Heavy-ion Collision Experiments Research Articles

Charmonium production in hot magnetized hyperonic matter: Effects of the baryonic Dirac sea and pseudoscalar-vector mixing

We investigate the medium modifications of the masses of pseudoscalar open charm (D and D¯) mesons and the charmonium state [ψ(3770)] in hot isospin asymmetric strange hadronic medium in the presence of an external magnetic field within a chiral effective model. The in-medium partial decay widths of ψ(3770) to charged and neutral DD¯ mesons are computed from the medium modifications of the masses of the initial and final state mesons. These are computed using two light quark pair creation models—(I) the P03 model and (II) a field theoretical (FT) model of composite hadrons with quark (and antiquark) constituents. The production cross sections of ψ(3770), arising from scattering of the D and D¯ mesons, are computed from the relativistic Breit-Wigner spectral function expressed in terms of the in-medium masses and the decay widths of the charmonium state. The effects of the magnetic field are considered due to the Dirac sea (DS) of the baryons, the mixing of the pseudoscalar (S=0) and vector (S=1) meson (PV mixing), the Landau level contributions for the charged hadrons. The anomalous magnetic moments (AMMs) of the baryons are also taken into account in the present study. For magnetized nuclear matter, at ρB=ρ0, the effect of Dirac sea leads to inverse magnetic catalysis (IMC), which is drop of the magnitude of the light quark condensates (proportional to the scalar fields) with increase in the magnetic field, contrary to the opposite effect of magnetic catalysis (MC) at ρB=0. The inclusion of hyperons to the nuclear medium is observed to lead to magnetic catalysis. There are observed to be significant effects from the DS and PV mixing on the properties of the charm mesons. The production cross sections of ψ(3770) arising due to scattering of D+D−(D0D¯0) mesons in the hot magnetized strange hadronic matter are observed to have distinct peak positions, when the magnetic field is large. This is because the production cross sections have contributions from the transverse as well as longitudinal components of ψ(3770), which have nondegenerate masses due to PV [ψ(3770)−ηc′] mixing. These can have observable consequences on the dilepton spectra as well as on the production of the charm mesons in ultrarelativistic peripheral heavy ion collision experiments, where the produced magnetic field is huge. Published by the American Physical Society 2024

Baryon Electric Charge Correlation as a Magnetometer of QCD.

The correlation between net baryon number and electric charge, χ_{11}^{BQ}, can serve as a magnetometer of QCD. This is demonstrated by lattice QCD computations using the highly improved staggered quarks with physical pion mass of M_{π}=135 MeV on N_{τ}=8 and 12 lattices. We find that χ_{11}^{BQ} along the transition line starts to increase rapidly with magnetic field strength eB≳2M_{π}^{2} and by a factor 2 at eB≃8M_{π}^{2}. Furthermore, the ratio of electric charge chemical potential to baryon chemical potential, μ_{Q}/μ_{B}, shows significant dependence on the magnetic field strength and varies from the ratio of electric charge to baryon number in the colliding nuclei in heavy ion collisions. These results can provide baselines for effective theory and model studies, and both χ_{11}^{BQ} and μ_{Q}/μ_{B} could be useful probes for the detection of magnetic fields in relativistic heavy ion collision experiments as compared with corresponding results from the hadron resonance gas model.

Novel instabilities in counter-streaming nonabelian fluids

The dynamics of strongly interacting particles are governed by Yang–Mills (Y–M) theory, which is a natural generalization of Maxwell Electrodynamics (ED). Its quantized version is known as quantum chromodynamics (QCD) (Gross and Wilczek, 1973; Politzer, 1973; ’t Hooft, 1972[1–3]) and has been very well studied. Classical Y–M theory is proving to be equally interesting because of the central role it plays in describing the physics of quark–gluon plasma (QGP) — which was prevalent in the early universe and is also produced in relativistic heavy ion collision experiments. This calls for a systematic study of classical Y–M theories. A good insight into classical Y–M dynamics would be best obtained by comparing and contrasting the Y–M results with their ED counterparts. In this article, a beginning has been made by considering streaming instabilities in Y–M fluids. We find that in addition to analogues of ED instabilities, novel nonabelian modes arise, reflecting the inherent nonabelian nature of the interaction. The new modes exhibit propagation/ growth, with growth rates that can be larger than what we find in ED. Interestingly, we also find a mode that propagates without getting affected by the medium.

Chiral magnetic waves in strongly coupled Weyl semimetals

Propagating chiral magnetic waves (CMW) are expected to exist in chiral plasmas due to the interplay between the chiral magnetic and chiral separation effects induced by the presence of a chiral anomaly. Unfortunately, it was pointed out that, because of the effects of electric conductivity and dissipation, CMW are overdamped and therefore their signatures are unlikely to be seen in heavy-ion collision experiments and in the quark gluon plasma. Nonetheless, the chiral anomaly plays a fundamental role in Weyl semimetals and their anomalous transport properties as well. Hence, CMW could be potentially observed in topological semimetals using table-top experiments. By using a holographic model for strongly coupled Weyl semimetals, we investigate in detail the nature of CMW in presence of Coulomb interactions and axial charge relaxation and estimate whether, and in which regimes, CMW could be observed as underdamped collective excitations in topological materials.

Transport Coefficients of Relativistic Matter: A Detailed Formalism with a Gross Knowledge of Their Magnitude

The present review article has attempted a compact formalism description of transport coefficient calculations for relativistic fluid, which is expected in heavy ion collision experiments. Here, we first address the macroscopic description of relativistic fluid dynamics and then its microscopic description based on the kinetic theory framework. We also address different relaxation time approximation-based models in Boltzmann transport equations, which make a sandwich between Macro and Micro frameworks of relativistic fluid dynamics and finally provide different microscopic expressions of transport coefficients like the fluid’s shear viscosity and bulk viscosity. In the numeric part of this review article, we put stress on the two gross components of transport coefficient expressions: relaxation time and thermodynamic phase-space part. Then, we try to tune the relaxation time component to cover earlier theoretical estimations and experimental data-driven estimations for RHIC and LHC matter. By this way of numerical understanding, we provide the final comments on the values of transport coefficients and relaxation time in the context of the (nearly) perfect fluid nature of the RHIC or LHC matter.

Quark Clusters, QCD Vacuum and the Cosmological 7Li, Dark Matter and Dark Energy Problems

We propose a non-exotic electromagnetic solution (within the standard model of particle physics) to the cosmological 7Li problem based upon a narrow 2 MeV photo-emission line from the decay of light glueballs (LGBs). These LGBs form within color superconducting quark clusters (SQCs), which are tens of Fermi in size, in the radiation-dominated post-BBN epoch. The mono-chromatic line from the LGB→γ+γ decay reduces Big Bang nucleosynthesis (BBN) 7Be by 2/3 without affecting other abundances or the cosmic microwave background (CMB) physics, provided the combined mass of the SQCs is greater than the total baryonic mass in the universe. Following the LGB emission, the in-SQC Quantum ChromoDynamics (QCD) vacuum becomes unstable and “leaks” (via quantum tunneling) into the external space-time (trivial) vacuum, inducing a decoupling of SQCs from hadrons. In seeking a solution to the 7Li problem, we uncovered a solution that also addresses the Dark Energy (DE) and dark matter (DM) problem, making these critical problems intertwined in our model. Being colorless, charge-neutral, optically thin, and transparent to hadrons, SQCs interact only gravitationally, making them a viable cold DM (CDM) candidate. The leakage (i.e., quantum tunneling) of the in-SQC QCD vacuum to the trivial vacuum offers an explanation of DE in our model and allows for a cosmology that evolves into a ΛCDM universe at a low redshift with a possible resolution of the Hubble tension. Our model distinguishes itself by proposing that the QCD vacuum within SQCs possesses the ability to tunnel into the exterior trivial vacuum, resulting in the generation of DE. This implies the possibility that DM and hadrons might represent distinct phases of quark matter within the framework of QCD, characterized by different vacuum properties. We discuss SQC formation in heavy-ion collision experiments at moderate temperatures and the possibility of detection of MeV photons from the LGB→γ+γ decay.

What ultracold atoms tell us about the real-time dynamics of QCD in extreme conditions

I review developments of how compact table-top setups with ultracold atoms can help us to understand the more complex real-time dynamics of QCD probed in heavy-ion collision experiments.

QCD material parameters at zero and non-zero chemical potential from the lattice

Using an eighth-order Taylor expansion in baryon chemical potential, we recently obtained the (2+1)-flavor QCD equation of state (EoS) at nonzero conserved charge chemical potentials from the lattice. We focused on strangeness-neutral, isospin-symmetric QCD matter, which closely resembles the situation encountered in heavy-ion collision experiments. Using this EoS, we present here results on various QCD material parameters; in particular we compute the specific heat, speed of sound, and compressibility along appropriate lines of constant physics. We show that in the entire range relevant for the beam energy scan at RHIC, the specific heat, speed of sound, and compressibility show no indication for an approach to critical behavior that one would expect close to a possibly existing critical endpoint.

Mapping the critical equation of state by resummations

Determining the existence and the location of the QCD critical point remains a major goal in the heavy-ion collision experiments. A crucial theoretical input for achieving this goal is mapping the QCD equation of state in the presence of baryon chemical potential (μ) which at the moment is limited to small values of μ, away from the critical point. We present a new framework for reconstructing the equation of state from a truncated Taylor series expansion for small μ by using novel resummation techniques. We show how this resummation method can be used to (i) determine the location of the critical point and (ii) constrain the form of the critical contribution to the QCD equation of state which has a direct impact on the shape of the experimental signatures of the critical point. Using the recent lattice data from the HotQCD collaboration we estimate that Tc ≈ 100 MeV, μc ≈ 580 MeV, the crossover slope at the critical point α1 ≈ 9°, and further constrain the non-universal mapping parameters between the Ising model and QCD (scaling) equations of state.

Experimental overview: Quarkonium production in relativistic particle collisions

Quarkonium provides a golden probe of the formation of the Quark Gluon Plasma (QGP). Its production in heavy-ion collisions can be affected by an interplay of different phenomena such as medium-induced dissociation and heavy-quark (re)combination in the QGP, and cold nuclear matter effects due to the presence of the nucleus. The measurement of quarkonium states in different collision systems and beam energies allows to gain further insights on these effects and probe the properties of the QGP. In these proceedings, an overview of the main experimental quarkonium measurements in proton-proton, protonnucleus and nucleus-nucleus collisions is presented, giving emphasis on the most recent results from the Large Hadron Collider and Relativistic Heavy Ion Collider experiments shown at the 2023 Quark Matter conference.

QCD Equation of State of Dense Nuclear Matter from a Bayesian Analysis of Heavy-Ion Collision Data.

Bayesian methods are used to constrain the density dependence of the QCD equation of state (EOS) for dense nuclear matter using the data of mean transverse kinetic energy and elliptic flow of protons from heavy ion collisions (HICs), in the beam energy range sqrt[s_{NN}]=2-10 GeV. The analysis yields tight constraints on the density dependent EOS up to 4 times the nuclear saturation density. The extracted EOS yields good agreement with other observables measured in HIC experiments and constraints from astrophysical observations both of which were not used in the inference. The sensitivity of inference to the choice of observables is also discussed.

Collective dynamics of polarized spin-half fermions in relativistic heavy-ion collisions

Standard relativistic hydrodynamics has been successful in describing the properties of the strongly interacting matter produced in the heavy-ion collision experiments. Recently, there has been a significant theoretical advancement in this field to explain spin polarization of hadrons emitted in these processes. Although current models have successfully explained some of the experimental data based on the coupling between spin polarization and vorticity of the medium, they still lack a clear understanding of the differential measurements. This is commonly interpreted as an indication that the spin needs to be treated as an independent degree of freedom whose dynamics is not entirely bound to flow circulation. In particular, if the spin is a macroscopic property of the system, in equilibrium its dynamics should follow hydrodynamic laws. Here, we develop a framework of relativistic hydrodynamics which includes spin degrees of freedom from the quantum kinetic theory for Dirac fermions and use it for modeling the dynamics of matter. Following experimental observations, we assume that the polarization effects are small and derive conservation laws for the net baryon current, the energy–momentum tensor and the spin tensor based on the de Groot–van Leeuwen–van Weert definitions of these currents. We present various properties of the spin polarization tensor and its components, analyze the propagation properties of the spin polarization components, and derive the spin-wave velocity for arbitrary statistics. We find that only the transverse spin components propagate, analogously to the electromagnetic waves. Finally, using our framework, we study the space–time evolution of the spin polarization for the systems respecting certain space–time symmetries and calculate the mean spin polarization per particle, which can be compared to the experimental data. We find that, for some observables, our spin polarization results agree qualitatively with the experimental findings and other model calculations.

Temperature dependence of the static quark diffusion coefficient

The propagation of a low momentum heavy quark in a deconfined quark-gluon plasma can be understood in terms of a Langevin description. The momentum exchange with the plasma in thermal equilibrium can then be parametrized in terms of a single heavy quark momentum diffusion coefficient κ, which needs to be determined nonperturbatively. In this work, we study the temperature dependence of κ for a static quark in a gluonic plasma, with a particular emphasis on the temperature range of interest for heavy ion collision experiments.

Heavy Quark Diffusion from 2+1 Flavor Lattice QCD with 320MeV Pion Mass.

We present the first calculations of the heavy flavor diffusion coefficient using lattice QCD with light dynamical quarks corresponding to a pion mass of around 320MeV. For temperatures 195 MeV<T<352 MeV, the heavy quark spatial diffusion coefficient is found to be significantly smaller than previous quenched lattice QCD and recent phenomenological estimates. The result implies very fast hydrodynamization of heavy quarks in the quark-gluon plasma created during ultrarelativistic heavy-ion collision experiments.

Observation of hydrodynamization and local prethermalization in 1D Bose gases.

Hydrodynamics accurately describe relativistic heavy-ion collision experiments well before local thermal equilibrium is established1. This unexpectedly rapid onset of hydrodynamics-which takes place on the fastest available timescale-is called hydrodynamization2-4. It occurs when an interacting quantum system is quenched with an energy density that is much greater than its ground-state energy density5,6. During hydrodynamization, energy gets redistributed across very different energy scales. Hydrodynamization precedes local equilibration among momentum modes5, which is local prethermalization to a generalized Gibbs ensemble7,8 in nearly integrable systems or local thermalization in non-integrable systems9. Although many theories of quantum dynamics postulate local prethermalization10,11, the associated timescale has not been studied experimentally. Here we use an array of one-dimensional Bose gases to directly observe both hydrodynamization and local prethermalization. After we apply a Bragg scattering pulse, hydrodynamization is evident in the fast redistribution of energy among distant momentum modes, which occurs on timescales associated with the Bragg peak energies. Local prethermalization can be seen in the slower redistribution of occupation among nearby momentum modes. We find that the timescale for local prethermalization in our system is inversely proportional to the momenta involved. During hydrodynamization and local prethermalization, existing theories cannot quantitatively model our experiment. Exact theoretical calculations in the Tonks-Girardeau limit12 show qualitatively similar features.

Determination of energy-dependent neutron backgrounds using shadow bars

Understanding the neutron background is essential for determining the neutron yield from nuclear reactions. The neutron backgrounds were determined for heavy-ion collision experiment using the shadow-bar method, where beams of 40,48Ca at 56, 140 MeV/u impinged on targets of 58,64Ni and 112,124Sn. In the analysis presented here, brass shadow bars are placed in front of organic liquid scintillator neutron detectors to determine the energy-dependent neutron background fractions. The measurement of neutron spectra with and without shadow bars is important to determine the neutron background more accurately. The neutron background, along with its sources and systematic uncertainties, are explored with a focus on the impact of background models and their dependence on neutron energy.

Model study on Υ(nS) modification in small collision systems

Quarkonium production has been studied extensively in relativistic heavy-ion collision experiments to understand the properties of the quark-gluon plasma. The experimental results on the yield modification in heavy-ion collisions relative to that in $p+p$ collisions can be described by several models considering dissociation and regeneration effects. A yield modification beyond initial-state effects has also been observed in small collision systems such as $p+\mathrm{Au}$ and $p+\mathrm{Pb}$ collisions, but it is still premature to claim any hot medium effect. A model study in various small collision systems such as $p+p$, $p+\mathrm{Pb}$, $p+\mathrm{O}$, and $\mathrm{O}+\mathrm{O}$ collisions will help quantitatively the understanding of nuclear effects on the $\mathrm{\ensuremath{\Upsilon}}(nS)$ production. A theoretical calculation considering the gluo-dissociation and inelastic parton scattering for dissociation, and their inverse reaction for regeneration, reasonably describes the modification of $\mathrm{\ensuremath{\Upsilon}}(1S)$ in $\mathrm{Pb}+\mathrm{Pb}$ collisions. Based on this calculation, a Monte Carlo simulation considering the dissociation effect is developed to more realistically incorporate the medium produced in heavy-ion collisions with event-by-event initial collision geometry and hydrodynamic evolution. We extend this framework to small systems to study the medium effects. In this work, we quantify the nuclear modification factor of $\mathrm{\ensuremath{\Upsilon}}(nS)$ as a function of charged particle multiplicity ($d{N}_{ch}/d\ensuremath{\eta}$) and transverse momentum. We also calculate the elliptic flow of $\mathrm{\ensuremath{\Upsilon}}(nS)$ in small collision systems.

Seebeck and Nernst coefficients of a magnetized hot QCD medium with a number conserving kernel

We study the thermoelectric response of a hot and magnetized QCD medium created in the noncentral events at heavy-ion collider experiments. The collisional aspects of the medium have been embedded in the relativistic Boltzmann transport equation (RBTE) using the Bhatnagar-Gross-Krook (BGK) collision integral, which insures the particle number conservation, unlike the commonly used relaxation time approximation (RTA). We have incorporated the thermal medium effects in the guise of a quasiparticle model, where the interaction among the quarks and gluons is assimilated in the medium dependent masses of the quarks, which have been evaluated using imaginary-time formalism of thermal QCD with a background magnetic field. In the absence of $B$, the Seebeck coefficient for individual quark flavors gets slightly reduced in the BGK term in comparison to naive RTA, while it gets enhanced for the composite partonic medium. In the strong magnetic field ($B$), the BGK term enhances the Seebeck coefficient for the individual flavors, as well as that for the medium. The medium Seebeck coefficient rises with the strength of quark chemical potential ($\ensuremath{\mu}$) in the absence, as well as that in the strong $B$. We observe chirality dependence in the transport coefficients in the weak $B$ as the masses of chiral modes become nondegenerate. In the case of the $L$ modes, the BGK collision term causes a slight reduction in the Seebeck coefficient, while for $R$ modes both the collision integrals produce the same results. The Nernst coefficient gets reduced (enhanced) for $L$ ($R$) chiral modes in the BGK term.

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.