We develop a holographic (bottom-up) gravity model for QCD to understand the connection between the peak in the trace anomaly and the magnitude of heavy quark energy loss in a strongly-coupled plasma. The potential of the scalar field on the gravity side is constructed to reproduce some properties of QCD at finite temperature and its parameters are constrained by fitting lattice gauge theory results. The energy loss of heavy quarks (as predicted by the holographic model) is found to be strongly sensitive to the medium properties.
We use our non-conformal holographic bottom-up model for QCD described in 1012.0116 to further study the effect of the QCD trace anomaly on the energy loss of both light and heavy quarks in a strongly coupled plasma. We compute the nuclear modification factor $R_{AA}$ for bottom and charm quarks in an expanding plasma with Glauber initial conditions. We find that the maximum stopping distance of light quarks in a non-conformal plasma scales with the energy with a temperature (and energy) dependent effective power.
We derive a new formula for jet energy loss using finite endpoint momentum shooting strings initial conditions in SYM plasmas to overcome the difficulties of previous falling string holographic scenarios. We apply the new formula to compute the nuclear modification factor RAA and the elliptic flow parameter v2 of light hadrons at RHIC and LHC. We show furthermore that Gauss-Bonnet quadratic curvature corrections to the AdS5 geometry improve the agreement with the recent data.
We show that the ``perfect fluid'' elliptic flow of the bulk hadrons and the unexpectedly strong quenching of heavy quark jet fragments in Au+Au reactions at 200 AGeV can be simultaneously accounted for within leading order AdS/CFT holography with a common large t'Hooft coupling $λ=g^2_{YM} N_c \sim 30$. In contrast, weakly coupled quark-gluon plasma models have so far failed to describe the observed correlation between these soft and hard observables even for couplings extrapolated to $α_s\sim 0.5$. We show that phenomenological applications of the classical trailing string AdS/CFT holographi…
In the presence of a gravitational contribution to the chiral anomaly, the chiral magnetic effect induces an energy current proportional to the square of the temperature in equilibrium. In holography the thermal state corresponds to a black hole. We numerically study holographic quenches in which a planar shell of scalar matter falls into a black hole and rises its temperature. During the process the momentum density (energy current) is conserved. The energy current has two components, a non-dissipative one induced by the anomaly and a dissipative flow component. The dissipative component can …
We study the non-linear propagation of radiation in {\cal N=4} SYM at zero and finite temperature using the refined radius/scale duality in AdS/CFT. We argue that a pulse radiation by a quark at the boundary should be described holographically by a "point like object" passing through the center of the AdS bulk. We find that at finite temperature, the radiation stalls at a distance of $1/πT$ with a natural geometric and holographic interpretation. Indeed, the stalling is the holographic analogue of the gravitational in-fall of light towards the black hole in the bulk. We suggest that these resu…
Finite-density QCD is difficult to study numerically because of the sign problem. We prove that, in a certain region of the phase diagram, the phase quenched approximation is exact to O(Nf/Nc). It is true for any physical observables. We also consider the implications for the lattice simulations and find a quantitative evidence for the validity of the phase quenching from existing lattice QCD results at Nc=3. Our results show that the phase-quench approximation is rather good already at Nc=3, and the 1/Nc correction can be incorporated by the phase reweighting method without suffering from the…
The effect of the complex phase of the fermion determinant is a key question related to the sign problem in finite-density QCD. Recently it has been shown that ignoring the complex phase -- the phase quenching -- does not change physics in a certain region of the phase diagram when a number of colors N_c is large. In this paper we study the effect of the phase quenching within the frameworks of effective models and holographic models. We show, in a unified manner, that the phase quenching gives exact results for any fermionic observables (e.g., chiral condensate) in the mean-field approximatio…
We consider second-order viscous hydrodynamics in conformal field theories at finite temperature. We show that conformal invariance imposes powerful constraints on the form of the second-order corrections. By matching to the AdS/CFT calculations of correlators, and to recent results for Bjorken flow obtained by Heller and Janik, we find three (out of five) second-order transport coefficients in the strongly coupled N=4 supersymmetric Yang-Mills theory. We also discuss how these new coefficents can arise within the kinetic theory of weakly coupled conformal plasmas. We point out that the Muelle…
We discuss holographic QCD in the Veneziano limit (the V-QCD models), concentrating on phenomena near the "conformal" phase transition taking place at a critical value of the ratio x = N_f/N_c. In particular, we review the results for the S-parameter, the technidilaton, and the masses of the mesons.
To a first semiclassical approximation one can reduce the multi-parton light-front problem in QCD to an effective one-dimensional quantum field theory, which encodes the fundamental conformal symmetry of the classical QCD Lagrangian. This procedure leads to a relativistic light-front wave equation for arbitrary spin which incorporates essential spectroscopic and non-perturbative dynamical features of hadron physics. The mass scale for confinement and higher dimensional holographic mapping to AdS space are also emergent properties of this framework.
We employ the string/gauge theory correspondence to study properties of strongly coupled quark-gluon plasmas in thermal gauge theories with a large number of colors and flavors. In particular, we analyze non-critical string duals of conformal (S)QCD, as well as ten dimensional wrapped fivebrane duals of SQCD-like theories. We study general properties of the dual plasmas, including the drag force exerted on a probe quark and the jet quenching parameter. We find that these plasma observables depend on the number of colors and flavors in the ``QCD dual''; in particular, we find that the jet quenc…
Inspired by the model of Jarvinen and Kiritsis, we present a simple holographic model for the on set of chiral symmetry breaking at the edge of the conformal window in QCD in the Veneziano limit. Our most naive model enforces the QCD two loop running coupling on a D3/D7 holographic brane system. The mass of the holographic field, describing the chiral condensate in the model, is driven below the BF bound when the running is sufficiently strong, triggering chiral symmetry breaking for N_f/N_c<2.9. This model though contains too great a remnant of supersymmetry and does not correctly encode the …
We argue that classical bulk gravitational radiation effects in AdS/CFT, previously ignored because of their subleading nature in the 1/Nc-expansion, are magnified by powers of large Lorentz factors gamma for ultrarelativistic jets, thereby dominating other forms of jet energy loss in holography at finite temperature. We make use of the induced gravitational self-force in thermal AdS5 to estimate its effects. In a thermal medium, relativistic jets may loose most of their energy through longitudinal drag caused by the energy accumulated in their nearby field as they zip through the strongly cou…
In this paper we obtain holographic formulas for the transport coefficients $κ$ and $τ_π$ present in the second-order derivative expansion of relativistic hydrodynamics in curved spacetime associated with a non-conformal strongly coupled plasma described holographically by an Einstein+Scalar action in the bulk. We compute these coefficients as functions of the temperature in a bottom-up non-conformal model that is tuned to reproduce lattice QCD thermodynamics at zero baryon chemical potential. We directly compute, besides the speed of sound, 6 other transport coefficients that appear at second…
Several arguments given in arXiv:1912.12316 suggest that braneworld effective models with fields strictly localized on $δ$-like branes cannot be UV-completed by a fundamental theory including gravity, i.e. they belong to the Swampland. It is concluded that one should instead only consider models with quasilocalized zero-modes of higher-dimensional fields. In this comment, the arguments are critically reviewed, and various loopholes are proposed. Beyond that, the main point that is discussed is the notion of effective brane form factors, which are required when the singular behavior of the bran…
Recently Liu, Rajagopal and Wiedemann (LRW) [hep-ph/0605178] proposed a first principle, nonperturbative quantum field theoretic definition of ``jet quenching parameter'' \hat{q} used in models of medium-induced radiative parton energy loss in nucleus-nucleus collisions at RHIC. Relating \hat{q} to a short-distance behavior of a certain light-like Wilson loop, they used gauge theory-string theory correspondence to evaluate \hat{q} for the strongly coupled N=4 SU(N_c) gauge theory plasma. We generalize analysis of LRW to strongly coupled non-conformal gauge theory plasma. We find that a jet que…
We initiate a non-perturbative study of anisotropic, non-conformal and confining gauge theories that are holographically realized in gravity by generic Einstein-Axion-Dilaton systems. In the vacuum our solutions describe RG flows from a conformal field theory in the UV to generic scaling solutions in the IR with generic hyperscaling violation and dynamical exponents $θ$ and $z$. We formulate a generalization of the holographic c-theorem to the anisotropic case. At finite temperature, we discover that the anisotropic deformation reduces the confinement-deconfinement phase transition temperature…
We show that in a general class of strongly interacting theories at high temperatures the speed of sound approaches the conformal value c_s^2=1/3 universally from_below_. This class includes theories holographically dual to a theory of gravity coupled to a single scalar field, representing the operator of the scale anomaly.
We give an overview of recent progress into the infrared structure of QCD based on the gauge/gravity correspondence and light-front quantization, where the color confining interaction for mesons and baryons is determined by an underlying superconformal algebraic structure. This new approach to hadron physics gives remarkable connections and predictions across the entire mass spectrum of hadrons and also describes the infrared behavior of the strong coupling. More recently, an extensive study of form factors, polarized and unpolarized parton distributions and the sea quark contribution to the n…
Braneworld theories are often described as low-energy effective field theories (EFTs) featuring an infinitely thin 3-brane and 4D fields exactly localized on it. We investigate whether an exactly localized braneworld can arise as a limit of a theory of 5D fields. Using a holographic formalism we argue that such limit does not exist in the presence of gravity, therefore implying a discontinuity in the space of EFTs. We then present specific models involving exactly localized fields in which inconsistencies appear, which are solved when fields are taken as quasilocalized. Part of our arguments r…
We study entanglement entropy inequalities in boundary conformal field theory (BCFT) by holographic correspondence. By carefully classifying all the configurations for different phases, we prove the strong subadditiviy and the monogamy of mutual information for holographic entanglement entropy in BCFT at both zero and finite temperatures
I review applications of superconformal algebra. light-front holography, and an extended form of conformal symmetry to hadron spectroscopy and dynamics. QCD is not supersymmetrical in the traditional sense -- the QCD Lagrangian is based on quark and gluonic fields -- not squarks nor gluinos. However, its hadronic eigensolutions conform to a representation of superconformal algebra. and provide a unified Regge spectroscopy of meson, baryon, and tetraquarks with a universal Regge slope. The pion $q \bar q$ eigenstate is composite but yet has zero mass for $m_q=0.$ Light-Front Holography also pre…
We compute the momentum broadening of an energetic parton as it moves through strongly interacting matter, using the gravity dual of a non conformal gauge theory with matter in the fundamental representation. Our approach defines a family of theories, all of which have logarithmic running of the couplings in the far IR but becomes almost conformal in the far UV. For a given set of degrees of freedom, we obtain average transverse momentum square of the parton as a function of temperature and compare with results obtained from the AdS/CFT correspondence.
Using the AdS/CFT correspondence, we study the heavy quark potential and the jet quenching parameter in the near horizon limit of D3-D(-1) background. The results are compared with those of conformal cases. It is shown that the presence of instantons tends to suppress the heavy quark potential and enhance the jet quenching parameter.
We present a class of field configurations that are forbidden in the quantum gravity because of inconsistency in the dual field theory from holography. Scale invariant but non-conformal field theories are impossible in (1+1) dimension, and so should be the corresponding gravity dual. In particular, the "spontaneous Lorentz symmetry breaking" models and the "ghost condensation" models, which are well-studied in phenomenology literatures, are forbidden in any consistent quantum theories of gravity in (1+2) dimension since they predict such inconsistent field configurations.
We investigate the speed of sound in $(d+1)$-dimensional field theory by studying its dual $(d+2)$-dimensional gravity theory from gauge/gravity correspondence. Instead of the well known conformal limit $c_s^2 \rightarrow 1/d$ at high temperature, we reveal two more universal quantities in various limits: $c_s^2 \rightarrow (d-1)/16π$ at low temperature and $c_s^2 \rightarrow (d-1)/16πd$ at large chemical potential.
We construct the holographic dual theory of unparticles. The Randall-Sundrum type hard wall model is shown to produce deconstructing particles, whose spectrum has a finite mass gap proportional to the inverse of the fifth direction segment. The introduction of new scale corresponds to setting a brane in ${\rm AdS}_5$. The broken conformal symmetry due to this brane is restored when it is moved to infinity. Unparticles then emerge as an infinite tower of massless particles.
