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Yuri V. Kovchegov

Publications and source records attributed to Yuri V. Kovchegov.

At least 109 records · Page 6Linked to original sources
arXiv2003

Saturation Physics in Heavy Ion Collisions

We discuss expectations of saturation physics for various observables in heavy ion collisions. We show how simple saturation-inspired assumptions about particle production in heavy ion collisions lead to Kharzeev-Levin-Nardi model. Comparing this model to RHIC data on particle multiplicities we conclude that saturation effects may play an important role in particle production and dynamics at the early stages of Au-Au collisions already at RHIC energies. We then estimate the contribution of the initial state two-particle azimuthal correlations to elliptic flow observable v_2 in Au-Au collisions by constructing a lower bound on these non-flow effects based on v_2 obtained from the analysis of proton-proton (pp) collisions.

hep-ph↗
arXiv2003

Diffractive Gluon Production in Proton-Nucleus Collisions and in DIS

We derive expressions for the gluon production cross sections in the single diffractive proton-nucleus scattering and DIS processes in a quasi-classical approximation. The resulting cross sections include the effects of all multiple rescatterings in the classical background field of the target proton or nucleus, which remains intact after the scattering. We also write down an expression for the inclusive gluon production cross section in DIS in the quasi-classical approximation.

hep-ph↗
arXiv2002

Correlation Functions and Cumulants in Elliptic Flow Analysis

We consider various methods of flow analysis in heavy ion collisions and compare experimental data on corresponding observables to the predictions of our saturation model proposed earlier. We demonstrate that, due to the nature of the standard flow analysis, azimuthal distribution of particles with respect to reaction plane determined from the second order harmonics should always be proportional to $\cos 2(ϕ-Ψ_R)$ independent of the physical origin of particle correlations (flow or non-flow). The amplitude of this distribution is always physical and proportional to $v_2$. Two-particle correlations analysis is therefore a more reliable way of extracting the shape of physical azimuthal anisotropy. We demonstrate that two-particle correlation functions generated in our minijet model of particle production are in good agreement with the data reported by PHENIX. We discuss the role of non-flow correlations in the cumulant flow analysis and demonstrate using a simple example that if the flow is weak, higher order cumulants analysis does not significantly reduce the contribution of non-flow correlations to elliptic flow observable $v_2$ in RHIC data.

nucl-th↗
arXiv2002

Elliptic Flow from Minijet Production in Heavy Ion Collisions

We calculate the contribution to the elliptic flow observable v2 from two-particle correlations in minijet production in ultrarelativistic heavy ion collisions. We use a minijet production cross section derived in a model inspired by saturation approach to high energy scattering. Resulting differential elliptic flow v2(pT) is an increasing function of pT for transverse momenta below the saturation scale Qs. At higher transverse momenta (pT > Qs) differential flow stops growing and becomes approximately constant, reproducing the elliptic flow saturation data reported by STAR. The centrality dependence of the minijet contribution to v2 is also in good agreement with the data.

hep-ph↗
arXiv2002

High Energy QCD and the Large Nc Limit

We review recent progress in understanding QCD at high energies and the role played in it by the large Nc limit. We discuss unitarization of total hadronic cross sections and saturation of structure functions at high energies.

hep-ph↗
arXiv2001

Inclusive Gluon Production in DIS at High Parton Density

We calculate the cross section of a single inclusive gluon production in deep inelastic scattering at very high energies in the saturation regime, where the parton densities inside hadrons and nuclei are large and the evolution of structure functions with energy is nonlinear. The expression we obtain for the inclusive gluon production cross section is generated by this nonlinear evolution. We analyze the rapidity distribution of the produced gluons as well as their transverse momentum spectrum given by the derived expression for the inclusive cross section. We propose an ansatz for the multiplicity distribution of gluons produced in nuclear collisions which includes the effects of nonlinear evolution in both colliding nuclei.

hep-ph↗
arXiv2001

Instantons in the Saturation Environment

We show that instanton calculations in QCD become theoretically well defined in the gluon saturation environment which suppresses large size instantons. The effective cutoff scale is determined by the inverse of the saturation scale. We concentrate on two most important cases: the small-x tail of a gluon distribution of a high energy hadron or a large nucleus and the central rapidity region in a high energy hadronic or heavy ion collision. In the saturation regime the gluon density in a single large ultrarelativistic nucleus is high and gluonic fields are given by the classical solutions of the equations of motion. We show that these strong classical fields do not affect the density of instantons in the nuclear wave function compared to the instanton density in the vacuum. A classical solution with non-trivial topological charge is found for the gluon field of a single nucleus at the lowest order in the instanton perturbation theory. In the case of ultrarelativistic heavy ion collisions a strong classical gluonic field is produced in the central rapidity region. We demonstrate that this field introduces a suppression factor of exp{-c ρ^4 Q_s^4 / [8 α^2 N_c (Q_s τ)^2]} in the instanton size distribution, where Q_s is the saturation scale of both (identical) nuclei, τis the proper time and c = 1 is the gluon liberation coefficient. This factor suggests that gluonic saturation effects at the early stages of nuclear collisions regulate the instanton size distribution in the infrared region and make the instanton density finite by suppressing large size instantons.

hep-ph↗
arXiv2001

Ioffe Time in Double Logarithmic Approximation

We analyze the light cone (Ioffe) time structure of the gluon distribution function in the double logarithmic approximation. We show that due to QCD evolution Ioffe equation is modified. The characteristic light cone time of the gluons does not increase as fast with increasing energy (decreasing Bjorken x) as predicted by the parton distributions exhibiting Bjorken scaling due to the increase of the transverse momenta of the gluons in the DGLAP ladder.

hep-ph↗
arXiv2001

Classical Initial Conditions for Nucleus-Nucleus Collisions

We present the results of an analytical calculation of the distribution of gluons in the state immediately following a heavy ion collision in the quasi-classical limit of QCD given by McLerran-Venugopalan model. We show that the typical transverse energy Et of the produced gluons is of the order of the saturation scale of the nuclei Qs, as predicted by Mueller. We demonstrate that due to multiple rescatterings the obtained gluon distribution remains finite (up to logarithms of k) in the soft transverse momentum limit of k << Q_s. We compare the gluon spectrum in the single nuclear wave function before the collision to the spectrum of the produced gluons. The total number of produced gluons turns out to be proportional to the total number of gluons inside the nuclear wave function before the collision with the proportionality coefficient c = 2 ln 2.

hep-ph↗
arXiv2000

Classical Initial Conditions for Ultrarelativistic Heavy Ion Collisions

We construct an analytical expression for the distribution of gluons in the state immediately following a heavy ion collision in the quasi-classical limit of QCD given by McLerran-Venugopalan model. The resulting gluon number distribution function includes the effects of all multiple rescatterings of gluons with the nucleons of both colliding nuclei. The typical transverse momentum k of the produced gluons is shown to be of the order of the saturation scale of the nuclei Qs, as predicted by Mueller. We analyze the properties of the obtained distribution and demonstrate that due to multiple rescatterings it remains finite (up to logarithms of k) in the soft transverse momentum limit of k << Qs unlike the usual perturbative initial conditions given by collinear factorization. We calculate the total number of produced gluons and show that it is proportional to the total number of gluons inside the nuclear wave function before the collision with the proportionality coefficient c = 2 ln2.

hep-ph↗
arXiv2000

QCD Instantons and the Soft Pomeron

We study the role of semi-classical QCD vacuum solutions in high energy scattering by considering the instanton contribution to hadronic cross sections. We propose a new type of instanton-induced interactions (``instanton ladder'') that leads to the rising with energy hadronic cross section of Regge type (the Pomeron). We argue that this interaction may be responsible for the structure of the soft Pomeron. The intercept is calculated. It has a non-analytic dependence on the strong coupling constant, allowing a non-singular continuation into the non-perturbative region. We derive the Pomeron trajectory, which appears to be approximately linear in some range of (negative) momentum transfer t, but exhibits a curvature at small t. Possible role of instantons in multiparticle production is also discussed.

hep-ph↗
arXiv2000

Diffractive Structure Function in a Quasi-Classical Approximation

We derive an expression for diffractive F_2 structure function which should be valid at small x for quasi-elastic scattering on a hadron and for quasi-elastic scattering on a large nucleus. This expression includes multiple rescatterings of the quark-antiquark pair produced by the virtual photon off the sources of color charge in a quasi-classical approximation. We find that there is a relation between such diffractive production and inclusive processes. In the former, one averages over all colors of sources before squaring the amplitude, and in the latter one first squares the amplitude and then averages it in the hadron's or nuclear wave function. We show that in the limit of a large virtuality of the photon Q^2 the diffractive structure function becomes linearly proportional to the gluon distribution of the hadron or nucleus, therefore proving that in this sense diffraction is a leading twist effect.

hep-ph↗
arXiv1999

Large Scale Rapidity Correlations in Heavy Ion Collisions

We discuss particle production mechanisms for heavy ion collisions. We present an argument demonstrating how the fluctuations of the number of produced particles in a series of classical emissions can account for KNO scaling. We predict rapidity correlations in the particle production in the event by event analysis of heavy ion collisions on the rapidity scales of the order of one over the strong coupling constant.

hep-ph↗
arXiv1999

Diffractive Dissociation Including Multiple Pomeron Exchanges in High Parton Density QCD

We derive an evolution equation describing the high energy behavior of the cross section for the single diffractive dissociation in deep inelastic scattering on a hadron or a nucleus. The evolution equation resums multiple BFKL pomeron exchanges contributing to the cross section of the events with large rapidity gaps. Analyzing the properties of an approximate solution of the proposed equation we point out that at very high energies there is a possibility that for a fixed center of mass energy the cross section will reach a local maximum at a certain intermediate size of the rapidity gap, or, equivalently, at some non-zero value of the invariant mass of the produced particles.

hep-ph↗
arXiv1999

Unitarization of the BFKL Pomeron on a Nucleus

We analyze the evolution equation describing all multiple hard pomeron exchanges in a hadronic or nuclear structure functions that was proposed earlier. We construct a perturbation series providing us with an exact solution to the equation outside of the saturation region. The series demonstrates how at moderately high energies the corrections to the single BFKL pomeron exchange contribution which are due to the multiple pomeron exchanges start unitarizing total deep inelastic scattering cross section. We show that as energy increases the scattering cross section of the quark-antiquark pair of a fixed transverse separation on a hadron or nucleus given by the solution of our equation inside of the saturation region unitarizes and becomes independent of energy. The corresponding F_2 structure function also unitarizes and becomes linearly proportional to ln s. We also discuss possible applications of the developed technique to diffraction.

hep-ph↗
arXiv1999

Small-x F_2 Structure Function of a Nucleus Including Multiple Pomeron Exchanges

We derive an equation determining the small-x evolution of the F_2 structure function of a large nucleus which includes all multiple pomeron exchanges in the leading logarithmic approximation using Mueller's dipole model. We show that in the double leading logarithmic limit this evolution equation reduces to the GLR equation.

hep-ph↗
arXiv1998

Running Coupling Effects in BFKL Evolution

We resum the recently calculated second order kernel of the BFKL equation. That kernel can be viewed as the sum of a conformally invariant part and a running coupling part. The conformally invariant part leads to a corrected BFKL intercept as found earlier. The running coupling part of the kernel leads to a non-Regge term in the energy dependence of high energy hard scattering, as well as a $Q^2-$dependent intercept.

hep-ph↗