Heavy-ion Collision Experiments Research Articles

Anatomy of octupole correlations in 96Zr with a symmetry-restored multidimensionally-constrained covariant density functional theory

A recent analysis based on the STAR measurement in relativistic heavy-ion collision experiments provides evidence of octupole correlation in the ground state of 96Zr, the description of which presents a challenge to nuclear structure models. In this work, we perform projection-after-variation calculations for 96Zr based on a multidimensionally-constrained relativistic Hartree-Bogoliubov model. Our results show that an octupole deformed shape is favored in energy after symmetry restoration, and this phenomenon cannot be reproduced in the pure mean-field calculations. These complex structures originate from the competition among various shell structures in this mass region. Our results suggest that the allowance of symmetry breaking at the mean-field level and the restoration of broken symmetries are essential elements for understanding the structure of 96Zr in density functional theories.

Dirac sea effects on heavy quarkonia decay widths in magnetized matter: A field theoretical model of composite hadrons

We study the partial decay widths of charmonium (bottomonium) states to $D\bar D (B\bar B)$ mesons in magnetized (nuclear) matter using a field theoretic model of composite hadrons with quark (and antiquark) constituents. These are computed from the mass modifications of the decaying and produced mesons, including the nucleon Dirac sea effects within a chiral effective model. The Dirac sea contributions are observed to lead to an rise (drop) in the quark condensates as the magnetic field is increased, an effect called the (inverse) magnetic catalysis. In the presence of a magnetic field, there are mixings of spin 0 (pseudoscalar) and spin 1 (vector) states (PV mixing) The nucleon Dirac sea effects are observed to be significant and the anomalous magnetic moments (AMMs) of the nucleons are observed to play an important role. For $\rho_B$=0, there is observed to be magnetic catalysis (MC) without and with AMMs, whereas, for $\rho_B=\rho_0$, the inverse magnetic catalysis (IMC) is observed when the AMMs are taken into account, contrary to MC, when the AMMs are ignored. The magnetic field effects on the heavy quarkonium decay widths should have observable consequences on the production of the hidden and open heavy flavour mesons from ultra-relativistic peripheral heavy ion collision experiments, at RHIC and LHC, where the produced magnetic fields are extremely large.

Heavy quarkonia in a baryon asymmetric strongly magnetized hot quark matter

Recently there is a resurrection in the study of heavy quark bound states in a hot and baryonless matter with an ambient magnetic field but the matter produced at heavy-ion collider experiments is not perfectly baryonless, so we wish to explore the effect of small baryon asymmetry on the in-medium properties of the heavy quark bound states submerged in a strongly magnetized hot quark matter. Therefore, we have first given a revisit to the general covariant tensor structure of gluon self-energy in the above environment to compute the resummed propagator for gluons. This resummed propagator embodies the properties of medium, which gets translated into the (complex) potential between Q and Q¯ placed in the medium. We observe that the baryon asymmetry makes the real-part of potential slightly more attractive and weakens the imaginary-part. This opposing effects thus lead to the enhancement of binding energies and the reduction of thermal widths of QQ¯ ground states, respectively. Finally, the properties of quarkonia thus deciphered facilitate to compute the dissociation points of J/ψ and ϒ, which are found to have slightly larger values in the presence of baryon asymmetry. For example, J/ψ gets dissociated at 1.64Tc, 1.69Tc, and 1.75Tc, whereas ϒ is dissociated at 1.95Tc, 1.97Tc and 2.00Tc, for μ=0,60 and 100 MeV, respectively. This observation prevents early dissociation of quarkonia in the matter produced at ultrarelativistic heavy ion collisions with a small net baryon number, compared to the ideal baryonless matter.

Dissociation of heavy quarkonium states in a rapidly varying strong magnetic field

Various heavy quarkonium states can be employed as a testing ground for the magnetic field created and later possibly persisted in heavy ion collision (HIC) experiments. In this paper, we have shown the modification in the binding energies, and ionization potentials categorically for different species of quarkonia in the medium in the presence of a magnetic field. For the study, we have considered quarkonium states which are well described by the real-valued effective potential in the thermal medium which is evolving by respecting the hydrodynamical power law. Furthermore while addressing the issue of their survival, we have not ignored the possibility of transitions between various bound-to-bound and bound-to-unbound states for different charmonium and bottomonium states. Our analysis shows that this phenomenon can be considered as a benchmark for understanding the presence, strength and persistence of the magnetic field which is expected to be produced through the high-energy nucleus–nucleus collisions.

Measuring deformed neutron skin with free spectator nucleons in relativistic heavy-ion collisions

The neutron skin in deformed nuclei is generally not uniformly distributed but has an angular distribution, depending on both the spin-dependent nuclear interaction and the nuclear symmetry energy. To extract the information of the deformed neutron skin, we have explored the possibility of using free spectator nucleons in central tip-tip and body-body collisions at top RHIC energy with four typical deformed nuclei. The density distributions of neutrons and protons are consistently obtained from the Skyrme-Hartree-Fock-Bogolyubov calculation, and the angular distribution of the neutron skin can be varied by adjusting the strength of the nuclear spin-orbit coupling. With the information of spectator nucleons obtained based on a Monte-Carlo Glauber model, the free spectator nucleons are generated from a multifragmentation process. By investigating the results from different systems and with different collision configurations, we found that although it is difficult to probe the deformed neutron skin in 96Zr and 238U by their collisions, it is promising to extract the polar angular distributions of the neutron skin in 96Ru and 197Au by comparing the yield ratios of free spectator neutrons to protons in their central tip-tip and body-body collisions. The proposed observables can be measured by dedicated zero-degree calorimeters in heavy-ion collision experiments that have been carried out in recent years by RHIC.

Constraining Neutron-Star Matter — Combination of heavy-ion experiments and multi-messenger astronomy

Describing supernova explosions or neutron-star collisions requires a deep understanding of properties of nuclear matter at supra-saturation densities, and extreme neutron over proton asymmetries. So far, our knowledge about dense matter provided by astrophysical observations in the cores of neutron stars remains limited. However, dense nuclear matter is also probed in terrestrial heavy-ion collision (HIC) experiments. We demonstrate how, within a novel approach, using Bayesian inference, combining data from astrophysical multi-messenger observations of neutron stars and from HICs at relativistic energies, one can improve our understanding of dense nuclear matter. The inclusion of HIC data probing the nuclear matter equation-of-state (EoS) at supra-saturation density has the effect of increasing the predicted pressure in the core of neutron stars relative to previous analyses, and shifts the neutron-star radii expectation towards larger values, in accordance with recent observations by the Neutron Star Interior Composition Explorer mission. More remarkable is that, though the sources and methods of observation are orthogonal, the constraints from HIC experiments and multimessenger observations are consistent with each other. It shows that both methods can be complementary at intermediate densities, and strengthen each other. Another conclusion is that in order to be even more constraining, the constraint of the EoS of asymmetric nuclear matter by HIC methods should be improved above twice saturation density, which should be feasible with future experiments with enhanced precision and higher bombarding energy.

Interaction of <sup>28</sup>Si with Emulsion Nuclei at 4.5 AGeV/c in View of Thermo-Statistical Approach

In this work, we study some changes of nuclear matter in the interactions of 28S with emulsion nuclei at 4.5 AGeV/c. From the experimental quantities investigated using Tsallis’ statistics, we deduced the temperature, entropy density and non-equilibrium factor of the nuclear medium. These obtained parameters were used to reveal variations in the nuclear matter at the stated interaction energy. The results that came up from this study were compared with their corresponding results obtained from other heavy ion collision experiments at wide energy range.

Retracted Article: Constraining Neutron-Star Matter — Combination of heavy-ion experiments, astronomy and theory

This paper has been formally retracted because it has been accidentally published twice in the same volume. Request approved by the proceedings Editor and the Publisher on January 29, 2024.

Measuring the Speed of Sound Using Cumulants of Baryon Number

We show that the values of the first three cumulants of the baryon number distribution can be used to calculate the isothermal speed of sound and its logarithmic derivative with respect to the baryon number density. We discuss applications of this result to heavy-ion collision experiments and address possible challenges, including effects due to baryon number conservation, differences between proton and baryon cumulants, and the influence of finite number statistics on fluctuation observables in both experiment and hadronic transport simulations. In particular, we investigate the relation between quantities calculated in infinite, continuous matter and observables obtained in simulations using a finite number of particles.

Relativistic spin hydrodynamics

In recent years, due to the discoveries of hyperon spin polarization and vector meson spin alignment in relativistic heavy-ion collision experiments, the spin transports in quark-gluon matter has received intensive studies. The relativistic spin hydrodynamics is one of the important theoretical frameworks to describe the spin transports, which encodes the spin degree of freedom into a hydrodynamic theory. The relativistic spin hydrodynamics have the conservation equations for energy-momentum tensor, currents and total angular momentum. In this article, we give an overview of the recent progresses of the relativistic spin hydrodynamics. We focus on the following topics: 1) The derivation of the relativistic spin hydrodynamic equations, including the phenomenological approach, the effective theory method, and the kinetic approach, 2) Some special properties of spin hydrodynamics, especially the asymmetric energy-momentum tensor and the pseudogauge transformation, and 3) The analytical solutions to the relativistic spin hydrodynamics for systems under Bjorken and Gubser expansion.

QCD sum rule analysis of heavy quarkonium states in magnetized matter: Effects of magnetic and inverse magnetic catalysis

In-medium masses of the $1S$ and $1P$ states of heavy quarkonia are investigated in the magnetized asymmetric nuclear medium, accounting for the Dirac sea effects, using a combined approach of chiral effective model and QCD sum rule method. Masses are calculated from the in-medium scalar and twist-2 gluon condensates, calculated within the chiral model. The gluon condensate is simulated through the scalar dilaton field, $\ensuremath{\chi}$ introduced in the model through a scale-invariance breaking logarithmic potential. Contribution of the Dirac sea is incorporated through the nucleonic tadpole diagrams. Treating the scalar fields as classical, the dilaton field, $\ensuremath{\chi}$, the isoscalar nonstrange $\ensuremath{\sigma}(\ensuremath{\sim}(⟨\overline{u}u⟩+⟨\overline{d}d⟩))$, strange $\ensuremath{\zeta}(\ensuremath{\sim}⟨\overline{s}s⟩)$ and isovector $\ensuremath{\delta}(\ensuremath{\sim}(⟨\overline{u}u⟩\ensuremath{-}⟨\overline{d}d⟩))$ fields, are obtained by solving their coupled equations of motion as derived from the chiral model Lagrangian. The effects of magnetic field are incorporated through the Dirac sea as well as the Landau energy levels of the protons, and the anomalous magnetic moments of the nucleons. The scalar fields modify appreciably with magnetic field due to the Dirac sea contribution. In-medium masses of the charmonium and bottomonium ground states are observed to have significant modifications with magnetic field due to the effects of (inverse) magnetic catalysis. In presence of an external magnetic field, there is mixing between the longitudinal component of the vector and the pseudoscalar mesons (PV mixing) in both quarkonia sectors, leading to a rise (drop) of the masses of $J/{\ensuremath{\psi}}^{||}({\ensuremath{\eta}}_{c})$ and ${\mathrm{\ensuremath{\Upsilon}}}^{||}(1S)({\ensuremath{\eta}}_{b})$ states. These might show in the experimental observables, e.g., the dilepton spectra in the noncentral, ultrarelativistic heavy ion collision experiments at RHIC and LHC, where the produced magnetic field is huge.

Rivet and the analysis preservation in heavy-ion collisions experiments

The comparison of experimental data and theoretical predictions is important for our understanding of the mechanisms for interactions and particle production in hadron collisions, both at the Large Hadron Collider and at the Relativistic Heavy-Ion Collider experiments. Several tools were ideated to help with that. Rivet (Robust Independent Validation of Experiment and Theory) is a framework that facilitates the comparison between measurements from high-energy physics experiments and Monte Carlo event generators able to produce outputs using the HepMC package. Rivet contains a repository with analysis algorithms developed by experiments, providing analysis documentation and preservation. The recent developments for the implementation of centrality and multiplicity classes in Rivet are presented in this contribution.

J/ψ yields in low energy nuclear collisions at SPS and FAIR: A baseline estimation

The yield of J/ψ mesons, produced in proton-nucleus (p+A) and nucleus-nucleus (A+A) collisions are estimated within a Glauber model ansatz for the upcoming low energy heavy-ion collision experiments at SPS and FAIR. A data driven parametrization is employed to incorporate the effects of Cold Nuclear Matter (CNM) on the J/ψ production cross-section.

Open bottom mesons and upsilon states in hot magnetized strange hadronic matter

The masses of open bottom mesons ([Formula: see text]([Formula: see text],[Formula: see text]), [Formula: see text]([Formula: see text],[Formula: see text]), [Formula: see text]([Formula: see text], [Formula: see text])) and upsilon states ([Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text]) are investigated in the isospin asymmetric strange hadronic medium at finite temperature in the presence of strong magnetic fields using a chiral effective Lagrangian approach. Here the chiral SU(3) Lagrangian is generalized to include the bottom sector to incorporate the interactions of the open bottom mesons with the magnetized medium. At finite temperature, the number density and scalar density of baryons are expressed in terms of thermal distribution functions. For charged baryons, the magnetic field introduces contribution from Landau energy levels. The masses of the open bottom mesons get modified through their interactions with the baryons and the scalar mesons, which undergo modifications in a magnetized medium. The charged [Formula: see text], [Formula: see text] mesons have additional positive mass shifts due to Landau quantization in the presence of the magnetic field. The medium mass shift of the upsilon states originates from the modification of the gluon condensates simulated by the variation of dilaton field ([Formula: see text]) and a quark mass term in the magnetized medium. The open bottom mesons and upsilon states experience a mass drop in the magnetized medium. The masses of these mesons initially increase with a rise in temperature, and beyond a high value of temperature, their masses are observed to drop. When the temperature is below 90[Formula: see text]MeV, the in-medium masses of the mesons increase with an increase in the magnetic field. However, at high temperatures (T > 90 MeV), the masses are observed to drop with an increase in the magnetic field. These in-medium modifications can have observable effects in asymmetric heavy-ion collisions experiments.

Evolutions in first-order viscous hydrodynamics

Motivated by the physics of the quark-gluon plasma created in heavy-ion collision experiments, we use holography to study the regime of applicability of various theories of relativistic viscous hydrodynamics. Using the microscopic description provided by holography of a system that relaxes to equilibrium, we obtain initial data with which we perform real-time evolutions in 2+1 dimensional conformal fluids using the first-order viscous relativistic hydrodynamics theory of Bemfica, Disconzi, Noronha and Kovtun (BDNK), BRSSS and ideal hydrodynamics. By initializing the hydrodynamics codes at different times, we can check the constitutive relations and assess the predictive power and accuracy of each of these theories.

Thermal and thermoelectric responses of hot QCD medium in time-varying magnetic fields

The thermal response of the hot QCD matter has been studied in the presence of a time-varying magnetic field. The impact of the magnetic field, its time dependence, and the collision aspects of the medium on thermal transport have been studied within the relativistic kinetic theory. The decay time of the magnetic field in the medium seems to have a strong dependence on thermal conductivity. The applicability of the Wiedemann-Franz law for the QCD medium has been investigated in the presence of time-varying external electromagnetic fields. The phenomenological significance of thermal transport in heavy-ion collision experiments has also been investigated by relating the thermal conductivity to the elliptic flow through the Knudsen number. The investigations are extended to study the thermoelectric behavior of hot QCD medium and its dependence on the magnetic field. The time dependent magnetic field is observed to significantly influence the thermoelectric behavior of the medium.

A topological realization of spin polarization through vortex formation in collisions of Bose–Einstein condensates

The global spin polarization of hadrons in heavy ion collisions has been measured in the solenoidal tracker at relativistic heavy ion collider experiments, which opens up a new window in the study of the hottest, least viscous, and most vortical fluid that has ever been produced in the laboratory. We present a different approach to spin polarization from conventional ones: a topological realization of spin polarization through quantum vortex formation in collisions of Bose–Einstein condensates. This approach is based on the observation that the vortex is a topological excitation in a superfluid in the presence of local orbital angular momentum and is an analog of spin degrees of freedom. The formation processes of vortices and vortex–antivortex pairs are investigated by solving the Gross–Pitaevskii equation with a large-scale parallel algorithm on a graphics processing unit to very high precision. In a rotating environment, the primary vortex with winding number one is stable against perturbation, which has minimal energy and fixed orbital angular momentum (OAM), but the vortices with larger winding numbers are unstable and will decay into primary vortices through a redistribution of the energy and vorticity. The injection of OAM can also be realized in non-central collisions of self-interacting condensates, part of the OAM of the initial state will induce the formation of vortices through concentration of energy and vorticity density around topological defects. Different from a hydrodynamical description, the interference of the wave function plays an important role in the transport of energy and vorticity, reflecting the quantum nature of the vortex formation process. The study of the vortex formation may shed light on the nature of particle spin and spin–orbit couplings in strong interaction matter produced in heavy-ion collisions.

Dynamic density correlations in baryon rich fluid using Mori–Zwanzig–Nakajima projection operator method

In this work, we calculate the dynamic density correlations using Mori–Zwanzig–Nakajima projection operator method. With a judicious choice of slow variables we derive the evolution equations for these slow variables starting from generalised Langevin equation. We get the hydrodynamic form of density correlations function which consist of two acoustic peaks: also called Brillouin peaks, and one thermal peak: also called Rayleigh peak. We then estimate the dynamic density correlations near the critical point using critical exponents extracted from the statistical bootstrap model of hadronic matter. We find that the bulk viscosity contributes to the sound attenuation at leading order sim |t|^{-frac{5}{4}} while the thermal conductivity contribute at sub-leading order sim |t|^{-frac{3}{4}}. On the other hand, only the thermal conductivity contributes at leading order sim |t|^{frac{1}{4}} to the thermal diffusivity. We discuss the implications of these results in search for the QCD critical point in the heavy-ion collision experiments.

Open charm mesons and charmonia in magnetized strange hadronic matter * *A.J.C.S acknowledges the support towards this work from the Department of Science and Technology, Government of India, via an INSPIRE fellowship (INSPIRE Code IF170745). AM acknowledges financial support from Department of Science and Technology (DST), Government of India (CRG/2018/002226)

We investigate the in-medium masses of open charm mesons (D( , ), ( , ), ( , )) and charmonium states ( , , , , ) in strongly magnetized isospin asymmetric strange hadronic matter using a chiral effective model. In the presence of a magnetic field, the number and scalar densities of charged baryons have contributions from Landau energy levels. The mass modifications of open charm mesons result from their interactions with nucleons, hyperons, and the scalar fields (the non-strange field σ, strange field ζ, and isovector field δ) in the presence of a magnetic field. The mass modifications of the charmonium states result from the modification of gluon condensates in a medium simulated by the variation in the dilaton field (χ) in the chiral effective model. The effects of finite quark masses are also incorporated in the trace of the energy-momentum tensor in quantum chromodynamics to investigate the mass shifts of charmonium states. The in-medium masses of open charm mesons and charmonia are observed to decrease with an increase in baryon density. The charged , , , and mesons have additional positive mass shifts due to Landau quantization in the presence of a magnetic field. The effects of the strangeness fraction are observed to be more dominant for mesons compared with D mesons. The mass shifts of charmonia are observed to be larger in hyperonic media compared with nuclear media when the effect of the finite quark mass term is neglected. These medium mass modifications can have observable consequences on the production of the open charm mesons and charmonia in high-energy asymmetric heavy-ion collision experiments.

Machine-learning model-driven prediction of the initial geometry in heavy-ion collision experiments

We demonstrate high prediction accuracy of three important properties that determine the initial geometry of the heavy-ion collision (HIC) experiments by using supervised Machine Learning (ML) methods. These properties are the impact parameter, the eccentricity and the participant eccentricity. Though ML techniques have been used previously to determine the impact parameter of these collisions, we study multiple ML algorithms, their error spectrum, and sampling methods using exhaustive parameter scans and ablation studies to determine a combination of efficient algorithm and tuned training set that gives multi-fold improvement in accuracy for all three different heavy-ion collision models. The three models chosen are a transport model, a hydrodynamic model and a hybrid model. The motivation of using three different heavy-ion collision models was to show that even if the model is trained using a transport model, it gives accurate results for a hydrodynamic model as well as a hybrid model. We show that the accuracy of the impact parameter prediction depends on the centrality of the collision. With the standard application of ML training methods, prediction accuracy is considerable low for central collisions. Our method increases this accuracy by multiple folds. We also show that the eccentricity prediction accuracy can be improved by inclusion of the impact parameter as a feature in all these algorithms. We discuss how the errors can be minimized and the accuracy can be improved to a great extent in all the ranges of impact parameter and eccentricity predictions.