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Yuri V. Kovchegov

Publications and source records attributed to Yuri V. Kovchegov.

At least 73 records · Page 4Linked to original sources
arXiv2011

Running coupling corrections to inclusive gluon production

We calculate running coupling corrections for the lowest-order gluon production cross section in high energy hadronic and nuclear scattering using the BLM scale-setting prescription. At leading order there are three powers of fixed coupling; in our final answer, these three couplings are replaced by seven factors of running coupling: five in the numerator and two in the denominator, forming a `septumvirate' of running couplings, analogous to the `triumvirate' of running couplings found earlier for the small-x BFKL/BK/JIMWLK evolution equations. It is interesting to note that the two running couplings in the denominator of the `septumvirate' run with complex-valued momentum scales, which are complex conjugates of each other, such that the production cross section is indeed real. We use our lowest-order result to conjecture how running coupling corrections may enter the full fixed-coupling k_T-factorization formula for gluon production which includes non-linear small-x evolution.

hep-ph↗
arXiv2011

Gravity Dual Corrections to the Heavy Quark Potential at Finite-Temperature

We apply gauge/gravity duality to compute $1/N^2_c$ corrections to the heavy quark potentials of a quark--anti-quark pair ($Q\bar Q$) and of a quark--quark pair ($QQ$) immersed into the strongly coupled N = 4 SYM plasma. On the gravity side these corrections come from the exchanges of supergravity modes between two string worldsheets stretching from the UV boundary of AdS space to the black hole horizon in the bulk and smeared over $S^5$. We find that the contributions to the $Q\bar Q$ potential coming from the exchanges of all of the relevant modes (such as dilaton, massive scalar, 2-form field, and graviton) are all attractive, leading to an attractive net $Q\bar Q$ potential. We show that at large separations $r$ and/or high-temperature $T$ the potential is of Yukawa-type, dominated by the graviton exchange, in agreement with earlier findings. On the other hand, at small-$r T$ the $Q\bar Q$ potential scales as $\sim (1/r) \ln (1/rT)$. In the case of $QQ$ potential the 2-form contribution changes sign and becomes repulsive: however, the net $QQ$ potential remains attractive. At large-$r T$ it is dominated by the graviton exchange, while at small-$r T$ the $QQ$ potential becomes Coulomb-like.

hep-th↗
arXiv2011

Long-Range Rapidity Correlations in Heavy Ion Collisions at Strong Coupling from AdS/CFT

We use AdS/CFT correspondence to study two-particle correlations in heavy ion collisions at strong coupling. Modeling the colliding heavy ions by shock waves on the gravity side, we observe that at early times after the collision there are long-range rapidity correlations present in the two-point functions for the glueball and the energy-momentum tensor operators. We estimate rapidity correlations at later times by assuming that the evolution of the system is governed by ideal Bjorken hydrodynamics, and find that glueball correlations in this state are suppressed at large rapidity intervals, suggesting that late-time medium dynamics can not "wash out" the long-range rapidity correlations that were formed at early times. These results may provide an insight on the nature of the "ridge" correlations observed in heavy ion collision experiments at RHIC and LHC, and in proton-proton collisions at LHC.

hep-th↗
arXiv2010

Running Coupling Corrections to High Energy Inclusive Gluon Production

We calculate running coupling corrections for the lowest-order gluon production cross section in high energy hadronic and nuclear scattering using the BLM scale-setting prescription. In the final answer for the cross section the three powers of fixed coupling are replaced by seven factors of running coupling, five in the numerator and two in the denominator, forming a 'septumvirate' of running couplings, analogous to the 'triumvirate' of running couplings found earlier for the small-x BFKL/BK/JIMWLK evolution equations. It is interesting to note that the two running couplings in the denominator of the 'septumvirate' run with complex-valued momentum scales, which are complex conjugates of each other, such that the production cross section is indeed real. We use our lowest-order result to conjecture how running coupling corrections may enter the full fixed-coupling kT-factorization formula for gluon production which includes non-linear small-x evolution.

hep-ph↗
arXiv2010

Shock Wave Collisions and Thermalization in AdS_5

We study heavy ion collisions at strong 't Hooft coupling using AdS/CFT correspondence. According to the AdS/CFT dictionary heavy ion collisions correspond to gravitational shock wave collisions in AdS_5. We construct the metric in the forward light cone after the collision perturbatively through expansion of Einstein equations in graviton exchanges. We obtain an analytic expression for the metric including all-order graviton exchanges with one shock wave, while keeping the exchanges with another shock wave at the lowest order. We read off the corresponding energy-momentum tensor of the produced medium. Unfortunately this energy-momentum tensor does not correspond to ideal hydrodynamics, indicating that higher order graviton exchanges are needed to construct the full solution of the problem. We also show that shock waves must completely stop almost immediately after the collision in AdS_5, which, on the field theory side, corresponds to complete nuclear stopping due to strong coupling effects, likely leading to Landau hydrodynamics. Finally, we perform trapped surface analysis of the shock wave collisions demonstrating that a bulk black hole, corresponding to ideal hydrodynamics on the boundary, has to be created in such collisions, thus constructing a proof of thermalization in heavy ion collisions at strong coupling.

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arXiv2010

R-Current DIS on a Shock Wave: Beyond the Eikonal Approximation

We find the DIS structure functions at strong coupling by calculating R-current correlators on a finite-size shock wave using AdS/CFT correspondence. We improve on the existing results in the literature by going beyond the eikonal approximation for the two lowest orders in graviton exchanges. We argue that since the eikonal approximation at strong coupling resums integer powers of 1/x (with x the Bjorken-x variable), the non-eikonal corrections bringing in positive integer powers of x can not be neglected in the small-x limit, as the non-eikonal order-x correction to the (n+1)st term in the eikonal series is of the same order in x as the nth eikonal term in that series. We demonstrate that, in qualitative agreement with the earlier DIS analysis based on calculation of the expectation value of the Wilson loop in the shock wave background using AdS/CFT, after inclusion of non-eikonal corrections DIS structure functions are described by two momentum scales: Q_1^2 ~ Λ^2 \, A^{1/3}/x and Q_2^2 ~ Λ^2 \, A^{2/3}, where Λis the typical transverse momentum in the shock wave and A is the atomic number if the shock wave represents a nucleus. We discuss possible physical meanings of the scales Q_1 and Q_2.

hep-ph↗
arXiv2010

Toward Thermalization in Heavy Ion Collisions at Strong Coupling

We find the trapped surface for a collision of two sourceless shock waves in AdS$_5$ and conclude that such collisions always lead to a creation of a black hole in the bulk. Due to holographic correspondence, in the boundary gauge theory this result proves that a thermalized medium (quark-gluon plasma) is produced in heavy ion collisions at strong coupling (albeit in ${\cal N} =4$ super-Yang-Mills theory). We present new evidence supporting the analytic estimate for the time of thermalization that exists in the literature and find that thermalization time is parametrically much shorter than the time of shock wave stopping, indicating that our result may be relevant for description of heavy ion collision experiments.

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arXiv2009

Comparing AdS/CFT Calculations to HERA F_2 Data

We show that HERA data for the inclusive structure function F_2(x,Q^2) at small Bjorken-x and Q^2 can be reasonably well described by a color-dipole model with an AdS/CFT-inspired dipole-proton cross section. The model contains only three free parameters fitted to data. In our AdS/CFT-based parameterization the saturation scale varies in the range of 1-3 GeV becoming independent of energy/Bjorken-x at very small x. This leads to the prediction of x-independence of the F_2 and F_L structure functions at very small x. We provide predictions for F_2 and F_L in the kinematic regions of future experiments. We discuss the limitations of our approach and its applicability region, and argue that our AdS/CFT-based model of non-perturbative physics could be viewed as complimentary to the perturbative description of data based on saturation/Color Glass Condensate physics.

hep-ph↗
arXiv2009

Early Time Dynamics in Heavy Ion Collisions from CGC and from AdS/CFT

We review two different theoretical approaches to the strong interaction dynamics at the early times immediately following heavy ion collisions. One approach is based on small-coupling physics of the Color Glass Condensate (CGC). The other approach is based on Anti-de Sitter space/Conformal Field Theory (AdS/CFT) correspondence and may be applicable to describing large-coupling QCD interactions. We point out that in terms of theoretical tools the two approaches are somewhat similar: in CGC one deals with classical gluon fields produced in a nuclear shock wave collision, while in AdS/CFT one studies classical gravity in a gravitational shock wave collision. We stress, however, that the resulting physics is different: the classical gluon fields in CGC lead to a free-streaming medium produced in heavy ion collisions, while the classical gravity in the 5-dimensional AdS bulk is likely to lead to ideal hydrodynamics description of the produced medium. Also, the valence quarks in colliding nuclei in CGC continue along their light cone trajectories after the collision with very little recoil, while we show that in AdS the colliding nuclei are likely to lose most of their energy in the collision and stop.

hep-ph↗
arXiv2009

Shock Treatment: Heavy Quark Drag in a Novel AdS Geometry

We calculate the drag force on a heavy quark hit by a shock wave, thus generalizing the strongly coupled AdS/CFT heavy quark drag calculations to both hot and cold nuclear matter. The derivation employs the trailing string configuration, similar to that used in the literature for a quark moving through a thermal medium, though in the shock metric the string profile is described by a much simpler analytic function. Our expression for the drag depends on the typical transverse momentum scale of the matter in the shock. For a thermal medium this scale becomes proportional to the temperature, making our drag coefficient and momentum limit of applicability identical to those found previously. As the shock wave can be composed of either thermalized or non-thermalized media, our derivation extends the existing drag calculations to the case of arbitrarily distributed matter.

hep-th↗
arXiv2009

Asymmetric Collision of Two Shock Waves in AdS_5

We consider high energy collisions of two shock waves in AdS_5 as a model of ultrarelativistic nucleus-nucleus collisions in the boundary theory. We first calculate the graviton field produced in the collisions in the NLO and NNLO approximations, corresponding to three- and four-graviton exchanges with the shock waves. We then consider the asymmetric limit where the energy density in one shock wave is much higher than in the other one. In the boundary theory this setup corresponds to proton-nucleus collisions, with the nucleus being the denser of the two shock waves and the proton being the less dense one. Employing the eikonal approximation we find the exact high energy analytic solution for the metric in AdS_5 for the asymmetric collision of two delta-function shock waves. The solution resums all-order graviton exchanges with the "nucleus" shock wave and a single-graviton exchange with the "proton" shock wave. Using the holographic renormalization prescription we read off the energy-momentum tensor of the matter produced in proton-nucleus collisions. We show in explicit detail that in the boundary theory the proton is completely stopped by strong-coupling interactions with the nucleus, in agreement with our earlier results. We also apply the eikonal technique to the asymmetric collision of two unphysical delta-prime shock waves, which we introduced in an earlier work as a means of modeling nuclear collisions with weak coupling initial dynamics. We obtain a surprising result that, for delta-prime shock waves, the multiple bulk graviton exchange series giving the leading energy-dependent contribution to the energy-momentum tensor terminates at the order of two graviton exchanges with the nucleus.

hep-th↗
arXiv2009

Subleading-N_c corrections in non-linear small-x evolution

We explore the subleading-N_c corrections to the large-N_c Balitsky-Kovchegov (BK) evolution equation by comparing its solution to that of the all-N_c Jalilian-Marian-Iancu-McLerran-Weigert-Leonidov-Kovner (JIMWLK) equation. In earlier simulations it was observed that the difference between the solutions of JIMWLK and BK is unusually small for a quark dipole scattering amplitude, of the order of 0.1%, which is two orders of magnitude smaller than the naively expected 1/N_c^2 or 11%. In this paper we argue that this smallness is not accidental. We show that saturation effects and correlator coincidence limits fixed by group theory constraints conspire with the particular structure of the dipole kernel to suppress subleading-N_c corrections reducing the difference between the solutions of JIMWLK and BK to 0.1%. We solve the JIMWLK equation with improved numerical accuracy and verify that the remaining 1/N_c corrections, while small, still manage to slow down the rapidity-dependence of JIMWLK evolution compared to that of BK. We demonstrate that a truncation of JIMWLK evolution in the form of a minimal Gaussian generalization of the BK equation captures some of the remaining 1/N_c contributions leading to an even better agreement with JIMWLK evolution. As the 1/N_c corrections to BK include multi-reggeon exchanges one may conclude that the net effect of multi-reggeon exchanges on the dipole amplitude is rather small.

hep-ph↗
arXiv2008

Heavy Quark Potential at Finite Temperature Using the Holographic Correspondence

We revisit the calculation of a heavy quark potential in N =4 supersymmetric Yang-Mills theory at finite temperature using the AdS/CFT correspondence. As is widely known, the potential calculated in the pioneering works of Rey et al. and Brandhuber et al. is zero for separation distances r between the quark and the anti-quark above a certain critical separation, at which the potential has a kink. We point out that by analytically continuing the string configurations into the complex plane, and using a slightly different renormalization subtraction, one obtains a smooth non-zero (negative definite) potential without a kink. The obtained potential also has a non-zero imaginary (absorptive) part for separations r > r_c = 0.870/πT . At large separations r the real part of the potential does not exhibit the exponential Debye falloff expected from perturbation theory and instead falls off as a power law, proportional to 1/r^4 for r > r_0 = 2.702 / πT.

hep-th↗
arXiv2008

DIS in AdS

We calculate the total cross section for the scattering of a quark--anti-quark dipole on a large nucleus at high energy for a strongly coupled N=4 super Yang-Mills theory using AdS/CFT correspondence. We model the nucleus by a metric of a shock wave in AdS_5. We then calculate the expectation value of the Wilson loop (the dipole) by finding the extrema of the Nambu-Goto action for an open string attached to the quark and antiquark lines of the loop in the background of an AdS_5 shock wave. We find two physically meaningful extremal string configurations. For both solutions we obtain the forward scattering amplitude N for the quark dipole--nucleus scattering. We study the onset of unitarity with increasing center-of-mass energy and transverse size of the dipole: we observe that for both solutions the saturation scale Q_s is independent of energy/Bjorken-x and depends on the atomic number of the nucleus as Q_s ~ A^{1/3}. Finally we observe that while one of the solutions we found corresponds to the pomeron intercept of α_P = 2 found earlier in the literature, when extended to higher energy or larger dipole sizes it violates the black disk limit. The other solution we found respects the black disk limit and yields the pomeron intercept of α_P = 1.5. We thus conjecture that the right pomeron intercept in gauge theories at strong coupling may be α_P = 1.5.

hep-th↗
arXiv2008

Modeling Heavy Ion Collisions in AdS/CFT

We construct a model of high energy heavy ion collisions as two ultrarelativistic shock waves colliding in AdS_5. We point out that shock waves corresponding to physical energy-momentum tensors of the nuclei completely stop almost immediately after the collision in AdS_5, which, on the field theory side, corresponds to complete nuclear stopping due to strong coupling effects, likely leading to Landau hydrodynamics. Since in real-life heavy ion collisions the large Bjorken x part of nuclear wave functions continues to move along the light cone trajectories of the incoming nuclei leaving the small-x partons behind, we conclude that a pure large coupling approach is not likely to adequately model nuclear collisions. We show that to account for small-coupling effects one can model the colliding nuclei by two (unphysical) ultrarelativistic shock waves with zero net energy each (but with non-zero energy density). We use this model to study the energy density of the strongly-coupled matter created immediately after the collision. We argue that expansion of the energy density in the powers of proper time squared corresponds on the gravity side to a perturbative expansion of the metric in graviton exchanges. Using such expansion we reproduce our earlier result that the energy density of produced matter at mid-rapidity starts out as a constant (of time) in heavy ion collisions at large coupling.

hep-th↗
arXiv2008

DIS on a Large Nucleus in AdS/CFT

We calculate the total cross section for deep inelastic scattering (DIS) on a nucleus at high energy for a strongly coupled N=4 super Yang-Mills theory using AdS/CFT correspondence. In analogy to the small coupling case we argue that at high energy the total DIS cross section is related to the expectation value of the Wilson loop formed by the quark-antiquark dipole. We model the nucleus by a metric of a shock wave in AdS_5. We then calculate the expectation value of the Wilson loop by finding the extrema of the Nambu-Goto action for an open string attached to the quark and antiquark lines of the loop in the background of an AdS_5 shock wave. We find three extrema of the Nambu-Goto action: the string coordinates at the extrema are complex-valued and are given by three different branches of the solution of a cubic equation. The physically meaningful solutions for the total DIS cross section are given either by the only branch with a purely imaginary string coordinate in the bulk or by a superposition of the two other branches. For both solutions we obtain the forward scattering amplitude N for the quark dipole-nucleus scattering. We study the onset of unitarity with increasing center-of-mass energy and transverse size of the dipole: we observe that for both solutions the saturation scale, while energy-dependent at lower energies, at very high energy becomes independent of energy/Bjorken-x. The saturation scale depends very strongly on the atomic number of the nucleus as Q_s ~ A^{1/3}.

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