Ultrarelativistic Nuclear Collisions Research Articles

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 parton model, which is phenomenologically successful in reproducing multiplicities of particle production at the 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 3He and 3H, or the α-clustered 7,9Be, 12C, and 16O, 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 parameterization 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. Program summaryProgram Title:GLISSANDO 3 ver. 3.42Program Files doi:http://dx.doi.org/10.17632/yfrzxkrcvr.1Licensing provisions: CC By 4.0Programming language:C++ with the ROOT librariesNature of problem: The code implements in a versatile way the Glauber modeling of the initial stages of ultra-relativistic nuclear collisions, including the wounded nucleon and wounded quark models, with possible admixture of binary collisions. A state-of-the art inelastic nucleon–nucleon collision profile is implemented. A statistical distribution of the strength of the sources can be overlaid. The α clustered structure of light nuclei is built in.Solution method: Monte-Carlo simulation of nuclear collisions, analyzed off-line with numerous ROOT scripts. The software allows for a straightforward event-by-event analysis of eccentricity coefficients and their correlations, size fluctuations, or multiplicity distributions.Additional comments including restrictions and unusual features: The input may consist only of the model type, the mass numbers of the nuclei and the collision energy. The output can also be used as initial conditions for further hydrodynamic studies.

The thermal proton yield anomaly in Pb-Pb collisions at the LHC and its resolution

We propose a resolution of the discrepancy between the proton yield predicted by the statistical hadronization approach and data on hadron production in ultra-relativistic nuclear collisions at the LHC. Applying the S-matrix formulation of statistical mechanics to include pion-nucleon interactions, we reexamine their contribution to the proton yield, taking resonance widths and the presence of nonresonant correlations into account. The effect of multi-pion-nucleon interactions is estimated using lattice QCD results on the baryon-charge susceptibility. We show that a consistent implementation of these features in the statistical hadronization model, leads to a reduction of the predicted proton yield, which then quantitatively matches data of the ALICE collaboration for Pb-Pb collisions at the LHC.

Fast resonance decays in nuclear collisions

In the context of ultra-relativistic nuclear collisions, we present a fast method for calculating the final particle spectra after the direct decay of resonances from a Cooper–Frye integral over the freeze-out surface. The method is based on identifying components of the final particle spectrum that transform in an irreducible way under rotations in the fluid-restframe. Corresponding distribution functions can be pre-computed including all resonance decays. Just a few of easily tabulated scalar functions then determine the Lorentz invariant decay spectrum from each space-time point, and simple integrals of these scalar functions over the freeze-out surface determine the final decay products. This by-passes numerically costly event-by-event calculations of the intermediate resonances. The method is of considerable practical use for making realistic data to model comparisons of the identified particle yields and flow harmonics, and for studying the viscous corrections to the freeze-out distribution function.

Forward-backward multiplicity fluctuations in ultrarelativistic nuclear collisions with wounded quarks and fluctuating strings

We analyze a generic model where wounded quarks are amended with strings in which both end-point positions fluctuate in spatial rapidity. With the assumption that the strings emit particles independently of one another and with a uniform distribution in rapidity, we are able to analyze the model semi-analytically, which allows for its detailed understanding. Using as a constraint the one-body string emission functions obtained from the experimental data for collisions at $\sqrt{s_{NN}}=200$ GeV, we explore the two-body correlations for various scenarios of string fluctuations. We find that the popular measures used to quantify the longitudinal fluctuations are limited with upper and lower bounds and assume close values for the most likely models of the end-point distributions, which may explain why various approaches yield here very similar results.

Partial Correlation Analysis in Ultra-relativistic Nuclear Collisions

We show that the method of partial covariance is a very efficient way to introduce constraints (such as the centrality selection) in data analysis in ultra-relativistic nuclear collisions. The technique eliminates spurious event-by-event fluctuations of physical quantities due to fluctuations of control variables. Moreover, in the commonly used superposition approach to particle production the method can be used to impose constraints on the initial sources rather than on the finally produced particles, thus separating out the trivial fluctuations from statistical hadronization or emission from sources and focusing strictly on the initial-state physics. As illustration, we use simulated data from hydrodynamics started on the wounded-quark event-by-event initial conditions, followed with statistical hadronization, to show the practicality of the approach in analyzing the forward-backward multiplicity fluctuations. We mention generalizations to the case with several constraints and other observables, such as the transverse momentum or eccentricity correlations.

Correlations with Fluctuating Strings

We present a semi-analytic approach to forward-backward multiplicity correlations in ultra-relativistic nuclear collisions, based on particle emission from strings with fluctuating end-points. We show that with the constraints from rapidity spectra, one can obtain bounds for the magnitude of the standard measures of the forward-backward fluctuations. The method is generic under the assumption of independent production from sources. For definiteness, we use the wounded quark model for Au+Au and d+Au collisions at the energy of $\sqrt{s_{NN}}=200$ GeV.

Strongly intensive fluctuations and correlations in ultrarelativistic nuclear collisions in the model with string fusion

The several types of strongly intensive correlation variables are studied in nuclear collisions at LHC energy. These quantities are expected not to depend on centrality class width. They have been calculated in the dipole-based parton-string Monte Carlo model with string fusion. The centrality dependence of the mean transverse momentum correlation coefficient and strongly intensive quantity Σ between multiplicity and PT have been obtained. Dynamical charge fluctuation vdyn has been also calculated and compared with experimental data. It is shown that string fusion improves agreement with the experiment.

Inverting the mass hierarchy of jet quenching effects with prompt b-jet substructure

In these proceedings, we discuss the role of heavy quark mass on the formation of parton showers. Mass effects are not well understood when parton branching occurs in nuclear matter, such as the quark-gluon plasma. Recently, a theoretically consistent picture of open heavy flavor production in ultra relativistic nuclear collisions has begun to emerge based on effective theories of QCD, such as soft collinear effective theory with Glauber gluons. We show that implementation of in-medium splitting processes containing heavy quarks into next-to-leading order calculations of heavy flavor production leads to larger cross section suppression when compared to traditional energy loss phenomenology. To better constrain the important mass dependence of in-medium splitting functions, we propose a new measurement in relativistic heavy ion collisions, based on a two-prong structure inside a reconstructed heavy flavor jet. In the region of jet transverse momenta where parton mass effects are leading, we predict a unique reversal of the mass hierarchy of jet quenching effects in heavy ion relative to proton collisions. We find that the momentum sharing distribution of prompt b-tagged jets is more strongly modified in comparison to the one for light jets. The work summarized here opens new directions of research on the substructure of heavy flavor jets.

Longitudinal decorrelation measures of flow magnitude and event-plane angles in ultrarelativistic 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.

Signatures of α clustering in ultrarelativistic collisions with light nuclei

We explore possible observable signatures of $\alpha$ clustering of light nuclei in ultra-relativistic nuclear collisions involving ${}^{7,9}$Be, ${}^{12}$C, and ${}^{16}$O. The clustering leads to specific spatial correlations of the nucleon distributions in the ground state, which are manifest in the earliest stage of the ultra-high energy reaction. The formed initial state of the fireball is sensitive to these correlations, and the effect influences, after the collective evolution of the system, the hadron production in the final stage. Specifically, we study effects on the harmonic flow in collisions of light clustered nuclei with a heavy target (${}^{208}$Pb), showing that measures of the elliptic flow are sensitive to clusterization in ${}^{7,9}$Be, whereas triangular flow is sensitive to clusterization in ${}^{12}$C and ${}^{16}$O. Specific predictions are made for model collisions at the CERN SPS energies. In another exploratory development we also examine the proton-beryllium collisions, where the $3/2^-$ ground state of ${}^{7,9}$Be nucleus is polarized by an external magnetic field. Clusterization leads to multiplicity distributions of participant nucleons which depend on the orientation of the polarization with respect to the collision axis, as well as on the magnetic number of the state. The obtained effects on multiplicities reach a factor of a few for collisions with a large number of participant nucleons.

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 $(dN_{\rm ch}/d\eta)^{1/2}$ 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.

Azimuthal pion fluctuation in ultra relativistic nuclear collisions and centrality dependence—a study with chaos based complex network analysis

Various works on multiplicity fluctuation have investigated the dynamics of particle production process and eventually have tried to reveal a signature of phase transition in ultra-relativistic nuclear collisions. Analysis of fluctuations of spatial patterns has been conducted in terms of conventional approach. However, analysis with fractal dynamics on the scaling behavior of the void has not been explored yet. In this work we have attempted to analyze pion fluctuation in terms of the scaling behavior of the void probability distribution in azimuthal space in ultra-relativistic nuclear collisions in the light of complex networks. A radically different and rigorous method viz. Visibility Graph was applied on the data of 32S-Ag/Br interaction at an incident energy of 200 GeV per nucleon. The analysis reveals strong scaling behavior of void probability distributions in azimuthal space and a strong centrality dependence.

Influence of the hadronic phase on observables in ultrarelativistic heavy ion collisions

The hadronic phase in ultrarelativistic nuclear collisions has a large influence on final state observables like multiplicity, flow and $p_t$ spectra, as studied in the UrQMD approach. In this model one assumes that a non-equilibrium decoupling phase follows a fluid dynamical description of the high density phase. Hadrons are produced assuming local thermal equilibrium and dynamically decouple during the hadronic rescattering until the particles are registered in the detectors. This rescattering of hadrons modifies every hadronic bulk observable. The apparent multiplicity of resonances is suppressed as compared to a chemical equilibrium freeze-out model, because the decay products rescatter. Therefore the resonances, which decay in the early hadronic phase, cannot be identified anymore by the invariant mass method. Stable and unstable particles change their momentum distribution by more than 30$\%$ through rescattering and their multiplicity is modified by resonance production and annihilation on a similar magnitude. These findings show that it is all but trivial to conclude from the final state observables on the properties of the system at an earlier time where it may have been in local equilibrium.

A new approach of chaos and complex network method to study fluctuation and phase transition in nuclear collision at high energy

In the endeavour to study fluctuation and a signature of phase transition in ultrarelativistic nuclear collision during the process of particle production, an approach based on chaos and complex network is proposed. In this work we have attempted an exhaustive study of pion fluctuation in $\eta$ space, $\phi$ space, their cross-correlation and finally two-dimensional fluctuation in terms of scaling of void probability distribution. The analysis is done on the $\eta$ values and their corresponding $ \phi$ values extracted from the 32S-Ag/Br interaction at an incident energy of 200GeV per nucleon. The methods used are Multifractal Detrended Cross-Correlation Analysis (MF-DXA) and a chaos-based rigorous complex network method --Visibility Graph. The analysis reveals that the highest degree of cross-correlation between pseudorapidity and azimuthal angles exists in the most central region of the interaction. The analysis further shows that two-dimensional void distribution corresponding to the $\eta$ - $\phi$ space reveals a strong scaling behaviour. Both cross-correlation coefficients of MF-DXA and PSVG (Power of the Scale-freeness in Visibility Graph, which is implicitly connected with the Hurst exponent) can be effectively used for the quantitative assessment of pion fluctuation in a very precise manner and have the capability to assess the tendency of approaching criticality for phase transitions.

Global hyperon polarization at local thermodynamic equilibrium with vorticity, magnetic field, and feed-down

The system created in ultrarelativistic nuclear collisions is known to behave as an almost ideal liquid. In non-central collisions, due to the large orbital momentum, such a system might be the fluid with the highest vorticity ever created under laboratory conditions. Particles emerging from such a highly vorticous fluid are expected to be globally polarized with their spins on average pointing along the system angular momentum. Vorticity-induced polarization is the same for particles and antiparticles, but the intense magnetic field generated in these collisions may lead to the splitting in polarization. In this paper we outline the thermal approach to the calculation of the global polarization phenomenon for particles with spin and we discuss the details of the experimental study of this phenomenon, estimating the effect of feed-down. A general formula is derived for the polarization transfer in two-body decays and, particularly, for strong and electromagnetic decays. We find that accounting for such effects is crucial when extracting vorticity and magnetic field from the experimental data.

On the possibility of thermalization of heavy mesons in ultrarelativistic nuclear collisions

The phenomenological analysis and interpretation of experimental data from RHIC and LHC on the production of J/ψ and D mesons in heavy-ion collisions are performed within the two-component HYDJET++ model including the thermal and hard mechanisms of hadron production. It is shown that the thermal freeze-out of charmed mesons at RHIC energies occurs earlier than the thermal freeze-out of light hadrons (assumingly, simultaneously with chemical freeze-out), which indicates that J/ψ and D mesons are not in kinetic equilibrium with the formed hadronic matter. At the same time, a significant part of D mesons at LHC energies are in kinetic equilibrium with the formed thermalized matter, but J/ψ mesons are still characterized by early freeze-out.

Recent development on heavy quark theory in heavy-ion collisions

A brief overview is presented for the theory of open heavy flavor dynamics in ultra-relativistic nuclear collisions, including a summary of different transport models, recent development on heavy quark phenomenology, in particular possible solutions to the “heavy vs. light flavor” puzzle and the “RAA vs. v2” puzzle in the field.

Flow anisotropy due to momentum deposition in ultra-relativistic nuclear collisions

Minijets and jets are produced in large numbers in nuclear collisions at TeV energies, so that there are many of them in a single fireball. They deposit non-negligible amount of momentum and energy into the hydrodynamically expanding bulk and cause anisotropies of the expansion. Moreover, due to their multiple production in a single event the resulting anisotropies are correlated with the collision geometry and thus contribute positively also to event-averaged anisotropies in non-central collisions. Using simulations with three-dimensional ideal hydrodynamic model we demonstrate the importance of this effect. It must be taken into account if conclusions about the properties of the hot matter are to be drawn.

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, $C(\eta_1,\eta_2)$. The model framework involves event-by-event fluctuations of the initial conditions, followed with 3D viscous hydrodynamic evolution and statistical hadronization.

Multiplicity fluctuation and phase transition in high-energy collision — A chaos-based study with complex network perspective

Multiplicity fluctuation provides enough information concerning the dynamics of particle production process and even signature of phase transition from hadronic to QGP phase expected in ultrarelativistic nuclear collision. Numerous analyses reported on the fluctuation pattern of pions have been studied from theoretical and phenomenological approaches. Also the fractal properties have been explored to characterize quantitative degree of fluctuation. The present work reports a study of pion fluctuation from a radically different perspective, using science of complexity. For this we have taken two different interactions — one hadron–nucleus and other nucleus–nucleus, namely [Formula: see text]-AgBr (350 GeV) and [Formula: see text]S-AgBr (200 A[Formula: see text]GeV). We have analyzed both data in the light of complex network analysis, viz. visibility graph method. The data reveal that power of the scale-freeness in visibility graph (PSVG), a quantitative parameter related to Hurst exponent, may provide information on the degree of fluctuation. Further, in a recent work, it was shown that phase transition can also be studied using the same methodology. Based on the result of the present study we further propose to use this methodology, where critical phenomena are to be assessed — even in case of pion fluctuation, for obtaining the QGP like phase transition.