observed azimuthal anisotropy of the flow and with the mass dependence of the average transverse momentum and of the elliptic flow. This raises the question of the mechanism explaining such a rapid build up of the collective flow and the large degree of local equilibration needed to justify this scenario.
We discuss eccentricities (ellipticity and triangularity) generated in nucleus-nucleus and proton-nucleus collisions. We define multi-particle eccentricities $ε_n\{m\}$ which are associated with the $n'th$ angular multipole moment for $m$ particles. We show that in the limit of fluctuation dominance all of the $ε_n\{m\}$'s are approximately equal for $m \ge 4$. For dynamics linearly responding to these eccentricities such as hydrodynamics or proposed in this paper weakly interacting field theory, these relations among eccentricities are translated into relations among flow moments $v_n\{m\}$. We explicitly demonstrate it with hydrodynamic calculations.
We review the signatures for the soft collective dynamics in highest-multiplicity ultrarelativistic p-Pb collisions and show that the effects are well described in a three-stage model, consisting of the event-by-event Glauber initial conditions, viscous 3+1D hydrodynamics, and statistical hadronization. In particular, we discuss the ridges in two-particle correlations in the relative azimuth and pseudorapidity, the elliptic and triangular flow coefficients, and the mass hierarchy of observables sensitive to flow.
We present an extended version of GLISSANDO, a Monte-Carlo generator for Glauber-like models of the initial stage of relativistic heavy-ion collisions. The increased functionality of the code incorporates a parametrization of shape of nuclei, including light nuclei needed in the NA61 experiment, the nuclear deformation, a possibility of using correlated distributions of nucleons in nuclei read from external files, an option of overlaying distributions of produced particles dependent on the space-time rapidity, the inclusion of the core-corona effect, or the output of the source distributions that can be used in event-by-event hydrodynamics. Together with other features, such as incorporation of various variants of Glauber models, or the implementation of a realistic NN collision profile, the generator offers a realistic and practical approach to describe the early phase of the collision in 3+1 dimensions; the predictions may later be used in modeling the intermediate evolution phase, e.g., with hydrodynamics. The software is integrated with the ROOT platform. The supplied scripts compute and plot numerous features of the distributions, such as the multiplicity distributions and centrality classes, harmonic asymmetry coefficients and their correlations, forward-backward correlations, etc. The code can also be used for the proton-nucleus and deuteron-nucleus collisions.
We study the mass dependence for identified-particle average transverse momentum and harmonic flow coefficients in proton-lead (p-Pb) collisions, recently measured at the LHC. The collective mechanism in the p-Pb system predicts a specific mass ordering in these observables: the growth of the average transverse momentum with the particle mass and a mass splitting of the elliptic flow coefficient, i.e., smaller differential elliptic flow of protons than pions for p_T<2 GeV. This provides an opportunity to distinguish between the collective scenario and the mechanism based on the initial gluon dynamics in the evolution of the p-Pb system.
Viscous hydrodynamics gives a satisfactory description of the transverse momentum spectra, of the elliptic and triangular flow, and of the femtoscopic correlations for particles produced in relativistic heavy-ion collisions. On general grounds, a similar collective behavior has been predicted for proton-lead (p-Pb) collisions at the LHC. We present results of the hydrodynamics calculation of the elliptic and triangular flow in p-Pb. We discuss the mass dependence of flow coefficients and of the average transverse momentum for identified particles.