Hot QCD Research Articles

Heavy quark potential in a static and strong homogeneous magnetic field

We have investigated the properties of quarkonia in a thermal QCD medium in the background of strong magnetic field. For that purpose, we employ the Schwinger proper-time quark propagator in the lowest Landau level to calculate the one-loop gluon self-energy, which in the sequel gives the effective gluon propagator. As an artifact of strong magnetic field approximation (eB>>T^2 and eB>>m^2), the Debye mass for massless flavors is found to depend only on the magnetic field which is the dominant scale in comparison to the scales prevalent in the thermal medium. However, for physical quark masses, it depends on both magnetic field and temperature in a low temperature and high magnetic field but the temperature dependence is very meager and becomes independent of the temperature beyond a certain temperature and magnetic field. With the above mentioned ingredients, the potential between heavy quark (Q) and anti-quark (bar{Q}) is obtained in a hot QCD medium in the presence of a strong magnetic field by correcting both short- and long-range components of the potential in the real-time formalism. It is found that the long-range part of the quarkonium potential is affected much more by magnetic field as compared to the short-range part. This observation facilitates us to estimate the magnetic field beyond which the potential will be too weak to bind Qbar{Q} together. For example, the J/psi is dissociated at eB sim 10 m_pi ^2 and Upsilon is dissociated at eB sim 100 m_pi ^2 whereas its excited states, psi ^prime and Upsilon ^prime are dissociated at smaller magnetic field eB= m_pi ^2, 13 m_pi ^2, respectively.

The dark-matter axion mass

We evaluate the efficiency of axion production from spatially random initial conditions in the axion field, so a network of axionic strings is present. For the first time, we perform numerical simulations which fully account for the large short-distance contributions to the axionic string tension, and the resulting dense network of high-tension axionic strings. We find nevertheless that the total axion production is somewhat less efficient than in the angle-averaged misalignment case. Combining our results with a recent determination of the hot QCD topological susceptibility [1], we find that if the axion makes up all of the dark matter, then the axion mass is ma = 26.2 ± 3.4 μeV.

Study of jet quenching in an asymmetric system of ultrarelativistic Cu + Au nuclei using neutral pions in the PHENIX experiment

An asymmetric system of colliding copper and gold nuclei (Cu + Au) has a specific overlap geometry when compared to symmetric systems (Au + Au, Cu + Cu). Studying asymmetric heavy systems allows us to obtain additional restrictions for existing models and more accurate descriptions of the processes of parton energy loss in hot and dense QCD matter produced in collisions of heavy nuclei. This work reports nuclear modification factors for neutral pions in Cu + Au, Сu + Cu, and Au + Au collisions at an energy of 200 GeV, measured in the PHENIX experiment on the RHIC.

Inclusive jets and jet substructure in 2.76 TeV and 5.02 TeV pp and Pb+Pb collisions with the ATLAS detector

We report on new measurements of jet yields and jet fragmentation in 2.76 TeV and 5.02 TeV Pb+Pb and pp collisions that were performed with the ATLAS detector at the LHC. These new measurements are expected to provide an insight into the mechanism of the energy loss of partons traversing hot and dense QCD matter created in heavy ion collisions.

Feasibility study of heavy-ion collision physics at NICA JINR

The project NICA (Nuclotron-based Ion Collider fAcility) is aimed to study hot and baryon rich QCD matter in heavy ion collisions in the energy range up to sNN=11GeV. The heavy ion program includes a study of collective phenomena, dilepton, hyperon and hypernuclei production under extreme conditions of highest baryonic density. This program will be performed at a fixed target experiment BM@N and with MPD detector at the NICA collider.

Direct photon measurements in pp and Pb–Pb collisions with the ALICE experiment

Direct photon production in heavy-ion collisions provides a valuable set of observables to study the hot QCD medium. The direct photons are produced at different stages of the collision and escape the medium unaffected. In heavy-ion collisions, the direct photon yield at high transverse momentum (pT>5GeV/c) is dominated by prompt photons produced in hard scattering of incoming partons and provides information on nuclear parton distribution functions and on the initial parton dynamics. The low momentum component (pT≲5GeV/c) of the direct photon production is dominated by thermal radiation by the hot and dense matter created, carrying information on its space-time evolution, collective flow and temperature.We present recent ALICE results on direct photon production Pb-Pb collisions at 2.76 TeV and on direct photon production in pp at 7 TeV using isolation techniques. The results are compared to theoretical predictions and previous measurements.

Heavy Flavor Production, Flow and Energy Loss

An overview of the current status of the heavy quarks as probe of the hot QCD in-medium interaction is presented. Heavy quarks that produced out-of-equilibrium but strongly interacting with the created quark-gluon plasma (QGP) bulk medium offer the unique opportunity to have a probe that thermalize in a time scale comparable with the lifetime of the QGP. Finally, the possible role of heavy quark for probing the initial strong electromagnetic field created in relativistic heavy-ion collisions is discussed.

Measurements of jet quenching with semi-inclusive hadron+jet distributions in Au+Au collisions at sNN=200 GeV

The STAR Collaboration reports the measurement of semi-inclusive distributions of charged-particle jets recoiling from a high transverse momentum hadron trigger, in central and peripheral Au+Au collisions at sNN=200 GeV. Charged jets are reconstructed with the anti-kT algorithm for jet radii R between 0.2 and 0.5 and with low infrared cutoff of track constituents (pT>0.2 GeV/c). A novel mixed-event technique is used to correct the large uncorrelated background present in heavy ion collisions. Corrected recoil jet distributions are reported at midrapidity, for charged-jet transverse momentum pT,jetch<30 GeV/c. Comparison is made to similar measurements for Pb+Pb collisions at s=2.76 TeV, to calculations for p+p collisions at s=200 GeV based on the pythia Monte Carlo generator and on a next-to-leading order perturbative QCD approach, and to theoretical calculations incorporating jet quenching. The recoil jet yield is suppressed in central relative to peripheral collisions, with the magnitude of the suppression corresponding to medium-induced charged energy transport out of the jet cone of 2.8±0.2(stat)±1.5(sys) GeV/c, for 10<pT,jetch<20 GeV/c and R=0.5. No medium-induced change in jet shape is observed for R<0.5. The azimuthal distribution of low-pT,jetch recoil jets may be enhanced at large azimuthal angles to the trigger axis, due to scattering off quasiparticles in the hot QCD medium. Measurement of this distribution gives a 90% statistical confidence upper limit to the yield enhancement at large deflection angles in central Au+Au collisions of 50±30(sys)% of the large-angle yield in p+p collisions predicted by pythia.14 MoreReceived 4 February 2017DOI:https://doi.org/10.1103/PhysRevC.96.024905©2017 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasHard scatteringJets & heavy flavor physicsQuark-gluon plasmaRelativistic heavy-ion collisionsNuclear Physics

Low mass dielectron measurements in pp, p–Pb, and Pb–Pb collisions with ALICE at the LHC

Low mass dielectrons are important probes of the hot and dense QCD medium created in heavy-ion collisions, since the production of low mass dielectrons is sensitive to the thermodynamical properties of the medium and chiral symmetry restoration in the medium.The ALICE experiment at the LHC has measured low mass dielectrons in pp collisions at s=7 TeV and p–Pb collisions at sNN=5.02 TeV to study and disentangle effects due to the cold nuclear matter. Recently, ALICE measured low mass dielectron mass spectra for different pair pT ranges in Pb–Pb collisions at sNN=2.76 TeV for the study of the properties of the hot and dense medium. In this paper, the status of the low-mass dielectron measurements in all collision systems produced at the LHC is summarized and the production of low mass dielectrons including virtual photons is discussed. Future prospects of the low-mass dielectron measurements in ALICE at the LHC Run3 and Run4 will be discussed.

Charged pions tagged with polarized photons probing strong CP violation in a chiral-imbalance medium

It is expected that in a hot QCD system, a local parity-odd domain can be produced due to nonzero chirality, which is induced from the difference of winding numbers carried by the gluon topological configuration (QCD sphaleron). This local domain is called the chiral-imbalance medium characterized by nonzero chiral chemical potential, which can be interpreted as the time variation of the strong CP phase. We find that the chiral chemical potential generates the parity breaking term in the electromagnetic form factor of charged pions. Heavy ion collision experiments could observe the phenomenological consequence of this parity-odd form factor through the elastic scattering of a pion and a photon in the medium. Then we quantify the asymmetry rate of the parity violation by measuring the polarization of the photon associated with the pion, and discuss how it could be measured in a definite lab frame. We roughly estimate the typical size of the asymmetry, just by picking up the pion resonant process, and find that the signal can be sufficiently larger than possible background events from parity-breaking electroweak process. Our findings might provide a novel possibility to make a manifest detection for the remnant of the strong CP violation.

Drag and diffusion of heavy quarks in a hot and anisotropic QCD medium

The propagation of heavy quarks (HQs) in a medium was quite often modeled by the Fokker-Plank (FP) equation. Since the transport coefficients, related to drag and diffusion processes are the main ingredients in the FP equation, the evolution of HQs is thus effectively controlled by them. At the initial stage of the relativistic heavy ion collisions, asymptotic weak-coupling causes the free-streaming motions of partons in the beam direction and the expansion in transverse directions are almost frozen, hence an anisotropy in the momentum space sets in. Since HQs are too produced in the same time therefore the study of the effect of momentum anisotropy on the drag and diffusion coefficients becomes advertently desirable. In this article we have thus studied the drag and diffusion of HQs in the anisotropic medium and found that the presence of the anisotropy reduces both drag and diffusion coefficients. In addition, the anisotropy introduces an angular dependence to both the drag and diffusion coefficients, as a result both coefficients get inflated when the partons are moving transverse to the direction of anisotropy than parallel to the direction of anisotropy.

Collective excitations of hot QCD medium in a quasiparticle description

Collective excitations of a hot QCD medium are the main focus of the present article. The analysis is performed within semi-classical transport theory with isotropic and anisotropic momentum distribution functions for the gluonic and quark-antiquark degrees of freedom that constitutes the hot QCD plasma. The isotropic/equilibrium momentum distributions for gluons and quarks are based on a recent quasi-particle description of hot QCD equations of state. The anisotropic distributions are just the extensions of isotropic ones by stretching or squeezing them in one of the directions. The hot QCD medium effects in the model adopted here enter through the effective gluon and quark fugacities along with non-trivial dispersion relations leading to an effective QCD coupling constant. Interestingly, with these distribution functions the tensorial structure of the gluon polarization tensor in the medium turned out to be similar to the one for the non-interacting ultra-relativistic system of quarks/antiquarks and gluons . The interactions mainly modify the Debye mass parameter and , in turn, the effective coupling in the medium. These modifications have been seen to modify the collective modes of the hot QCD plasma in a significant way.

Momentum anisotropy effects for quarkonium in a weakly coupled quark-gluon plasma below the melting temperature

In the early stages of heavy-ion collisions, the hot QCD matter expands more longitudinally than transversely. This imbalance causes the system to become rapidly colder in the longitudinal direction and a local momentum anisotropy appears. In this paper, we study the heavy-quarkonium spectrum in the presence of a small plasma anisotropy. We work in the framework of pNRQCD at finite temperature. We inspect arrangements of non-relativistic and thermal scales complementary to those considered in the literature. In particular, we consider temperatures larger and Debye masses smaller than the binding energy, which is a temperature range relevant for presently running LHC experiments. In this setting we compute the leading thermal corrections to the binding energy and the thermal width induced by quarkonium gluo-dissociation.

Experimental overview on heavy-flavor production

Hadrons containing heavy-flavors are unique probes of the properties of the hot and dense QCD medium produced in heavy-ion collisions. Due to their large masses, heavy quarks are produced at the initial stage of the collision, almost exclusively via hard partonic scattering processes. Therefore, they are expected to experience the full collision history propagating through and interacting with the QCD medium. The parton energy loss, which is sensitive to the transport coefficients of the produced medium, can be studied experimentally by measuring the nuclear modification factor which accounts for the modification of the heavy–flavored hadron yield in Pb–Pb collisions with respect to pp collisions. In semi-central Pb–Pb collisions, the degree of thermalization of charm quarks in the QCD medium can be accessed via the measurement of the heavy flavor elliptic flow v2 at low pT. Furthermore, the measurement of heavy-flavors production in pp collisions allows testing the perturbative QCD calculations. The PHENIX and STAR Collaborations at the Relativistic Heavy-Ion Collider and ALICE, CMS and ATLAS Collaborations at the Large Hadron Collider have measured the production of charmonium and bottonium states as well as open heavy flavor hadrons via their hadronic and semi-leptonic decays at mid-rapidity and in the semi-muonic decay channel at forward rapidity in pp, p-A and A–A collisions in an energy domain that ranges from = 0.2 TeV to = 13 TeV in pp collisions and from = 0.2 TeV to = 5.02 TeV in A–A collisions. In this contribution the latest experimental results will be reviewed.

Jet fragmentation and multijet studies in heavy ion collisions at ATLAS

We report on two measurements done by ATLAS in a context of exploring the jet quenching physics at LHC: the jet fragmentation measurement and the measurement of neighbouring jet production. Both of these measurements were done using the data collected in 2011 Pb+Pb run providing Pb+Pb collisions at . These measurements are expected to provide an insight into the modifications of jets initiated by partons passing through hot and dense QCD matter created in heavy ion collisions.

The Gribov mode in hot QCD

In this proceedings, I summarize recent findings of a novel massless dubbed as mode, generated by the (chromo)magnetic scale g 2 T in hot QCD. The Gribov mode is a genuine non-Abelian mode inducing effects such as positivity violation.

Heavy flavor production in heavy-ion collisions from soft collinear effective theory

We review the approach to calculate open heavy flavor production in heavy-ion collisions based on Soft Collinear Effective Theory (SCET). We include both finite heavy quark masses in the SCET Lagrangian as well as Glauber gluons that describe the interaction of collinear partons with the hot and dense QCD medium. From the new effective field theory, we derive massive in-medium splitting kernels and we propose a new framework for including in-medium interactions consistent with next-to-leading order calculations in QCD. We present numerical results for the suppression of both D- and B-mesons and compare to results obtained within the traditional approach to parton energy loss. We find good agreement when comparing to existing data from the LHC at and 2.76 TeV.

Transverse momentum spectra and nuclear modification factors of charged particles at √sNN = 5.02 TeV measured by ALICE at the LHC

The ALICE experiment at the LHC is designed to study the properties of the Quark-Gluon Plasma (QGP) based on high energy pp, p-Pb and Pb-Pb collisions. Certain properties of these collisions can be studied by measuring the production of charged particles. A suppression of the yield of charged particles was observed at high pT by comparing central Pb-Pb events scaled by the number of binary collisions to pp collisions, in terms of the nuclear modification factor RAA. This suppression can be an effect of the energy loss of partons as they propagate in a hot and dense QCD medium (QGP).In the end of 2015, pp and Pb-Pb collisions at √sNN = 5.02 TeV were measured by ALICE. Here, transverse momentum distributions of inclusive charged particles in pp and Pb-Pb collisions at √sNN = 5.02 TeV as well as the nuclear modification factor in six centrality classes are presented and compared with model predictions.

Bulk viscous corrections to screening and damping in QCD at high temperatures

Non-equilibrium corrections to the distribution functions of quarks and gluons in a hot and dense QCD medium modify the “hard thermal loops” (HTL). The HTLs determine the retarded, advanced, and symmetric (time-ordered) propagators for gluons with soft momenta as well as the Debye screening and Landau damping mass scales. We compute such corrections to a thermal as well as to a non-thermal fixed point. The screening and damping mass scales are sensitive to the bulk pressure and hence to (pseudo-) critical dynamical scaling of the bulk viscosity in the vicinity of a second-order critical point. This could be reflected in the properties of quarkonium bound states in the deconfined phase and in the dynamics of soft gluon fields.

Open heavy-flavour measurements in p-Pb and Pb-Pb collisions with ALICE at the LHC

Heavy flavours are sensitive probes of the hot and dense QCD medium formed in high-energy heavy-ion collisions. Measurements of their production in p-Pb collisions are crucial for the interpretation of heavy-ion results, by investigating the cold nuclear matter effects. The open heavy-flavour production studied with ALICE at the LHC in p-Pb collisions at and in Pb-Pb collisions at are presented. Emphasis is given to the recent measurements of D0 production cross section down to pT=0, the nuclear modification factor of heavy-flavour hadron decay electrons in p-Pb collisions, the nuclear modification factor of D-meson, and heavy-flavour hadron decay electron elliptic flow in Pb-Pb collisions, as a function of centrality.