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Dmitri Kharzeev

Publications and source records attributed to Dmitri Kharzeev.

At least 37 records · Page 2Linked to original sources
arXiv2016

Self-similar inverse cascade of magnetic helicity driven by the chiral anomaly

For systems with charged chiral fermions, the imbalance of chirality in the presence of magnetic field generates an electric current - this is the Chiral Magnetic Effect (CME). We study the dynamical real-time evolution of electromagnetic fields coupled by the anomaly to the chiral charge density and the CME current by solving the Maxwell-Chern-Simons equations. We find that the CME induces the inverse cascade of magnetic helicity towards the large distances, and that at late times this cascade becomes self-similar, with universal exponents. We also find that in terms of gauge field topology the inverse cascade represents the transition from linked electric and magnetic fields (Hopfions) to the knotted configuration of magnetic field (Chandrasekhar-Kendall states). The magnetic reconnections are accompanied by the pulses of the CME current directed along the magnetic field lines. We devise an experimental signature of these phenomena in heavy ion collisions, and speculate about implications for condensed matter systems.

hep-th↗
arXiv2014

Magnetohydrodynamics and charged currents in heavy ion collisions

The hot QCD matter produced in any heavy ion collision with a nonzero impact parameter is produced within a strong magnetic field. We study the imprint the magnetic fields produced in non-central heavy ion collisions leave on the azimuthal distributions and correlations of the produced charged hadrons. The magnetic field is time-dependent and the medium is expanding, which leads to the induction of charged currents due to the combination of Faraday and Hall effects. We find that these currents result in a charge-dependent directed flow $v_1$ that is odd in rapidity and odd under charge exchange. It can be detected by measuring correlations between the directed flow of charged hadrons at different rapidities, $\langle v_1^\pm (y_1) v_1^\pm (y_2) \rangle$.

hep-ph↗
arXiv2014

Magnetohydrodynamics, charged currents and directed flow in heavy ion collisions

The hot QCD matter produced in any heavy ion collision with a nonzero impact parameter is produced within a strong magnetic field. We study the imprint that these fields leave on the azimuthal distributions and correlations of the produced charged hadrons. The magnetic field is time-dependent and the medium is expanding, which leads to the induction of charged currents due to the combination of Faraday and Hall effects. We find that these currents result in a charge- dependent directed flow v1 that is odd in rapidity and odd under charge exchange. It can be detected by measuring correlations between the directed flow of charged hadrons at different rapidities, $\langle v_1^\pm(y_1)v_1^\pm(y_2)\rangle$.

hep-ph↗
arXiv2012

Conformal anomaly as a source of soft photons in heavy ion collisions

We introduce a novel photon production mechanism stemming from the conformal anomaly of QCDxQED and the existence of strong (electro)magnetic fields in heavy ion collisions. Using the hydrodynamical description of the bulk modes of QCD plasma, we show that this mechanism leads to the photon production yield that is comparable to the yield from conventional sources. This mechanism also provides a significant positive contribution to the azimuthal anisotropy of photons, $v_2$, as well as to the radial "flow". We compare our results to the data from the PHENIX Collaboration.

hep-ph↗
arXiv2009

J/Psi production in heavy ion collisions and gluon saturation

We calculate the inclusive J/Psi production in heavy ion collisions including the effects of gluon saturation in the wave functions of the colliding nuclei. We argue that the dominant production mechanism in proton--nucleus and nucleus--nucleus collisions for heavy nuclei is different from the one in hadron-hadron interactions. We find that the rapidity distribution of primary J/Psi production is more peaked around midrapidity than the analogous distribution in elementary pp collisions. We discuss the consequences of this fact on the experimentally observed J/Psi suppression in Au-Au collisions at RHIC energies.

hep-ph↗
arXiv2009

Gluon saturation effects on J/Psi production in heavy ion collisions

We consider a novel mechanism for J/Psi production in nuclear collisions arising due to the high density of gluons. We calculate the resulting J/Psi production cross section as a function of rapidity and centrality. We evaluate the nuclear modification factor and show that the rapidity distribution of the produced J/Psi's is significantly more narrow in AA collisions due to the gluon saturation effects. Our results indicate that gluon saturation in the colliding nuclei is a significant source of J/Psi suppression that can be disentangled from the quark-gluon plasma effects.

hep-ph↗
arXiv2009

Broken scale invariance, massless dilaton and confinement in QCD

Classical conformal invariance of QCD in the chiral limit is broken explicitly by scale anomaly. As a result, the lightest scalar particle (scalar glueball, or dilaton) in QCD is not light, and cannot be described as a Goldstone boson. Nevertheless basing on an effective low-energy theory of broken scale invariance we argue that inside the hadrons the non-perturbative interactions of gluon fields result in the emergence of a massless dilaton excitation (which we call the "scalaron"). We demonstrate that our effective theory of broken scale invariance leads to confinement. This theory allows a dual formulation as a classical Yang-Mills theory on a curved conformal space-time background. Possible applications are discussed, including the description of strongly coupled quark-gluon plasma and the spin structure of hadrons.

hep-ph↗
arXiv2009

In search of the QCD-Gravity correspondence

Quantum Chromodynamics (QCD) is the fundamental theory of strong interactions. It describes the behavior of quarks and gluons which are the smallest known constituents of nuclear matter. The difficulties in solving the theory at low energies in the strongly interacting, non-perturbative regime have left unanswered many important questions in QCD, such as the nature of confinement or the mechanism of hadronization. In these lectures oriented towards the students we introduce two classes of dualities that attempt to reproduce many of the features of QCD, while making the treatment at strong coupling more tractable: (1) the AdS/CFT correspondence between a specific class of string theories and a conformal field theory and (2) an effective low-energy theory of QCD dual to classical QCD on a curved conformal gravitational background. The hope is that by applying these dualities to the evaluation of various properties of the strongly-interacting matter produced in heavy ion collisions one can understand how QCD behaves at strong coupling. We give an outline of the applications, with emphasis on two transport coefficients of QCD matter -- shear and bulk viscosities.

hep-ph↗
arXiv2008

Mass ordering of differential elliptic flow and its violation for phi mesons

We simulate the dynamics of Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC) with a hybrid model that treats the dense early quark-gluon plasma (QGP) stage macroscopically as an ideal fluid, but models the dilute late hadron resonance gas (HG) microscopically using a hadronic cascade. By comparing with a pure hydrodynamic approach we identify effects of hadronic viscosity on the transverse momentum spectra and differential elliptic flow v_2(p_T). We investigate the dynamical origins of the observed mass-ordering of v_2(p_T) for identified hadrons, focusing on dissipative effects during the late hadronic stage. We find that, at RHIC energies, much of the finally observed mass-splitting is generated during the hadronic stage, due to build-up of additional radial flow. The phi meson, having a small interaction cross section, does not fully participate in this additional flow. As a result, it violates the mass-ordering pattern for v_2(p_T) that is observed for other hadron species. We also show that the early decoupling of the phi meson from the hadronic rescattering dynamics leads to interesting and unambiguous features in the p_T-dependence of the nuclear suppression factor R_AA and of the phi/p ratio.

nucl-th↗
arXiv2007

Elliptic Flow from a Hybrid CGC, Full 3D Hydro and Hadronic Cascade Model

We investigate the robustness of the discovery of the perfect fluid through comparison of hydrodynamic calculations with the elliptic flow coefficient v_2 at midrapidity in Au+Au collisions at sqrt{s_{NN}}=200 GeV. Employing the Glauber model for initial entropy density distributions, the centrality dependence of v_2 is reasonably reproduced by using an ideal fluid description of the early QGP stage followed by a hadronic cascade in the late hadronic stage. On the other hand, initial conditions based on the Colour Glass Condensate model are found to generate larger elliptic flow due to larger initial eccentricity epsilon. We further predict v_2/epsilon at a fixed impact parameter as a function of collision energy sqrt{s_{NN}} up to the LHC energy.

nucl-th↗
arXiv2007

Multi-particle Production and Thermalization in High-Energy QCD

We argue that multi-particle production in high energy hadron and nuclear collisions can be considered as proceeding through the production of gluons in the background classical field. In this approach we derive the gluon spectrum immediately after the collision and find that at high energies it is parametrically enhanced by ln(1/x) with respect to the quasi-classical result (x is the Bjorken variable). We show that the produced gluon spectrum becomes thermal (in three dimensions) with an effective temperature determined by the saturation momentum Qs, T= c Qs/2pi during the time ~1/T; we estimate c=sqrt{2pi}/2 ~ 1.2. Although this result by itself does not imply that the gluon spectrum will remain thermal at later times, it has an interesting applications to heavy ion collisions. In particular, we discuss the possibility of Bose-Einstein condensation of the produced gluon pairs and estimate the viscosity of the produced gluon system.

hep-ph↗
arXiv2007

Vacuum self-focussing of very intense laser beams

We argue that long-range photon-photon attraction induced by the dipole interaction of two electron-positron loops can lead to "vacuum self-focussing" of very intense laser beams. The focussing angle theta(F) is found to increase with the beam intensity I as theta(F) I^(4/3); for the laser beams available at present or in the near future, theta(F) 10^(-10)--10^(-7).

hep-ph↗
arXiv2006

Hadronic dissipative effects on elliptic flow in ultrarelativistic heavy-ion collisions

We study the elliptic flow coefficient v_2(eta,b) in Au+Au collisions at sqrt(s)=200 A GeV as a function of pseudorapidity eta and impact parameter b. Using a hybrid approach which combines early ideal fluid dynamical evolution with late hadronic rescattering, we demonstrate strong dissipative effects from the hadronic rescattering stage on the elliptic flow. With Glauber model initial conditions, hadronic dissipation is shown to be sufficient to fully explain the differences between measured v_2 values and ideal hydrodynamic predictions. Initial conditions based on the Color Glass Condensate model generate larger elliptic flow and seem to require additional dissipation during the early quark-gluon plasma stage in order to achieve agreement with experiment.

nucl-th↗
arXiv2005

Signatures of the Color Glass Condensate in J/Psi production off nuclear targets

We consider the J/Psi production in proton (deuteron) -- nucleus collisions at high energies. We argue that the production mechanism in this case is different from that in pp collisions due to gluon saturation in the nucleus and formation of the Color Glass Condensate. At forward rapidities (in the proton fragmentation region), the production of J/Psi is increasingly suppressed both as a function of rapidity and centrality. On the other hand, at backward rapidities at RHIC (in the fragmentation region of the nucleus) the coherent effects lead to a modest enhancement of the production cross section, with the nuclear modification factor R(J/Psi) increasing with centrality. We find that the J/Psi production cross section exhibits at forward rapidities the limiting fragmentation scaling established previously for soft processes; in the energy range studied experimentally, it manifests itself as an approximate "xF scaling".

hep-ph↗
arXiv2005

Chaos in the Color Glass Condensate

The number of gluons in the hadron wave function is discrete, and their formation in the chain of small $x$ evolution occurs over discrete rapidity intervals of $Δy \simeq 1/\as$. We therefore consider the evolution as a discrete quantum process. We show that the discrete version of the mean-field Kovchegov evolution equation gives rise to strong fluctuations in the scattering amplitude, not present in the continuous equation. We find that if the linear evolution is as fast as predicted by the perturbative BFKL dynamics, the scattering amplitude at high energies exhibits a chaotic behavior. As a consequence, the properties of diffraction at high energies become universal.

hep-ph↗
arXiv2005

From Color Glass Condensate to Quark Gluon Plasma through the event horizon

We propose a new thermalization scenario for heavy ion collisions which at sufficiently high energies implies the phase transition to the quark--gluon plasma. The key ingredient of our approach is the Hawking--Unruh effect: an observer moving with an acceleration $a$ experiences the influence of a thermal bath with an effective temperature $T = a / 2π$, similar to the one present in the vicinity of a black hole horizon. For electric charges moving in external electromagnetic fields of realistic strength, the resulting temperature appears too small to be detected. However for partons in strong color fields the effect should be observable: in the Color Glass Condensate picture, the strength of the color-electric field is $E \sim Q_s^2/g$ ($Q_s$ is the saturation scale, and $g$ is the strong coupling), the typical acceleration is $a \sim Q_s$, and the heat bath temperature is $T = Q_s / 2π\sim 200$ MeV. In nuclear collisions at sufficiently high energies the effect can induce a rapid thermalization over the time period of $τ\simeq 2π/Q_s \simeq 1\ {\rm fm}$ accompanied by phase transitions. We consider a specific example of chiral symmetry restoration induced by a rapid deceleration of the colliding nuclei. We argue that parton saturation in the initial nuclear wave functions is a necessary pre--condition for the formation of quark--gluon plasma. We discuss the implications of our "black hole thermalization" scenario for various observables in relativistic heavy ion collisions.

hep-ph↗