Dense Quark-gluon Plasma Research Articles

Leading order track functions in a hot and dense QGP

We study the modifications to the fragmentation pattern of partons into charged particles in the presence of a hot and dense quark gluon plasma. To this end, we analyze the perturbative renormalization group equations of the track functions, which describe the energy fraction carried by charged hadrons. Focusing on pure Yang-Mills theory, we compute the lowest-order moments of the medium-modified track functions, which are found to be sensitive to the reduced phase space for emissions in the medium and to energy loss. We use the extracted moments to calculate the energy energy correlator (EEC) on tracks in the collinear limit. The EEC on medium-evolved tracks does not differ qualitatively from the EEC on vacuum tracks despite being sensitive to the color decoherence transition and suppressing the distribution due to quenching, as seen in other jet observables. Published by the American Physical Society 2024

Holographic transport coefficients and jet energy loss for the hot and dense quark-gluon plasma

We employ an Einstein-Maxwell-dilaton model, based on the gauge/gravity correspondence, to obtain the thermodynamics and transport properties for the hot and dense quark-gluon plasma. The model, which is constrained to reproduce lattice QCD thermodynamics at zero density, predicts a critical point and a first order line at finite temperature and density, is used to quantify jet energy loss through simulations of high-energy collision events.

Hard probes of heavy ion collisions with ATLAS

This proceeding gives an overview of the latest hard process measurements in heavy ion collision systems with the ATLAS detector at the LHC, utilizing the high statistics of 5.02 TeV Pb+Pb data collected in 2015 and 2018. These include multiple measurements of jet production and structure, which probe the dynamics of the hot, dense quark-gluon plasma (QGP) formed in relativistic nucleus-nucleus collisions. New results sensitive to the role of path length, color charge, parton mass, and parton fragmentation in the jet quenching will also be shown. Furthermore, new measurements of quarkonia and heavy flavor production to probe the QGP medium properties will be discussed. A particular focus of the measurements is the systematic comparison of fully unfolded data to state-of-the-art theoretical models.

Holographic thermodynamics and transport for the hot and baryon dense quark-gluon plasma

This is a contribution to the Proceedings of the XLIV Brazilian Workshop on Nuclear Physics, held in virtual format during 9-11 November, 2021. In this contribution I briefly review some of the main results obtained with collaborators regarding quantitative predictions from an Einstein-Maxwell-Dilaton holographic model for the thermodynamics and some of the transport coefficients of the hot and baryon dense quark-gluon plasma produced in relativistic heavy ion collisions.

Transport properties and equation-of-state of hot and dense QGP matter near the critical endpoint in the phenomenological dynamical quasiparticle model

We extend the effective dynamical quasiparticle model (DQPM)---constructed for the description of nonperturbative QCD phenomena of the strongly interacting quark-gluon plasma (QGP)---to large baryon chemical potentials, ${\ensuremath{\mu}}_{B}$, including a critical endpoint (CEP) and a first-order phase transition. The DQPM description of quarks and gluons is based on partonic propagators with complex self-energies where the real part of the self-energies is related to the quasiparticle mass and the imaginary part to a finite width of their spectral functions (i.e., the imaginary parts of the propagators). In DQPM the determination of complex self-energies for the partonic degrees of freedom at zero and finite ${\ensuremath{\mu}}_{B}$ has been performed by adjusting the entropy density to the lattice QCD data. The temperature-dependent effective coupling (squared) ${g}^{2}(T/{T}_{c})$, as well as the effective masses and widths of the partons are based on this adjustments. The novel extended dynamical quasiparticle model, named ``DQPM-CP,'' makes it possible to describe thermodynamical and transport properties of quarks and gluons in a wide range of temperature, $T$, and baryon chemical potential, ${\ensuremath{\mu}}_{B}$, and reproduces the equation of state of lattice QCD calculations in the crossover region of finite $T,{\ensuremath{\mu}}_{B}$. We apply a scaling ansatz for the strong coupling constant near the CEP, located at $({T}^{\mathrm{CEP}},{\ensuremath{\mu}}_{B}^{\mathrm{CEP}})=(0.100,0.960)\text{ }\text{ }\mathrm{GeV}$. We show the equation of state as well as the speed of sound for $T>{T}_{c}$ and for a wide range of ${\ensuremath{\mu}}_{B}$, which can be of interest for hydrodynamical simulations. Furthermore, we consider two settings for the strange quark chemical potentials, (I) ${\ensuremath{\mu}}_{q}={\ensuremath{\mu}}_{u}={\ensuremath{\mu}}_{s}={\ensuremath{\mu}}_{B}/3$ and (II) ${\ensuremath{\mu}}_{s}=0,{\ensuremath{\mu}}_{u}={\ensuremath{\mu}}_{d}={\ensuremath{\mu}}_{B}/3$. The isentropic trajectories of the quark-gluon plasma matter are compared for these two cases. The phase diagram of DQPM-CP is close to PNJL calculations. The leading order pQCD transport coefficients of both approaches differ. This elucidates that the knowledge of the phase diagram alone is not sufficient to describe the dynamical evolution of strongly interacting matter.

Hot and dense quark-gluon plasma thermodynamics from holographic black holes

We present new results on the equation of state and transition line of hot and dense strongly interacting QCD matter, obtained from a bottom-up Einstein-Maxwell-Dilaton holographic model. We considerably expand the previous coverage in baryon densities in this model by implementing new numerical methods to map the holographic black hole solutions onto the QCD phase diagram. We are also able to obtain, for the first time, the first-order phase transition line in a wide region of the phase diagram. Comparisons with the most recent lattice results for the QCD thermodynamics are also presented.

A modified in-medium evolution equation with color coherence

QCD jets produced in heavy-ion collisions at LHC or RHIC energies partially evolve inside the produced hot and dense quark gluon plasma, offering unique opportunities to study QCD splitting processes in different backgrounds. Induced (modified) splittings are expected to be the most relevant mechanism driving the modifications of in-medium jets compared to vacuum jets for a wide sets of observables. Although color coherence among different emitters has been identified as an essential mechanism in studies of the QCD antenna radiation, it is usually neglected in the multi-gluon medium-induced cascade. This independent gluon emission approximation can be analytically proved to be valid in the limit of very large media, but corrections or modifications to it have not been computed before in the context of the evolution (or rate) equation describing the gluon cascade. We propose a modified evolution equation that includes corrections due to the interference of subsequent emitters. In order to do so, we first compute a modified splitting kernel following the usual procedure of factorizing it from the subsequent Brownian motion. The calculation is performed in the two-gluon configuration with no overlapping formation times, that is expected to provide the first correction to the completely independent picture.

Jet radiation in a longitudinally expanding medium

In a series of previous papers, we have presented a new approach, based on perturbative QCD, for the evolution of a jet in a dense quark-gluon plasma. In the original formulation, the plasma was assumed to be homogeneous and static. In this work, we extend our description and its Monte Carlo implementation to a plasma obeying Bjorken longitudinal expansion. Our key observation is that the factorisation between vacuum-like and medium-induced emissions, derived in the static case, still holds for an expanding medium, albeit with modified rates for medium-induced emissions and transverse momentum broadening, and with a modified phase-space for vacuum-like emissions. We highlight a scaling relation valid for the energy spectrum of medium-induced emissions, through which the case of an expanding medium is mapped onto an effective static medium. We find that scaling violations due to vacuum-like emissions and transverse momentum broadening are numerically small. Our new predictions for the nuclear modification factor for jets RAA, the in-medium fragmentation functions, and substructure distributions are very similar to our previous estimates for a static medium, maintaining the overall good qualitative agreement with existing LHC measurements. In the case of RAA, we find that the agreement with the data is significantly improved at large transverse momenta pT ≳ 500 GeV after including the effects of the nuclear parton distribution functions.

Anisotropic flow of photons in relativistic heavy ion collisions

Electromagnetic radiations are one of the potential probes to study the initial state of the hot and dense quark-gluon plasma (QGP) produced during the collision of heavy nuclei at relativistic energies. Photons are emitted throughout the lifetime of the evolving system and carry undistorted information from the production point to the detector. The observation of large anisotropic flow of charged particles provides a strong confirmation of QGP formation and collective behaviour of the produced matter in these collisions. However, the theoretical model calculations which explain the hadronic spectra and anisotropic flow successfully, underpredict the experimental data of elliptic as well as triangular flow of photons by a large margin. This discrepancy between data and theory results is known as direct photon puzzle. In this article, we review the anisotropic flow of photons calculated using hydrodynamical model framework for different collision systems and beam energies. In addition, we propose some new ideas which can be valuable for understanding direct photon puzzle.

Highlights from the ATLAS experiment

The ATLAS experiment at the LHC has undertaken an extensive program to probe and characterize the hot and dense quark-gluon plasma created in relativistic heavy ion collisions and, in general, to study non-perturbative and collective properties of the strong interaction. In this report we summarize the most recent results on the collectivity in pp, p+Pb and nucleus-nucleus collisions, results on jets, quarkonia, heavy flavor and electroweak boson production, measured in Pb+Pb and p+Pb collisions. This report covers also new results on electromagnetic processes studied in ultra-peripheral collisions.

Nuclear modification factors for jet fragmentation

Using a recently-developed perturbative-QCD approach for jet evolution in a dense quark-gluon plasma, we study the nuclear modification factor for the jet fragmentation function. The qualitative behaviour that we find is in agreement with the respective experimental observations in Pb+Pb collisions at the LHC: a pronounced nuclear enhancement at both ends of the spectrum. Our Monte Carlo simulations are supplemented with analytic estimates which clarify the physical interpretation of the results. The main source of theoretical uncertainty is the sensitivity of our calculations to a low-momentum cutoff which mimics confinement. To reduce this sensitivity, we propose a new observable, which describes the jet fragmentation into subjets and is infrared-and-collinear safe by construction. We present Monte Carlo predictions for the associated nuclear modification factor together with their physical interpretation.

Electrical conductivity and Hall conductivity of a hot and dense quark gluon plasma in a magnetic field: A quasiparticle approach

We estimate here the electrical and Hall conductivity using a quasiparticle approach for quark matter. We use a Boltzmann kinetic approach in presence of external magnetic field. We confront the results of model calculations with Lattice QCD simulations for vanishing magnetic field. In general electrical conductivity decreases with magnetic field. The Hall conductivity on the other hand can show a non monotonic behaviour with magnetic field due to an intricate interplay of behaviour of relaxation time and strength of the magnetic field. We argue for vanishing quark chemical potential Hall conductivity vanishes and quark gluon plasma with finite quark chemical potential can show Hall effect. Both electrical conductivity and Hall conductivity increases with increasing quark chemical potential.

Electrical conductivity of a hot and dense QGP medium in a magnetic field

We compute the electrical conductivity ($ \sigma_{el} $) in the presence of constant and homogeneous external electromagnetic field for the static quark-gluon plasma (QGP) medium, which is among the important transport coefficients of QGP. We present the derivation of the electrical conductivity by solving the relativistic Boltzmann kinetic equation in the relaxation time approximation in the presence of magnetic field ($ B $). We investigate the dependence of electrical conductivity on the temperature and finite chemical potential in magnetic field. We find that electrical conductivity decreases with the increase in the presence of magnetic field. We observe that $ \sigma_{el} $ at a nonzero $ B $ remains within the range of the lattice and model estimate at $ B\neq 0 $. Further, we extend our calculation at finite chemical potential.

Calculating the equation of state of dense quark-gluon plasma using the complex Langevin equation

The pressure and energy density of the quark-gluon plasma at finite baryon chemical potential are calculated using the Complex Langevin equation. The stout smearing procedure is generalized for the SL(3,$\mathcal{C}$) manifold allowing the usage of an improved action in the Complex Langevin setup. Four degenerate flavors of staggered quarks with $m_\pi=500-700$ MeV are used with a tree-level Symanzik improved gauge action on $16^3 \times 8 $ lattices. Results are compared to the Taylor expansion and good agreement is found for small chemical potentials.

Deciphering the \U0001d4cfg distribution in ultrarelativistic heavy ion collisions

Within perturbative QCD, we develop a new picture for the parton shower generated by a jet propagating through a dense quark-gluon plasma. This picture combines in a simple, factorised, way multiple medium-induced parton branchings and standard vacuum-like emissions, with the phase-space for the latter constrained by the presence of the medium. We implement this picture as a Monte Carlo generator that we use to study two phenomenologically important observables: the jet nuclear modification factor RAA and the \U0001d4cfg distribution reflecting the jet substructure. In both cases, the outcome of our Monte Carlo simulations is in good agreement with the LHC measurements. We provide basic analytic calculations that help explaining the main features observed in the data. We find that the energy loss by the jet is increasing with the jet transverse momentum, due to a rise in the number of partonic sources via vacuum-like emissions. This is a key element in our description of both RAA and the \U0001d4cfg distribution. For the latter, we identify two main nuclear effects: incoherent jet energy loss and hard medium-induced emissions. As the jet transverse momentum increases, we predict a qualitative change in the ratio between the \U0001d4cfg distributions in PbPb and pp collisions: from increasing at small \U0001d4cfg, this ratio becomes essentially flat, or even slightly decreasing.

Holographic Renormalization Group Flows

We briefly review recent applications of holographic renormalization group flow equations in a hot and dense quark-gluon plasma (QGP). We especially focus on presenting a few examples typically used in the holographic description of the QGP. We study the influence of the chemical potential on the holographic β-function in these examples.

Effect of the chiral phase transition on axion mass and self-coupling

We compute the effect of the chiral phase transition of QCD on the axion mass and self-coupling; the coupling of the axion to the quarks at finite temperature is described within the Nambu--Jona-Lasinio model. We find that the axion mass decreases with temperature, following the response of the topological susceptibility, in agreement with previous results obtained within chiral perturbation theory at low and intermediate temperatures. As expected, the comparison with lattice data shows that chiral perturbation theory fails to reproduce the topological susceptibility around the chiral critical temperature, while the Nambu--Jona-Lasinio model offers a better qualitative agreement with these data, hence a more reliable estimate of the temperature dependence of the axion mass in the presence of a hot quark medium. We complete our study by computing the temperature dependence of the self-coupling of the axion, finding that this coupling decreases at and above the phase transition. The model used in our work as well as the results presented here pave the way to the computation of the in-medium effects of hot and/or dense quark-gluon plasma on the axion properties.

Production and Hadronic Decays of Higgs Bosons in Heavy-Ion Collisions.

We examine Higgs boson production and decay in heavy-ion collisions at the LHC and future colliders. Owing to the long lifetime of the Higgs boson, its hadronic decays may experience little or no screening from the hot and dense quark-gluon plasma, whereas jets from hard scattering processes and from decays of the electroweak gauge bosons and the top quark suffer significant energy loss. This distinction can lead to enhanced signal to background ratios in hadronic decay channels and thus, for example, provide alternative ways to probe the Yukawa coupling of the Higgs boson to the bottom quark and its lifetime.

Adding vacuum branching to jet evolution in a dense medium

We study the fragmentation of a jet propagating in a dense quark-gluon plasma. We show that the “vacuum-like” emissions triggered by the parton virtualities can be factorized from the medium-induced radiation responsible for the energy loss within a controlled, “double-logarithmic”, approximation in perturbative QCD. We show that the collisions with the plasma constituents modify the vacuum-like parton shower already at leading twist, in two ways: the radiation phase-space is reduced and the first emission outside the medium can violate angular ordering. We compute the jet fragmentation function and find results in qualitative agreement with measurements at the LHC.

Resolving discrepancies in the estimation of heavy quark transport coefficients in relativistic heavy-ion collisions

Heavy flavor observables provide valuable information on the properties of the hot and dense Quark-Gluon Plasma (QGP) created in ultra-relativistic nucleus-nucleus collisions. Various microscopic models have successfully described many of the observables associated with its formation. Their transport coefficients differ, however, due to different assumptions about the underlying interaction of the heavy quarks with the plasma constituents, different initial geometries and formation times, different hadronization processes and a different time evolution of the QGP. In this study we present the transport coefficients of all these models and investigate systematically how some of these assumptions influence the heavy quark properties at the end of the QGP expansion. For this purpose we impose on these models the same initial condition and the same model for the QGP expansion and show that both have considerable influence on $R_{AA}$ and $v_2$.