Hot QCD Research Articles

Effect of magnetic field on the charge and thermal transport properties of hot and dense QCD matter

We have studied the effect of strong magnetic field on the charge and thermal transport properties of hot QCD matter at finite chemical potential. For this purpose, we have calculated the electrical conductivity (sigma _mathrm{el}) and the thermal conductivity (kappa ) using kinetic theory in the relaxation time approximation, where the interactions are subsumed through the distribution functions within the quasiparticle model at finite temperature, strong magnetic field and finite chemical potential. This study helps to understand the impacts of strong magnetic field and chemical potential on the local equilibrium by the Knudsen number (Omega ) through kappa and on the relative behavior between thermal conductivity and electrical conductivity through the Lorenz number (L) in the Wiedemann–Franz law. We have observed that, both sigma _mathrm{el} and kappa get increased in the presence of strong magnetic field, and the additional presence of chemical potential further increases their magnitudes, where sigma _mathrm{el} shows decreasing trend with the temperature, opposite to its increasing behavior in the isotropic medium, whereas kappa increases slowly with the temperature, contrary to its fast increase in the isotropic medium. The variation in kappa explains the decrease of the Knudsen number with the increase of the temperature. However, in the presence of strong magnetic field and finite chemical potential, Omega gets enhanced and approaches unity, thus, the system may move slightly away from the equilibrium state. The Lorenz number (kappa /(sigma _mathrm{el} T)) in the abovementioned regime of strong magnetic field and finite chemical potential shows linear enhancement with the temperature and has smaller magnitude than the isotropic one, thus, it describes the violation of the Wiedemann–Franz law for the hot and dense QCD matter in the presence of a strong magnetic field.

Thermal transport in a weakly magnetized hot QCD medium

The thermal transport coefficients in a weakly magnetized quark-gluon plasma have been investigated within the ambit of a quasiparticle model to encode the effects of the realistic equation of state. The presence of a weak magnetic field leads to the Hall-type conductivity associated with thermal transport in the medium. An effective covariant kinetic theory has been employed to quantify the thermal dissipation while incorporating the mean field contributions in the medium. The interplay of thermal transport and electric charge transport in the weakly magnetized medium has been explored in terms of the Wiedemann-Franz law. Strong violation of the Wiedemann-Franz law has been observed in temperature regimes near to the transition temperature. The behaviour of thermal conductivity in the strong magnetic field limit has also been studied. It is observed that both the magnetic field and equation of state have a significant impact on the thermal dissipation in the medium.

Can we observe jet PT-broadening in heavy-ion collisions at the LHC?

We investigate the effect of PT-broadening on jet substructure observables in heavy-ion collisions at the LHC. As an example, we focus on the opening angle of the two branches that satisfy the soft drop grooming condition in a highly energetic jet. The medium modification of the angular distribution can provide important information on the jet transport properties of hot QCD matter. In addition, we take into account a change of the overall fraction of quark and gluon jets in heavy-ion collisions. We comment on the comparison to a recent measurement from the ALICE Collaboration.

Melting of quarkonium in an anisotropic hot QCD medium in the presence of a generalized Debye screening mass and Nikiforov–Uvarov’s method

Generalized temperature and anisotropy dependent Debye screening mass is introduced into the real part of a potential in an anisotropic plasma. The N-radial Schrödinger equation (SE) is approximately solved by using the Nikiforov–Uvarov (NU) method which based on the expansion of power series. Binding energies and dissociation temperatures of charmonium and bottomonium are calculated. In addition, we have calculated the screening mass values for different parameters. Comparing to their values in an isotropic medium, the charmonium and bottomonium binding energies within an anisotropic medium are found to be increased. Also, the dissociation temperatures of both the charmonium and the bottomonium within anisotropic environments appear larger relative to those found within an isotropic medium. Finally, one observes that in any medium the bottomonium dissociation temperature is higher than the charmonium one.

Topological susceptibility, divergent chiral density, and phase diagram of chirally imbalanced QCD medium at finite temperature

We show that the nonlocal two-flavor Nambu--Jona-Lasinio model predicts the enhancement of both chiral and axial symmetry breaking as the chiral imbalance of hot QCD matter, regulated by a chiral chemical potential $\mu_5$, increases. The two crossovers are reasonably close to each other in the range of $\mu_5$ examined here and the pseudocritical temperatures rise with $\mu_5$. The curvatures of the chiral and axial crossovers for the chiral quark chemical potential approximately coincide and give $\kappa_5 \simeq - 0.011$. We point out that the presence of $\mu_5$ in thermodynamic equilibrium is inconsistent with the fact that the chiral charge is not a Noether-conserved quantity for massive fermions. The chiral chemical potential should not, therefore, be considered as a true chemical potential that sets a thermodynamically stable environment in the massive theory, but rather than as a new coupling that may require a renormalization in the ultraviolet domain. The divergence of an unrenormalized chiral density, \corr{coming from zero-point fermionic fluctuations,} is a consequence of this property. We propose a solution to this problem via a renormalization procedure.

First order dissipative hydrodynamics and viscous corrections to the entropy four-current from an effective covariant kinetic theory

The first order hydrodynamic evolution equations for the shear stress tensor, the bulk viscous pressure and the charge current have been studied for a system of quarks and gluons, with a non-vanishing quark chemical potential and finite quark mass. The first order transport coefficients have been obtained by solving an effective Boltzmann equation for the grand-canonical ensemble of quasiquarks and quasigluons. We adopted temperature dependent effective fugacity for the quasiparticles to encode the hot QCD medium effects. The non-trivial energy dispersion of the quasiparticles induces mean field contributions to the transport coefficients whose origin could be directly related to the realization of conservation laws from the effective kinetic theory. Both the QCD equation of state and chemical potential are seen to have a significant impact on the quark-gluon plasma evolution. The shear and bulk viscous corrections to the entropy-four current have been investigated in the framework of the effective kinetic theory. The effect of viscous corrections to the entropy density have been quantified in the case of one dimensional boost-invariant expansion of the system. Further, the first order viscous corrections to the time evolution of temperature along with the description of pressure anisotropy and Reynolds number of the system have been explored for the longitudinal boost-invariant expansion.

Chiral transition and the chiral charge density of the hot and dense QCD matter.

We study the chirally imbalanced hot and dense strongly interacting matter by means of the Dyson-Schwinger equations (DSEs). The chiral phase diagram is studied in the presence of chiral chemical potential μ5. The chiral quark condensate leftlangle overline{psi}psi rightrangle is obtained with the Cornwall-Jackiw-Tomboulis (CJT) effective action in concert with the Rainbow truncation. Catalysis effect of dynamical chiral symmetry breaking (DCSB) by μ5 is observed. We examine with two popular gluon models and consistency is found within the DSE approach, as well as in comparison with lattice QCD. The critical end point (CEP) location (μE, TE ) shifts toward larger TE but constant μE as μ5 increases. A technique is then introduced to compute the chiral charge density n5 from the fully dressed quark propagator. We find the n5 generally increases with temperature T , quark number chemical potential μ and μ5. Since the chiral magnetic effect (CME) is typically investigated with peripheral collisions, we also investigate the finite size effect on n5 and find an increase in n5 with smaller system size.

Impact of longitudinal bulk viscous effects to heavy quark transport in a strongly magnetized hot QCD medium

The effects of longitudinal bulk viscous pressure on the heavy quark dynamics have been estimated in a strongly magnetized quark-gluon plasma within the Fokker-Planck approach. The bulk viscous modification to the momentum distribution of bulk degrees of freedom has been obtained in the presence of a magnetic field while incorporating the realistic equation of state of the hot magnetized QCD medium. As the magnetic field breaks the isotropy of the medium, the analysis is done along the directions longitudinal and transverse to the field. The longitudinal bulk viscous contribution is seen to have sizable effects in the heavy quark momentum diffusion in the magnetized medium. The dependence of higher Landau levels and the equation of state on the viscous correction to the heavy quark transport has been explored in the analysis.

Heavy - light flavor correlations of anisotropic flows at LHC energies within event-by-event transport approach

The heavy quarks (HQs) are a unique probe of the hot QCD matter properties and their dynamics are coupled to the locally thermalized expanding quark gluon plasma. We present here a novel study of the event by event correlations between light and heavy flavor flow harmonics at LHC energy within a transport approach. Interaction between heavy quarks and light quarks have been taken into account to explore the impact of different temperature dependence for transport coefficients Ds and Γ. Our study indicates that vnheavy−vnlight correlation and the relative fluctuations of anisotropic flows, σvn/〈vn〉, are novel observables to understand the heavy quark-bulk interaction and are sensitive to the temperature dependence even to moderate differences of Ds(T), or Γ(T). Hence, the comparison of such new HQ observables to upcoming experimental data at both RHIC and LHC energies can put further constraints on heavy quark transport coefficients and in particular on its temperature dependence toward a solid comparison between phenomenological determination and lattice QCD calculations.

Dissociation of nucleon and heavy baryon in an anisotropic hot and dense QCD medium using Nikiforov–Uvarov method

By using the Nikiforov–Uvarov method, the hyper-radial Schrodinger equation is analytically solved, in which the real modified potential is employed at finite temperature and baryon chemical potential. The eigenvalue of energy and corresponding wave function are obtained in the isotropic and anisotropic media in the hot and dense media. The present results show that the binding energy of nucleon and some heavy baryon decreases strongly in hot medium and decreases slightly with increasing baryon chemical potential. In addition, the binding energy for each baryon is more bound in an anisotropic medium in comparison with its value in an isotropic medium. The temperature dissociation of each baryon is above a critical temperature, and it increases in the anisotropic medium. The dissociation of temperature is slightly decreased in the hot medium when the baryon of chemical potential is considered. A comparison is studied with the other available studies. We conclude that the present study provides a good description of the nucleon and some heavy baryon in the hot and dense media in the isotropic and anisotropic systems.

Novel tools and observables for jet physics in heavy-ion collisions

Studies of fully-reconstructed jets in heavy-ion collisions aim at extracting thermodynamical and transport properties of hot and dense QCD matter. Recently, a plethora of new jet substructure observables have been theoretically and experimentally developed that provide novel precise insights on the modifications of the parton radiation pattern induced by a QCD medium. This report, summarizing the main lines of discussion at the 5th Heavy Ion Jet Workshop and CERN TH institute ‘Novel tools and observables for jet physics in heavy-ion collisions’ in 2017, presents a first attempt at outlining a strategy for isolating and identifying the relevant physical processes that are responsible for the observed medium-induced jet modifications. These studies combine theory insights, based on the Lund parton splitting map, with sophisticated jet reconstruction techniques, including grooming and background subtraction algorithms.

Traces of the nuclear liquid-gas phase transition in the analytic properties of hot QCD

The nuclear liquid-gas transition at normal nuclear densities, $n\ensuremath{\approx}{n}_{0}=0.16\phantom{\rule{4pt}{0ex}}{\mathrm{fm}}^{\ensuremath{-}3}$, and small temperatures, $T\ensuremath{\approx}20\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$, has a large influence on analytic properties of the QCD grand-canonical thermodynamic potential. A classical van der Waals equation is used to determine qualitatively these unexpected features due to dense cold matter. The existence of the nuclear matter critical point results in thermodynamic branch points, which are located at complex chemical potential values, for $T>{T}_{c}\ensuremath{\simeq}20\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$, and exhibit a moderate model dependence up to rather large temperatures $T\ensuremath{\lesssim}100\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$. The behavior at higher temperatures is studied using the van der Waals hadron resonance gas (vdW-HRG) model. The baryon-baryon interactions have a decisive influence on the QCD thermodynamics close to ${\ensuremath{\mu}}_{B}=0$. In particular, nuclear matter singularities limit the radius of convergence ${r}_{{\ensuremath{\mu}}_{B}/T}$ of the Taylor expansion in ${\ensuremath{\mu}}_{B}/T$, with ${r}_{{\ensuremath{\mu}}_{B}/T}\ensuremath{\approx}2--3$ values at $T\ensuremath{\approx}140--170\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$ obtained in the vdW-HRG model.

Equation of state for hot QCD and compact stars from a mean-field approach

The thermodynamic properties of high temperature and high density QCD-matter are explored within the Chiral SU(3)-flavor parity-doublet Polyakov-loop quark-hadron mean-field model, CMF. The quark sector of the CMF model is tuned to describe the $\mu_B=0$ thermodynamics data of lattice QCD. The resulting lines of constant physical variables as well as the baryon number susceptibilities are studied in some detail in the temperature/chemical potential plane. The CMF model predicts three consecutive transitions, the nuclear first-order liquid-vapor phase transition, chiral symmetry restoration, and the cross-over transition to a quark-dominated phase. All three phenomena are cross-over, for most of the $T-\mu_B$-plane. The deviations from the free ideal hadron gas baseline at $\mu_B=0$ and $T\approx 100-200$ MeV can be attributed to remnants of the liquid-vapor first order phase transition in nuclear matter. The chiral crossing transition determines the baryon fluctuations at much higher $\mu_B\approx1.5$ GeV, and at even higher baryon densities $\mu_B\approx2.4$ GeV, the behavior of fluctuations is controlled by the deconfinement cross-over. The CMF model also describe well the static properties of high $\mu_B$ neutron stars as well as the new neutron star merger observations. The effective EoS presented here describes simultaneously lattice QCD results at $\mu_B=0$, as well as observed physical phenomena (nuclear matter and neutron star matter) at $T\cong0$ and high densities, $\mu_B>1$ GeV.

Optical properties of an anisotropic hot QCD medium

The present investigation involves explorations on the chromo-dielectric properties of the hot QCD medium produced in relativistic heavy-ion collisions in terms of refractive index. The isotropic/equilibrium modelling is done within an effective quasi-particle model of hot QCD medium. The possibilities of negative refraction in the medium are also explored in terms of the Depine–Lakhtakia index. The anisotropic aspects of the hot QCD medium are incorporated by introducing the anisotropy in a particular direction. That makes the medium quite similar to uniaxial crystals, and hence we observe the phenomenon of birefringence (two distinct refractive indices in the anisotropic case). Interestingly, both anisotropy and medium effects play significant roles in deciding the optical properties of the hot QCD/quark–gluon-plasma medium.

Landau damping in a strong magnetic field: Dissociation of quarkonia

In this article we have investigated the effects of strong magnetic field on the properties of quarkonia immersed in a thermal medium of quarks and gluons and then studied the quasi-free dissociation of quarkonia due to the Landau-damping. Thermalising the Schwinger propagator for quarks in the lowest Landau levels and the Feynman propagator for gluons in real-time formalism, we have calculated the resummed retarded and symmetric propagators, which in turn give the real and imaginary components of dielectric permittivity, respectively. Finally the inverse Fourier transform of the permittivities encrypt the effect of hot QCD medium in the presence of strong magnetic field into both real and imaginary parts of heavy quark potential. We have found that the magnetic field largely affects the large-distance interaction, as a result, the real part of potential becomes more attractive and the magnitude of imaginary part too becomes larger, compared to its counterpart in the absence of magnetic field. The real part of the potential is thereafter solved numerically by the Schrödinger equation to obtain the energy eigenvalues and energy eigenfunctions of the charmonium states. We have noticed that in the presence of strong magnetic field, the size (r2) of J/ψ and ψ′ are swelled whereas χc gets shrunk, unless the temperature is very high. Similarly the magnetic field affects the binding of J/ψ and χc differently, i.e. it decreases the binding of J/ψ but increases for χc. On contrary the magnetic field increases the width of the resonances, unless the temperature is sufficiently high. We have finally studied the dissociation due to the Landau damping and found that the dissociation temperatures become higher in the presence of magnetic field. For example, with eB=6mπ2 the J/ψ is dissociated at 2Tc, and with eB=4mπ2 the χc is dissociated at 1.1Tc in comparison, with eB = 0 the J/ψ is dissociated at T=1.6Tc and χc is dissociated at T=0.8Tc. However, with the further increase of magnetic field the dissociation temperatures decrease.

Modified Boltzmann approach for modeling the splitting vertices induced by the hot QCD medium in the deep Landau-Pomeranchuk-Migdal region

Hard probes produced in perturbative processes are excellent probes for the study of the hot and dense QCD matter created in relativistic heavy-ion collisions. Transport theory, allowing for coupling to an evolving medium with fluctuating initial conditions, has become a powerful tool in this endeavor. However, the implementation of the Landau-Pomeranchuk-Migdal (LPM) effect for medium-induced parton bremsstrahlung and pair production, poses a challenge to semi-classical transport models based on Boltzmann-type transport equations. In this work, we investigate a possible solution to approximate the LPM effect in a "modified Boltzmann transport" approach, including a prescription for the running coupling constant. By fixing a numerical parameter, this approach quantitatively reproduces the rates of medium-induced parton splitting predicted by the next-to-lead-log solution of the AMY equation which is valid in the deep-LPM regime of an infinite medium. We also find qualitative agreement of our implementation with calculations in a finite and expanding medium, but future improvements are needed for added precision at small path length. This work benefits transport model-based studies and the usage of these models in the phenomenological extraction of the jet transport coefficient.

Measurement of angular and momentum distributions of charged particles within and around jets in Pb+Pb and pp collisions at sNN=5.02 TeV with the ATLAS detector

Studies of the fragmentation of jets into charged particles in heavy-ion collisions can provide information about the mechanism of jet-quenching by the hot and dense QCD matter created in such collisions, the quark-gluon plasma. This paper presents a measurement of the angular distribution of charged particles around the jet axis in $\sqrt{s_{\mathrm{NN}}}=$ 5.02 TeV Pb+Pb and $pp$ collisions, using the ATLAS detector at the LHC. The Pb+Pb and $pp$ data sets have integrated luminosities of 0.49 nb$^{-1}$ and 25 pb$^{-1}$, respectively. The measurement is performed for jets reconstructed with the anti-$k_{t}$ algorithm with radius parameter $R = 0.4$ and is extended to an angular distance of $r= 0.8$ from the jet axis. Results are presented as a function of Pb+Pb collision centrality and distance from the jet axis for charged particles with transverse momenta in the 1$-$63 GeV range, matched to jets with transverse momenta in the 126$-$316 GeV range and an absolute value of jet rapidity of less than 1.7. Modifications to the measured distributions are quantified by taking a ratio to the measurements in $pp$ collisions. Yields of charged particles with transverse momenta below 4 GeV are observed to be increasingly enhanced as a function of angular distance from the jet axis, reaching a maximum at $r=0.6$. Charged particles with transverse momenta above 4 GeV have an enhanced yield in Pb+Pb collisions in the jet core for angular distances up to $r = 0.05$ from the jet axis, with a suppression at larger distances.

Quarkonium masses in a hot QCD medium using conformable fractional of the Nikiforov–Uvarov method

By using the conformable fractional of the Nikiforov–Uvarov (CF–NU) method, the radial Schrödinger equation is analytically solved. The energy eigenvalues and corresponding functions are obtained, in which the dependent temperature potential is employed. The effect of fraction-order parameter is studied on the heavy-quarkonium masses such as charmonium and bottomonium in a hot QCD medium in the 3D and the higher-dimensional space. This paper discusses the flavor dependence of their binding energies and explores the nature of dissociation by employing the perturbative, nonperturbative, and the lattice-parametrized form of the Debye masses in the medium-modified potential. A comparison is studied with recent works. We conclude that the fractional-order plays an important role in a hot QCD medium in the 3D with consideration of a form of the Debye mass.

Thermomagnetic properties and Bjorken expansion of hot QCD matter in a strong magnetic field

In this work we have studied the effects of an external strong magnetic field on the thermodynamic and magnetic properties of a hot QCD matter and then explored these effects on the subsequent hydrodynamic expansion of the said matter once produced in the ultrarelativistic heavy ion collisions. For that purpose, we have computed the quark and gluon self-energies up to one loop in the strong magnetic field, using the HTL approximation with two hard scales - temperature and magnetic field, which in turn compute the effective propagators for quarks and gluons, respectively. Hence the quark and gluon contributions to the free energy are obtained from the respective propagators and finally derive the equation of state (EOS) by calculating the pressure and energy density. We have found that the speed of sound is enhanced due to the presence of strong magnetic field and this effect will be later exploited in the hydrodynamics. Thereafter the magnetic properties are studied from the free energy of the matter, where the magnetization is found to increase linearly with the magnetic field, thus hints the paramagnetic behavior. The temperature dependence of the magnetization is also studied, where the magnetization is found to increase slowly with the temperature. Finally, to see how a strong magnetic field could affect the hydrodynamic evolution, we have revisited the Bjorken boost-invariant picture with our paramagnetic EOS as an input in the equation of motion for the energy-momentum conservation. We have noticed that the energy density evolves faster than in the absence of strong magnetic field, i.e. cooling becomes faster, which could have implications on the heavy-ion phenomenology. As mentioned earlier, this observation can be understood by the enhancement of the speed of sound.

First Observation of the Directed Flow of D^{0} and D^{0}[over ¯] in Au+Au Collisions at sqrt[s_{NN}]=200 GeV.

We report the first measurement of rapidity-odd directed flow (v_{1}) for D^{0} and D^{0}[over ¯] mesons at midrapidity (|y|<0.8) in Au+Au collisions at sqrt[s_{NN}]=200 GeV using the STAR detector at the Relativistic Heavy Ion Collider. In 10-80% Au+Au collisions, the slope of the v_{1} rapidity dependence (dv_{1}/dy), averaged over D^{0} and D^{0}[over ¯] mesons, is -0.080±0.017(stat)±0.016(syst) for transverse momentum p_{T} above 1.5 GeV/c. The absolute value of D^{0} meson dv_{1}/dy is about 25 times larger than that for charged kaons, with 3.4σ significance. These data give a unique insight into the initial tilt of the produced matter, and offer constraints on the geometric and transport parameters of the hot QCD medium created in relativistic heavy-ion collisions.