Analysis Of Heavy-ion Collisions Research Articles

Nuclear viscosity estimated by dynamics of neck formation in the early stage of nuclear collision

The very early stage of the coalescence of two nuclei is studied and used to estimate the nuclear viscosity. The time evolution of the neck region has been simulated by the unified Langevin equation method, which is used in the analysis of heavy-ion collisions from the approaching stage to the fusion-fission stage. It is found that the transition from viscous to inertial coalescence that appeared in the neck growth of macroscopic drops can also be seen in the present simulation in nucleus–nucleus collisions. The dynamics of neck growth is analyzed in terms of the hydrodynamical formula and the viscosity coefficient of nuclear matter is estimated using the analogy of macroscopic drops.

Overview of intermittency analysis in heavy-ion collisions

In this paper, a search for power-law fluctuations with fractality and intermittency analysis to explore the QCD phase diagram and the critical point is summarized. Experimental data on self-similar correlations and fluctuations with respect to the size of phase–space volume in various high energy heavy-ion collisions are presented, with special emphasis on background subtraction and efficiency correction of the measurement. Phenomenological modeling and theoretical work on the subject are discussed. Finally, we highlight possible directions for future research.

Effect of non-uniform efficiency on higher-order cumulants in heavy-ion collisions * *Supported by National Key Research and Development Program of China (2018YFE0205200, 2018YFE0104700, 2020YFE0202002), Natural Science Foundation of China (11890711, 11890712, 12122505, 11828501, 11861131009), Anhui Provincial Natural Science Foundation (1808085J02) and Innovation Fund of Key Laboratory of Quark and Lepton Physics (QLPL2020P01)

We perform a systematic study on the effect of non-uniform track efficiency correction in higher-order cumulant analysis in heavy-ion collisions. Through analytical derivation, we find that the true values of cumulants can be successfully reproduced by the efficiency correction with an average of the realistic detector efficiency for particles with the same charges within each single phase space. The theoretical conclusions are supported by a toy model simulation by tuning the non-uniformity of the efficiency employed in the track-by-track efficiency correction method. The valid averaged efficiency is found to suppress the statistical uncertainties of the reproduced cumulants dramatically. Thus, usage of the averaged efficiency requires a careful study of phase space dependence. This study is important for carrying out precision measurements of higher-order cumulants in heavy-ion collision experiments at present and in future.

Bayesian analysis of heavy ion collisions with the heavy ion computational framework Trajectum

We introduce a model for heavy ion collisions named Trajectum, which includes an expanded initial stage with a variable free streaming velocity $v_{\rm fs}$ and a hydrodynamic stage with three varying second order transport coefficients. We describe how to obtain a Gaussian Emulator for this 20-parameter model and show results for key observables. This emulator can be used to obtain Bayesian posterior estimates on the parameters, which we test by an elaborate closure test as well as a convergence study. Lastly, we employ the optimal values of the parameters found in [1] to perform a detailed comparison to experimental data from PbPb and $p$Pb collisions. This includes both observables that have been used to obtain these values as well as wider transverse momentum ranges and new observables such as correlations of event-plane angles.

Influence of direct Urca on the r-mode spin down features of newborn neutron star pulsars

We perform a study on the time duration, final frequency, and exit temperature of the r-mode instability in newborn neutron stars of normal fluid composition under the assumption that the r-mode is the dominating spin-down mechanism. It is found that these spin-down features will vary widely depending on the star mass when direct Urca becomes operative. The occurrence of direct Urca is decided from the stiffness of the symmetry energy of the underlying equation of state, which is still uncertain. The change in the spin-down features in stars of different masses has been studied by computing the magnitude of the bulk viscosity due to the extent of direct Urca coverage in the core. The range for the variation of the symmetry energy stiffness, characterized by the slope parameter L, is decided from the tidal deformability data of the GW170817 event and the maximum mass constraint. The study is done by applying a family of equation of states obtained with the finite range simple effective interaction, constrained at low and intermediate densities by nuclear data and the flow analysis in heavy-ion collisions and at high densities by causality.

Reexamining an extended-mean-field approach in heavy-ion collisions near the Fermi energy

Static and dynamical aspects of nuclear systems are described through an extended time-dependent mean-field approach. The foundations of the formalism are presented, with highlights on the estimation of average values and their corresponding dispersions. In contrast to semiclassical transport models, the particular interest of this description lies on its intrinsic quantal character. The reliability of this approach is discussed by means of stopping-sensitive observables analysis in heavy-ion collisions in the range of 20 to 120 MeV per nucleon.

A general procedure for detector–response correction of higher order cumulants

We propose a general procedure for the detector–response correction (including efficiency correction) of higher order cumulants observed by the event-by-event analysis in heavy-ion collisions. This method makes use of the moments of the response matrix characterizing the property of a detector, and is applicable to a wide variety of response matrices such as those having non-binomial responses and including the effects of ghost tracks. A procedure to carry out the detector–response correction of realistic detectors is discussed. In test analyses, we show that this method can successfully reconstruct the cumulants of true distribution for various response matrices including the one having multiplicity-dependent efficiency.

Estimates of the collision symmetry planes in HADES experiment at GSI

Estimate of the collision symmetry planes is a crucial part of the anisotropic flow analysis in heavy-ion collisions. HADES experiment at GSI has different possibilities for symmetry plane estimation. In this letter different methods of the symmetry plane resolution calculation are compared and the differences are explained in terms of non-flow contribution.

Recent femtoscopy results from ALICE

ALICE, an experiment dedicated to the analysis of heavy-ion collisions is ideally suited for femtoscopic studies, with its excellent particle identification capabilities at low and intermediate momenta. It measures correlations of pions which provide crucial information on the size and the dynamics of the system. Recent results for femtoscopic radii from p+p, p–Pb and Pb–Pb collisions will be discussed with emphasis on similarities and differences between small and large systems. In particular the multiplicity and transverse momentum dependence of the radii will be discussed.

Statistics of initial density perturbations in heavy ion collisions and their fluid dynamic response

An interesting opportunity to determine thermodynamic and transport properties in more detail is to identify generic statistical properties of initial density perturbations. Here we study event-by-event fluctuations in terms of correlation functions for two models that can be solved analytically. The first assumes Gaussian fluctuations around a distribution that is fixed by the collision geometry but leads to non-Gaussian features after averaging over the reaction plane orientation at non-zero impact parameter. In this context, we derive a three-parameter extension of the commonly used Bessel-Gaussian event-by-event distribution of harmonic flow coefficients. Secondly, we study a model of N independent point sources for which connected n-point correlation functions of initial perturbations scale like 1/N^(n-1). This scaling is violated for non-central collisions in a way that can be characterized by its impact parameter dependence. We discuss to what extent these are generic properties that can be expected to hold for any model of initial conditions, and how this can improve the fluid dynamical analysis of heavy ion collisions.

Mode-by-mode fluid dynamics for relativistic heavy ion collisions

We propose to study the fluid dynamic propagation of fluctuations in relativistic heavy ion collisions differentially with respect to their azimuthal, radial and longitudinal wavelength. To this end, we introduce a background-fluctuation splitting and a Bessel–Fourier decomposition of the fluctuating modes. We demonstrate how the fluid dynamic evolution of realistic events can be built up from the propagation of individual modes. We describe the main elements of this mode-by-mode fluid dynamics, and we discuss its use in the fluid dynamic analysis of heavy ion collisions. As a first illustration, we quantify to what extent only fluctuations of sufficiently large radial wave length contribute to harmonic flow coefficients. We find that fluctuations of short wave length are suppressed not only due to larger dissipative effects, but also due to a geometrical averaging over the freeze-out hyper-surface. In this way, our study further substantiates the picture that harmonic flow coefficients give access to a coarse-grained version of the initial conditions for heavy ion collisions, only.

Two-particle correlations from RHIC to LHC with a new method based on the fourier transform

An extension of the study of two particle correlations used at RHIC to LHC energies (Pb + Pb at \( \sqrt {s_{NN} } \) = 5.5 TeV) is presented. The method is based on the two-dimensional Fourier Transform used to calculate the angular autocorrelation function in (η,ϕ) space. It does not require the identification of a leading particle and is, hence, optimized to the low-pt and medium-pt region at LHC where many minijets are overlapping. We discuss the feasibility of the analysis in heavy ion collisions at the LHC using HIJING and Pythia simulations.

Fine structure in the azimuthal transverse momentum correlations at $\sqrt{s_{NN}}=200$ GeV using the event shape analysis

The experimental results on transverse momentum azimuthal hadron correlations at RHIC have opened a rich field for parton energy loss analysis in heavy-ion collisions. Recently, a considerable amount of work has been devoted to study the shapes of the “away-side” jet which exhibit an interesting and unexpected “double hump” structure not observed in the analogous treatment of the pp data. Driven by the possibility that the latter result might just mean that such a structure exists already in the case of pp collisions, but that its relative intensity could be small, here we use the Event Shape Analysis to show that it is possible to identify and select well defined event topologies in pp collisions, among which a double hump structure for the away-side jet emerges. Using two shape parameters, the sphericity in the transverse plane and the recoil to analyze a sample of PYTHIA generated pp collisions at \(\sqrt{s_{NN}}=200\) GeV, we show that this structure corresponds to two jets emitted in the backward hemisphere. Finally, we show that Q-PYTHIA qualitatively reproduces the decrease in the yield of dijet events and the increase of the double hump structure in the away side observed in heavy-ion collisions. The implications for the treatment of parton energy loss in heavy-ion collisions are discussed.

Hydro-kinetic approach to relativistic heavy ion collisions

We develop a combined hydro-kinetic approach which incorporates a hydrodynamical expansion of the systems formed in $A+A$ collisions and their dynamical decoupling described by escape probabilities. The method corresponds to a generalized relaxation time (${\ensuremath{\tau}}_{\mathrm{rel}}$) approximation for the Boltzmann equation applied to inhomogeneous expanding systems; at small ${\ensuremath{\tau}}_{\mathrm{rel}}$ it also allows one to catch the viscous effects in hadronic component--hadron-resonance gas. We demonstrate how the approximation of sudden freeze-out can be obtained within this dynamical picture of continuous emission and find that hypersurfaces, corresponding to a sharp freeze-out limit, are momentum dependent. The pion ${m}_{T}$ spectra are computed in the developed hydro-kinetic model, and compared with those obtained from ideal hydrodynamics with the Cooper-Frye isothermal prescription. Our results indicate that there does not exist a universal freeze-out temperature for pions with different momenta, and support an earlier decoupling of higher ${p}_{T}$ particles. By performing numerical simulations for various initial conditions and equations of state we identify several characteristic features of the bulk QCD matter evolution preferred in view of the current analysis of heavy ion collisions at RHIC energies.

Heavy ion collisions: Correlations and Fluctuations in particle production

Correlations and fluctuations (the latter are directly related to the 2-particle correlations) is one of the important directions in analysis of heavy ion collisions. At the current stage of RHIC exploration, when the details matter, basically any physics question is addressed with help of correlation techniques. In this talk I start with a general introduction to the correlation and fluctuation formalism and discuss weak and strong sides of different type of observables. In more detail, I discuss the two-particle pTcorrelations/⟨pT⟩ fluctuations. In spite of not observing any dramatic changes in the event-by-event fluctuations with energy, which would indicate a possible phase transition, such correlations measurements remain an interesting and important subject, bringing valuable information. Lastly, I show how radial flow can generate characteristic azimuthal, transverse momentum and rapidity correlations, which could qualitatively explain many of recently observed phenomena in nuclear collisions.

Jet Physics in Heavy Ion Collisions with Compact Muon Solenoid detector at the LHC

The status of CMS jet simulations and physics analysis in heavy ion collisions is presented. Jet reconstruction and high-pT particle tracking in the high multiplicity environment of heavy ion collisions at the LHC using the CMS calorimetry and tracking system are described. The Monte Carlo tools used to simulate jet quenching are discussed.

Correlation functions and cumulants in elliptic flow analysis

We consider various methods of flow analysis in heavy ion collisions and compare experimental data on corresponding observables to the predictions of our saturation model proposed earlier [Nucl. Phys. A 708 (2002) 413]. We demonstrate that, due to the nature of the standard flow analysis, azimuthal distribution of particles with respect to reaction plane determined from the second order harmonics should always be proportional to cos2( φ− Ψ R ) independent of the physical origin of particle correlations (flow or non-flow). The amplitude of this distribution is always physical and proportional to v 2. Two-particle correlations analysis is, therefore, a more reliable way of extracting the shape of physical azimuthal anisotropy. We demonstrate that two-particle correlation functions generated in our minijet model of particle production [Nucl. Phys. A 708 (2002) 413] are in good agreement with the data reported by PHENIX. We discuss the role of non-flow correlations in the cumulant flow analysis and demonstrate using a simple example that if the flow is weak, higher order cumulants analysis does not significantly reduce the contribution of non-flow correlations to elliptic flow observable v 2 in RHIC data.

Partial wave contributions to the antikaon potential at finite momentum

The momentum dependence of the antikaon optical potential in nuclear matter is obtained from a microscopic and self-consistent calculation using the meson-exchange Jülich KN interaction. Two self-consistent schemes are discussed, which would lead to substantially different predictions for the width of K nuclear bound states. The effect of higher partial waves of the KN interaction, beyond the L=0 component, is studied and found to have moderate but nonnegligible effects on the K nuclear potential at zero momentum. At momenta as large as 500 MeV /c, relevant in the analysis of heavy-ion collisions, the higher partial partial waves modify the K optical potential by nearly a factor of two.

Large scale rapidity correlations in heavy ion collisions

We discuss particle production mechanisms for heavy ion collisions. We present an argument demonstrating how the fluctuations of the number of produced particles in a series of classical emissions can account for Koba-Nielsen-Olesen scaling. We predict rapidity correlations in the particle production in the event-by-event analysis of heavy ion collisions on the rapidity scales of the order of $1/{\ensuremath{\alpha}}_{s}.$

Hadronic matter compressibility from event-by-event analysis of heavy-ion collisions

We propose a method to measure the hadronic matter compressibility by means of the event-by-event analysis of heavy-ion collisions at high energies. The method, which utilizes the thermodynamical relation between the compressibility and the particle number fluctuations, requires a simultaneous measurement of the particle source size, temperature and particle multiplicity.