Quark-gluon Plasma Research Articles

Heavy-flavor anisotropic flow at RHIC and LHC energies within a full transport approach

The propagation of heavy quarks (HQs) in the quark-gluon plasma (QGP) is described by means of a full Boltzmann transport approach. The nonperturbative dynamics and the interaction between HQs and the bulk is taken into account by means of a Quasi-Particle Model. Including the intense electromagnetic and vortical fields, we discuss their impact on the directed flow of neutral D mesons at RHIC energy, clarifying the role of this observable in giving information on QGP transport properties. We also show a novel study of the correlations between different flow coefficients of D mesons and soft hadrons at LHC energy within a coalescence plus fragmentation hadronization scheme and including event-by-event initial state fluctuations. This investigations can put further constraints on HQ transport coefficients towards a solid determination in comparison to the lattice QCD calculations.

Highlights from the NA61/SHINE

NA61/SHINE is a multipurpose fixed-target facility at the CERN Super Proton Synchrotron. The main goals of the NA61/SHINE stronginteractions program are to discover the critical point of strongly interacting matter as well as to determine the properties of produced particles relevant for the study of the onset of deconfinement - the transition between the state of hadronic matter and the quark-gluon plasma. An analysis of hadron production properties is performed in nucleus-nucleus, proton-proton, and proton-nucleus interactions as a function of collision energy and size of the colliding nuclei to achieve these goals. The selected NA61/SHINE results from this program are presented. In particular, the latest results from different reactions p+p, Be+Be, Ar+Sc, and Pb+Pb on hadron spectra and fluctuations are discussed. The NA61/SHINE results are compared with worldwide experiments and predictions from various theoretical models, like EPOS, PHSD, UrQMD, and others.

Impact parameter dependence of photon-photon interactions in relativistic heavy-ion collisions

The Lorentz-boosted electromagnetic fields surrounding relativistic heavy ions with large charges can be treated as a flux of linearly polarized quasireal photons, which can interact via the photon-photon scattering to produce lepton antilepton pairs. Those photon-photon interactions can happen even in heavy-ion collisions with hadronic overlap, making an opportunity to probe the electromagnetic properties of the produced deconfined quark-gluon plasma. In this paper, we review the recent experimental progress of the impact parameter dependent photon-photon interactions in heavy-ion collisions, and discuss their essential role in probing the possible electromagnetic properties of quark-gluon plasma produced in hadronic heavy-ion collisions.

Investigation of in-medium effects of charmonia using azimuthal anisotropy and jet fragmentation function in PbPb collisions at √SNN = 5.02 TeV with the CMS experiment

Quarkonia have been long proposed as the golden probes to study quark-gluon plasma (QGP) in heavy-ion collisions. In this presentation, we present the second- and third-order Fourier coefficients, v2 and v3, for prompt and nonprompt J/ψ mesons in PbPb collisions as functions transverse momentum (pT) and PbPb collision centrality. Also, we report the first measurements of v2 and v3 for the prompt ψ(2S) mesons in PbPb collisions. The results provide v2 and v3 values over the wide studied kinematics regions of transverse momentum and collision centrality.

Jet quenching effect in relativistic heavy-ion collisions

One of the main goals of high-energy nuclear physics is to explore the fundamental properties of quark-gluon plasma (QGP), a new state of quantum chromodynamics (QCD) matter created in relativistic heavy-ion collisions, in which the energetic quarks and gluons, known as fast partons, created prior to the formation of the QGP, traverse the hot-dense medium and experience strong interactions with the constituents of the medium, and eventually lead to the attenuation of jet energy. Such a novel phenomenon, referred to as jet quenching, plays an essential role in probing the transport properties of the QGP. The objective of this paper is to review some of the latest experimental and theoretical progress of jet quenching, such as medium modification on the large <inline-formula><tex-math id="M11111">\begin{document}$ p_{\rm T} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20230993_M11111.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20230993_M11111.png"/></alternatives></inline-formula> hadrons, full jets, and jet substructures in heavy-ion collisions, as well as the challenges in the forefront theoretical investigations.

Interplay between core and corona from small to large systems

We present new results in p+p and Pb+Pb collisions at the LHC energies from the updated dynamical core–corona initialization framework (DCCI2). The fractions of final hadron yields originating from equilibrated and non-equilibrated components are extracted as functions of multiplicity. We find that the contributions from non-equilibrated components are non-negligible even in Pb+Pb collisions and affect pT-integrated multi-particle correlations. These suggest the importance of non-equilibrated components for the sophisticated extraction of properties of the quark gluon plasma from comparisons between dynamical frameworks and experimental data.

Bayesian Quantification of the Quark–Gluon Plasma: Improved Design and Closure Demonstration

Bayesian Quantification of the Quark–Gluon Plasma: Improved Design and Closure Demonstration

Insights into the initial conditions and evolution of hadronic collisions with flow observables

Studies of anisotropic flow in heavy-ion collisions are one of the critical elements in understanding the production and evolution of the Quark Gluon Plasma. These proceedings review the most recent results on anisotropic flow measurements at the LHC energies. In particular, recent measurements of pT-differential v2{4} in Pb–Pb collisions for a plethora of light-flavour particles are discussed and compared to models with and without partonic coalescence. The measurements of v2{2} and v2{4} allow for the calculation of relative flow fluctuations, which for the first time are also reported for (multi)strange baryons. The study is further extended to higher moments of the underlying charged-particle v2 PDF, where a transition from non-Gaussian to Gaussian fluctuations is observed. Finally, the correlation between v2 and mean transverse momentum [pT] is studied in terms of the Pearson Correlation Coefficient. This opens new venues to study nuclear deformation in ultra-central heavy-ion collisions and could provide a tool to identify processes in the pre-QGP phase.

Spin-orbital coupling in strong interaction and global spin polarization

In non-central relativistic heavy ion collisions, the colliding nuclear system possesses a huge global orbital angular momentum in the direction opposite to the normal of the reaction plane. Due to the spin-orbit coupling in strong interaction, such a huge orbital angular momentum leads to a global spin polarization of the quark matter system produced in the collision process. The global polarization effect in high energy heavy ion collisions was first predicted theoretically and confirmed by STAR experiments at the Relativistic Heavy Ion Collider in Brookhaven National Laboratory. The discovery has attracted much attention to the study of spin effects in heavy ion collision and leads to a new direction in high energy heavy ion physics—Spin Physics in Heavy Ion Collisions. In this paper, we briefly review the original ideas, the calculation methods, the main results and recent theoretical developments in last years. First, we present a short discussion of the spin-orbit coupling which is an intrinsic property for a relativistic fermionic quantum system. Then we review how the global orbital angular momentum can be generated in non-central heavy ion collisions and how the global orbital angular momentum can be transferred to the local orbital angular momentum distribution in two limit model---Landan fireball model and Bjorken scaling model. After that, we review how we can describe the scattering process with initial local orbital angular momentum in the formalism of scattering cross section in impact parameter space and how we calculate the polarization of the quarks and antiquarks in quark gluon plasma produced in non-central heavy ion collisions after single or multiple scattering. We also give a brief review on how the global polarization can be predicted from the formalism of relativistic hydrodynamics with the generalized Cooper-Frye formula with spin. Finally, we discuss how the quark's polarization can be transferred to the final hadron's polarization. We focus on the hyperon's polarization and vector meson's spin alignment produced in heavy-ion collisions.

White Paper on Forward Physics, BFKL, Saturation Physics and Diffraction

The goal of this whitepaper is to give a comprehensive overview of the rich field of forward physics. We discuss the occurrences of BFKL resummation effects in special final states, such as Mueller-Navelet jets, jet gap jets, and heavy quarkonium production. It further addresses TMD factorization at low x and the manifestation of a semi-hard saturation scale in (generalized) TMD PDFs. More theoretical aspects of low x physics, probes of the quark gluon plasma, as well as the possibility to use photon-hadron collisions at the LHC to constrain hadronic structure at low x, and the resulting complementarity between LHC and the EIC are also presented. We also briefly discuss diffraction at colliders as well as the possibility to explore further the electroweak theory in central exclusive events using the LHC as a photon-photon collider.

Search for the Chiral Magnetic Wave in Pb–Pb collisions using the ALICE detector

The interplay of the chiral anomaly and the strong magnetic field (∼1015 T) created in heavy-ion collisions could give rise to a collective excitation in the quark–gluon plasma called the Chiral Magnetic Wave (CMW). This effect can be experimentally sought by the charge asymmetry (Ach) dependence of elliptic flow v2. However, non-CMW mechanisms such as local charge conservation (LCC) coupled with collective flow can also lead to a similar dependence of v2 on Ach. The triangular flow (v3) measurement serves as a reference as it is not expected to be affected by the CMW. The v2 and v3 of charged hadrons as a function of Ach measured in Pb–Pb collisions at √SNN=2.76 and 5.02 TeV are presented. In addition, the event-shape engineering (ESE) technique is adopted for the first time to quantitatively disentangle the CMW signal and the LCC background. The results indicate that the background effects dominate the CMW measurements.

Multiplicity-dependent production of heavy mesons with strangeness in small systems at LHCb

The phenomenon of strangeness enhancement, originally proposed as a signature of quark-gluon plasma formation, has received considerable new interest following recent observations in small collision systems. LHCb is uniquely well suited to study such effects in the heavy quark sector, down to very low transverse momentum. Here we will present new LHCb results on the production rates of B0s relative to B0 mesons versus multiplicity in pp collisions. Potential implications for the hadronization mechanism of heavy quarks and our understanding of the factorization of fragmentation functions will be discussed.

Effective Shear and Bulk Viscosities of the Quark–Gluon Plasma: QCD Versus Heavy-ion Data

In recent years, there has been a significant effort to extract the temperature-dependent shear ($\eta/s$) and bulk ($\zeta/s$) viscosity over entropy ratios of the quark-gluon plasma from a global comparison of heavy-ion data with results of hydrodynamic simulations. However, anisotropic flow, which is arguably the most sensitive probe of viscosity, is only sensitive to an {\it effective\/} viscosity over entropy ratio, which is obtained by taking a weighted average over the temperature, and summing the contributions of shear and bulk. We estimate this effective viscosity using existing first-principles calculations, which give $0.17<(\eta/s)_{\rm eff}<0.21$, and $(\zeta/s)_{\rm eff}<0.08$, implying that the damping of anisotropic flow at the LHC is mostly due to shear viscosity. The values extracted from global data analyses are compatible with these theory predictions.

Local equilibrium and Lambda polarization in high energy heavy ion collisions

The polarization of the Λ hyperon has become an important probe of the Quark-Gluon Plasma produced in relativistic heavy-ion collisions. Recently, it has been found that polarization receives a substantial contribution from a local equilibrium term proportional to the symmetric derivative of the four-temperature vector, the thermal shear tensor. We show that, at very high energies, this term can restore the agreement between the experimental measurements and the predictions of the hydrodynamic model, provided that the hadronization hypersurface is isothermal. We review the theoretical derivation of this new term, discuss numerical computations at RHIC and LHC energies, and compare them with the experimental data. We also present the effect of feed-down corrections.

Status and Performance of sPHENIX Experiment

sPHENIX is a state-of-the-art experiment at the Relativistic Heavy Ion Collider. Hard probes are to be measured in p+p, p+Au and Au+Au collisions to understand the properties of the quark-gluon plasma. sPHENIX covers a wide range of physics programs: jet correlations and substructure, upsilon spectroscopy, open heavy flavor and cold quantum chromodynamics. It provides unique opportunities in the low transverse momentum region and also offer kinematic overlap with the Large Hadron Collider experiments. In these proceedings, the scientific mission of sPHENIX, detector design and key performance parameters are described, and expected projections of some measurements under consideration are presented.

Transport Properties of the QCD Medium

I present an overview of recent developments in the microscopic description of the quark-gluon plasma. I will concentrate on medium-induced emission and transverse momentum broadening. These are two key ingredients of the theory of jet modifications in the QCD medium and of the kinetic theory used for transport and thermalisation. The main focus is on progress towards a better understanding of theory and of its uncertainties.

PHENIX Probing QCD Matter Through Heavy Flavor and Quarkonium at RHIC

Recent results from the PHENIX experiment on heavy flavors and quarkonia production in p + Al, p + Au, d + Au, and Au + Au collision systems at √ SNN = 200 GeV are summarized. The results are carried out by the measurements of the nuclear modification factors and elliptic flow. The nuclear modification factors measurements give insight into the energy loss of heavy quarks in the quark-gluon plasma medium along their path lengths. The elliptic flow measurements are a good tool to investigate the coupling of heavy quarks with the medium. The measurements are presented as a function of centrality, rapidity, and transverse momentum. The interpretations of the results in light of our current theoretical models, and comparison to LHCb and ALICE measurements are presented.

Phase Transitions under Strong Magnetic Fields in Neutron Star

A comprehensive study is carried out on the impact of strong magnetic fields on the deconfinement phase transition inside massive neutron stars. The matter equation of state and the general relativity solutions, which also fulfill Maxwell’s equations, are modified when taking magnetic-field effects into account. We observe that the maximum mass and canonical-mass radius of stars computed using spherically-symmetric TOV equations and axisymmetric solutions obtained through the LORENE library differ significantly for large values of magnetic dipole moment. The discrepancies depend on the stellar mass being studied, as well as the stiffness of the equation of state. This indicates that the matter composition and interactions determine the magnetic field thresholds for the acceptable approximation of isotropic stars and the appropriate application of TOV equations.

Magnetic-field Induced Deformation in Hybrid Stars

The effects of strong magnetic fields on the deconfinement phase transition expected to take place in the interior of massive neutron stars are studied in detail for the first time. For hadronic matter, the very general density-dependent relativistic mean field model is employed, while the simple, but effective vector-enhanced bag model is used to study quark matter. Magnetic-field effects are incorporated into the matter equation of state and in the general-relativity solutions, which also satisfy Maxwell’s equations. We find that for large values of magnetic dipole moment, the maximum mass, canonical mass radius, and dimensionless tidal deformability obtained for stars using spherically symmetric Tolman–Oppenheimer–Volkoff (TOV) equations and axisymmetric solutions attained through the LORENE library differ considerably. The deviations depend on the stiffness of the equation of state and on the star mass being analyzed. This points to the fact that, unlike what was assumed previously in the literature, magnetic field thresholds for the approximation of isotropic stars and the acceptable use of TOV equations depend on the matter composition and interactions.

Chiral symmetry restoration and properties of Goldstone bosons at finite temperature* *Supported in part by the National Natural Science Foundation of China (12075117, 11775112, 11535005, 11690030, 11905104, 11805024, 11947406) and Jiangsu Provincial Natural Science Foundation of China (BK20180323)

We study chiral symmetry restoration by analyzing thermal properties of QCD's(pseudo-) Goldstone bosons, especially the pion. The meson properties are obtained from the spectral densities of mesonic imaginary-time correlation functions. To obtain the correlation functions, we solve the Dyson-Schwinger equations and the inhomogeneous Bethe-Salpeter equations in the leading symmetry-preserving rainbow-ladder approximation. In chiral limit, the pion and its partner sigma degenerate at the critical temperature . At , it was found that the pion rapidly dissociates, which signals deconfinement phase transition. Beyond the chiral limit, the pion dissociation temperature can be used to define the pseudo-critical temperature of the chiral phase crossover, which is consistent with that obtained by the maximum point of chiral susceptibility. A parallel analysis for kaon and pseudoscalar suggests that heavy mesons may survive above .