arXiv subjects

Vladimir Skokov

Publications and source records attributed to Vladimir Skokov.

At least 37 records · Page 2Linked to original sources

arXiv2018

Measuring the Weizsaecker-Williams distribution of linearly polarized gluons at an EIC through dijet azimuthal asymmetries

The production of a hard dijet with small transverse momentum imbalance in semi-inclusive DIS probes the conventional and linearly polarized Weizsaecker-Williams (WW) Transverse Momentum Dependent (TMD) gluon distributions. The latter, in particular, gives rise to an azimuthal dependence of the dijet cross-section. In this paper we analyze the feasibility of a measurement of these TMDs through dijet production in DIS on a nucleus at an Electron-Ion Collider. We introduce the MCDijet Monte-Carlo generator to sample quark-antiquark dijet configurations based on leading order parton level cross-sections with WW gluon distributions that solve the non-linear small-x QCD evolution equations. These configurations are fragmented to hadrons using PYTHIA, and final state jets are reconstructed. We report on background studies and on the effect of kinematic cuts introduced to remove beam jet remnants. We estimate that with an integrated luminosity of 20 fb^{-1}/A one can determine the distribution of linearly polarized gluons with a statistical accuracy of approximately 5%.

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arXiv2018

Odd Azimuthal Anisotropy of the Glasma for pA Scattering

In this paper we analytically extract the odd azimuthal anisotropy in the Classical Yang-Mills equations for the Glasma for pA collisions. We compute the first non-trivial term in the expansion of the proton sources of color charge. The computation is valid in the limit of a large nucleus when the produced particle momenta are larger than the saturation momentum of the proton.

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arXiv2018

Cos(4 phi) azimuthal anisotropy in small-x DIS dijet production beyond the leading power TMD limit

We determine the first correction to the quadrupole operator in high-energy QCD beyond the TMD limit of Weizsaecker-Williams and linearly polarized gluon distributions. These functions give rise to isotropic resp. ~ cos 2 phi angular distributions in DIS dijet production. On the other hand, the correction produces a ~ cos 4 phi angular dependence which is suppressed by one additional power of the dijet transverse momentum scale (squared) P^2.

hep-ph↗

arXiv2018

The small-x gluon distribution in centrality biased pA and pp collisions

The nuclear modification factor $R_{pA}(p_T)$ provides information on the small-$x$ gluon distribution of a nucleus at hadron colliders. Several experiments have recently measured the nuclear modification factor not only in minimum bias but also for central $pA$ collisions. In this paper we analyze the bias on the configurations of soft gluon fields introduced by a centrality selection via the number of hard particles. Such bias can be viewed as reweighting of configurations of small-$x$ gluons. We find that the biased nuclear modification factor ${\cal Q}_{pA}(p_T)$ for central collisions is above $R_{pA}(p_T)$ for minimum bias events, and that it may redevelop a "Cronin peak" even at small $x$. The magnitude of the peak is predicted to increase approximately like $1/{A_{\perp}}^ν$, $ν\sim0.6\pm0.1$, if one is able to select more compact configurations of the projectile proton where its gluons occupy a smaller transverse area $A_\perp$. We predict an enhanced ${\cal Q}_{pp}(p_T)-1 \sim 1/(p_T^2)^ν$ and a Cronin peak even for central $pp$ collisions.

hep-ph↗

arXiv2017

The Weizsäcker-Williams distribution of linearly polarized gluons (and its fluctuations) at small x

The conventional and linearly polarized Weizsäcker-Williams gluon distributions at small x are defined from the two-point function of the gluon field in light-cone gauge. They appear in the cross section for dijet production in deep inelastic scattering at high energy. We determine these functions in the small-x limit from solutions of the JIMWLK evolution equations and show that they exhibit approximate geometric scaling. Also, we discuss the functional distributions of these WW gluon distributions over the JIMWLK ensemble at rapidity $Y\sim 1/α_s$. These are determined by a 2d Liouville action for the logarithm of the covariant gauge function $g^2 \mathrm{tr}\,A^+(q)A^+(-q)$. For transverse momenta on the order of the saturation scale we observe large variations across configurations (evolution trajectories) of the linearly polarized distribution up to several times its average, and even to negative values.

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arXiv2017

Fluctuations of the gluon distribution from the small-x effective action

The computation of observables in high energy QCD involves an average over stochastic semiclassical small-x gluon fields. The weight of various configurations is determined by the effective action. We introduce a method to study fluctuations of observables, functionals of the small-x fields, which does not explicitly involve dipoles. We integrate out those fluctuations of the gluon field under which a given observable is invariant. Thereby we obtain the effective potential for that observable describing its fluctuations about the saddle point. We determine explicitly the effective potential for the covariant gauge gluon distribution both for the McLerran-Venugopalan (MV) model and for a (non-local) Gaussian approximation for the small-x effective action. This provides insight into the correlation of fluctuations of the number of hard gluons versus their typical transverse momentum. We find that the spectral shape of the fluctuations of the gluon distribution is fundamentally different in the MV model, where there is a pile-up of gluons near the saturation scale, versus the solution of the small-x JIMWLK renormalization group, which generates essentially scale invariant fluctuations above the absorptive boundary set by the saturation scale.

hep-ph↗

arXiv2017

Polyakov loop modeling for hot QCD

We review theoretical aspects of quantum chromodynamics (QCD) at finite temperature. The most important physical variable to characterize hot QCD is the Polyakov loop, which is an approximate order parameter for quark deconfinement in a hot gluonic medium. Additionally to its role as an order parameter, the Polyakov loop has rich physical contents in both perturbative and non-perturbative sectors. This review covers a wide range of subjects associated with the Polyakov loop from topological defects in hot QCD to model building with coupling to the Polyakov loop.

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arXiv2017

Exploring correlations in the CGC wave function: odd azimuthal anisotropy

We extend the CGC approach to calculation of the double inclusive gluon production by including high-density effect in the CGC wave function of the projectile (proton). Our main result is that these effects lead to the appearance of odd harmonics in the two-particle correlation C(k,p). We find that in the high momentum limit, |k|,|p|>>Q_s, this results in a positive c_1{2}. Additionally when the magnitudes of the two momenta are approximately equal, |k|/|p| ~ 1, the density effects also generate a positive third harmonic c_3{2}, which translates into a non-vanishing v_3 when the momenta of the trigger and associated particle are in the same momentum bin. The sign of c_3{2} becomes negative when |k|/|p|>1.1 suggesting an interesting experimental signature.

hep-ph↗

arXiv2017

Initial state qqg correlations as a background for the Chiral Magnetic Effect in collision of small systems

Motivated by understanding the background to Chiral Magnetic Effect in proton-nucleus collisions from first principles, we compute the three particle correlation in the projectile wave function. We extract the correlations between two quarks and one gluon in the framework of the Color Glass Condensate. This is related to the same-charge correlation of the conventional observable for the Chiral Magnetic Effect. We show that there are two different contributions to this correlation function. One contribution is rapidity-independent and as such can be identified with the pedestal; while the other displays rather strong rapidity dependence. The pedestal contribution and the rapidity-dependent contribution at large rapidity separation between the two quarks result in the negative same charge correlations, while at small rapidity separation the second contribution changes sign. We argue that the computed initial state correlations might be partially responsible for the experimentally observed signal in proton-nucleus collisions.

hep-ph↗

arXiv2017

The Large N Limit with Vanishing Leading Order Condensate for Zero Pion Mass

It is conventionally assumed that the negative mass squared term in the linear sigma model version of the pion Lagrangian is $M^2 \sim Λ_{\rm QCD}^2$ in powers of $N_c$. We consider the case where $M^2 \sim Λ^2_{\rm QCD}/N_c$ so that to leading order in $N_c$ this symmetry breaking term vanishes. We present some arguments why this might be plausible. One might think that such a radical assumption would contradict lattice Monte Carlo data on QCD as function of $N_c$. We show that the linear sigma model gives a fair description of the data of DeGrand and Liu both for $N_c = 3$, and for variable $N_c$. The values of quark masses considered by DeGrand and Liu, and by Bali et. al. turn out to be too large to resolve the case we consider from that of the conventional large $N_c$ limit. We argue that for quark masses $m_{q} \ll Λ_{\rm QCD}/N_c^{3/2}$, both the baryon mass and nucleon size scale as $\sqrt{N_c}$. For $m_{q} \gg Λ_{\rm QCD}/N_c^{3/2}$ the conventional large-$N_c$ counting holds. The physical values of quark masses for QCD correspond to the small quark mass limit. We find pion-nucleon coupling strengths are reduced to order ${\cal O}(1)$ rather than ${\cal O}(N_c)$. Under the assumption that in the large $N_c$ limit the sigma meson mass is larger than that of the omega, and that the omega-nucleon coupling constant is larger than that of the sigma, we argue that the nucleon-nucleon large range potential is weakly attractive and admits an interaction energy of order $Λ_{\rm QCD}/N_c^{5/2} \sim 10$ MeV. With these assumptions on coupling and masses, there is no strong long range attractive channel for nucleon-nucleon interactions, so that nuclear matter at densities much smaller than that where nucleons strongly interact is a weakly interacting configuration of nucleons with strongly interacting localized cores.

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arXiv2016

Correlated stopping, proton clusters and higher order proton cumulants

We investigate possible effects of correlations between stopped nucleons on higher order proton cumulants at low energy heavy-ion collisions. We find that fluctuations of the number of wounded nucleons $N_{\mathrm{part}}$ lead to rather nontrivial dependence of the correlations on the centrality; however, this effect is too small to explain the large and positive four-proton correlations found in the preliminary data collected by the STAR collaboration at $\sqrt{s}=7.7$ GeV. We further demonstrate that, by taking into account additional proton clustering, we are able to qualitatively reproduce the preliminary experimental data. We speculate that this clustering may originate either from collective/multi-collision stopping which is expected to be effective at lower energies or from a possible first-order phase transition, or from (attractive) final state interactions. To test these ideas we propose to measure a mixed multi-particle correlation between stopped protons and a produced particle (e.g. pion, antiproton).

nucl-th↗

arXiv2016

Chiral Magnetic Effect Task Force Report

In this report, we briefly examine the current status of the study of the chiral magnetic effect including theory and experimental progress. We recommend future strategies for resolving uncertainties in interpretation including recommendations for theoretical work, recommendations for measurements based on data collected in the past five years, and recommendations for beam use in the coming years of RHIC. We have specifically investigated the case for colliding nuclear isobars (nuclei with the same mass but different charge) and find the case compelling. We recommend that a program of nuclear isobar collisions to isolate the chiral magnetic effect from background sources be placed as a high priority item in the strategy for completing the RHIC mission.

nucl-th↗

arXiv2015

Finite Numbers of Sources, Particle Correlations and the Color Glass Condensate

We show that for a finite number of emitting sources, the Color Glass Condensate produces substantial elliptic azimuthal anisotropy, characterized by $v_2$, for two and four particle correlations for momentum greater than or of the order the saturation momentum. The flow produced has the correct semi-quantitative features to describe flow seen in LHC experiments with p-Pb and pp collisions. This flow is induced by quantum mechanical interference between the waves of produced particles, and the flow itself is coupled to fluctuations in the positions of emitting sources. We shortly discuss generalizing these results to odd $v_n$, to correlations involving larger number of particles, and to transverse momentum scales $Λ_{\rm QCD} \ll p_T \ll Q_{\rm sat}$.

hep-ph↗

arXiv2015

Centrality dependence of high energy jets in p+Pb collisions at the LHC

The recently measured centrality dependence of high energy jets in proton-lead collisions at the LHC is investigated. We hypothesize that events with jets of very high energy (a few hundred GeV) are characterized by a suppressed number of soft particles, thus shifting these events into more peripheral bins. This naturally results in the suppression (enhancement) of the nuclear modification factor, $R_{pA}$, in central (peripheral) collisions. Our calculations suggest that a moderate suppression of the order of $20\%$, for $10^{3}$ GeV jets, can quantitatively reproduce the experimental data. We further extract the suppression factor as a function of jet energy and test our conjecture using available $R_{pA}$ data for various centralities.

hep-ph↗

arXiv2015

Multi-particle eccentricities in collisions dominated by fluctuations

We compute analytically the multi-particle eccentricities, ε_m {2n}, for systems dominated by fluctuations, such as proton-nucleus collisions at the Large Hadron Collider. In particular, we derive a general relation for <ε_2^{2n}>. We further discuss the relations between various multi-particle eccentricities and demonstrate that ε_2{2} > ε_2{4} \simeq ε_2{6} \simeq ε_2{8}, in agreement with recent numerical calculations in a Glauber model.

hep-ph↗

arXiv2015

The distribution of linearly polarized gluons and elliptic azimuthal anisotropy in DIS dijet production at high energy

We determine the distribution of linearly polarized gluons of a dense target at small $x$ by solving the B-JIMWLK rapidity evolution equations. From these solutions we estimate the amplitude of $\sim \cos 2ϕ$ azimuthal asymmetries in DIS dijet production at high energies. We find sizeable long-range in rapidity azimuthal asymmetries with a magnitude in the range of $v_2=\langle\cos 2ϕ\rangle \sim 10\%$.

hep-ph↗

arXiv2015

Anisotropy of the semi-classical gluon field of a large nucleus at high energy

The McLerran-Venugopalan model describes a highly boosted hadron/nucleus as a sheet of random color charges which source soft classical Weizsäcker-Williams gluon fields. We show that due to fluctuations, individual configurations are azimuthally anisotropic. We present initial evidence that impact parameter dependent small-x JIMWLK resummation preserves such anisotropies over several units of rapidity. Finally, we compute the first four azimuthal Fourier amplitudes of the S-matrix of a fundamental dipole in such background fields.

hep-ph↗

arXiv2015

Anisotropic particle production and azimuthal correlations in high-energy pA collisions

We summarize some recent ideas relating to anisotropic particle production in high-energy collisions. Anisotropic gluon distributions lead to anisotropies of the single-particle azimuthal distribution and hence to disconnected contributions to multi-particle cumulants. When these dominate, the four-particle elliptic anisotropy $c_2\{4\}$ changes sign. On the other hand, connected diagrams for $m$-particle cumulants are found to quickly saturate with increasing $m$, a ``coherence'' quite unlike conventional ``non-flow'' contributions such as decays. Finally, we perform a first exploratory phenomenological analysis in order to estimate the amplitude ${\cal A}$ of the $\cos(2φ)$ anisotropy of the gluon distribution at small $x$, and we provide a qualitative prediction for the elliptic asymmetry from three-particle correlations, $c_2\{3\}$.

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