arXiv subjects

Adrian Dumitru

Publications and source records attributed to Adrian Dumitru.

At least 55 records · Page 3Linked to original sources

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 $v_2=\langle\cos 2ϕ\rangle \sim 10\%$.

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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.

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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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arXiv2015

Azimuthal asymmetries and the emergence of "collectivity" from multi-particle correlations in high-energy pA collisions

We show how angular asymmetries $\sim\cos 2ϕ$ can arise in dipole scattering at high energies. We illustrate the effects due to anisotropic fluctuations of the saturation momentum of the target with a finite correlation length in the transverse impact parameter plane, i.e.\ from a domain-like structure. We compute the two-particle azimuthal cumulant in this model including both one-particle factorizable as well as genuine two-particle non-factorizable contributions to the two-particle cross section. We also compute the full BBGKY hierarchy for the four-particle azimuthal cumulant and find that only the fully factorizable contribution to $c_2\{4\}$ is negative while all contributions from genuine two, three and four-particle correlations are positive. Our results may provide some qualitative insight into the origin of azimuthal asymmetries in p+Pb collisions at the LHC which reveal a change of sign of $c_2\{4\}$ in high-multiplicity events.

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arXiv2014

Initial state angular asymmetries in high energy p+A collisions: spontaneous breaking of rotational symmetry by a color electric field and C-odd fluctuations

We present a simple model for generating initial-state azimuthal asymmetries in pA collisions from dipole scattering on an anisotropic dense target. Parity even angular harmonics arise from the C-even real part of the dipole S-matrix which spontaneously breaks rotational symmetry in two dimensions due to a condensate for the color electric field. This leads to an angular correlation with the direction of $\vec E$. Parity odd harmonics are generated by the C-odd imaginary part (odderon) due to coupling to coherent target fluctuations which again break rotational invariance. We perform a first qualitative extraction of the amplitude and cutoff of C-odd fluctuations in the dense target.

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arXiv2014

Azimuthal asymmetries in high-energy collisions of protons with holographic shockwaves

Large azimuthal quadrupole and octupole asymmetries have recently been found in p+Pb collisions at the LHC. We argue that these might arise from a projectile dipole scattering off fluctuations in the target with a size on the order of the dipole. In a holographic scenario, parity even angular moments $v_{2n}$ are generated by the real part of the light-like Wilson loop due to the contribution from the background metric to the Nambu-Goto action. On the other hand, parity odd moments $v_{2n+1}$ must arise from the imaginary part of the light-like Wilson loop, which is naturally induced by a fluctuating Neveu-Schwarz 2-form.

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arXiv2014

Two-loop perturbative corrections to the thermal effective potential in gluodynamics

The thermodynamics of pure glue theories can be described in terms of an effective action for the Polyakov loop. This effective action is of the Landau-Ginzburg type and its variables are the angles parametrizing the loop. In this paper we compute perturbative corrections to this action. Remarkably, two-loop corrections turn out to be proportional to the one-loop action, independent of the eigenvalues of the loop. By a straightforward generalization of the 't Hooft coupling this surprisingly simple result holds for any of the classical and exceptional groups.

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arXiv2013

Are the Angular Correlations in pA Collisions due to a Glasmion or Bose Condensation ?

Experiments at the LHC have recently reported results on the angular asymmetry coefficients $v_n[m]$, for various angular moments $n$ and orders of cumulants $m$, in high multiplicity p+Pb collisions. These coefficients are large, and have both even and odd moments. We discuss here some of the implications of these results for our understanding of the initial state of the collision (Color Glass Condensate) and for the evolution in the final state (Glasma and thermalized Quark Gluon Plasma). We show the Color Glass Condensate predicts large even moments, $v_n$ with $n$ an even integer. Odd moments are generated by final state interactions or fragmentation. For a multi-particle determination of $v_2[m]$, where m is the number of particles used to determine the correlation, we argue that if these coefficients approach equality for large $m$ in high multiplicity events, this may imply the existence of either solitonic solutions or Bose condensation either for the JIMWLK action that describes the CGC, or for the Glasma that might be produced in such a collision.

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arXiv2013

Magnetic flux loop in high-energy heavy-ion collisions

We consider the expectation value of a magnetic flux loop in the immediate forward light cone of collisions of heavy nuclei at high energies. Such collisions are characterized by a non-linear scale Q_s where color fields become strong. We find that loops of area greater than ~2/Q_s^2 exhibit area law behavior, which determines the scale of elementary flux excitations ("vortices"). We also estimate the magnetic string tension, sigma_M = 0.12 Q_s^2. By the time t ~ 1/Q_s even small loops satisfy an area law. We describe corrections to the propagator of semi-hard gluons at very early times in the background of fluctuating magnetic fields.

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arXiv2013

Effective potential for SU(2) Polyakov loops and Wilson loop eigenvalues

We simulate SU(2) gauge theory at temperatures ranging from slightly below $T_c$ to roughly $2T_c$ for two different values of the gauge coupling. Using a histogram method, we extract the effective potential for the Polyakov loop and for the phases of the eigenvalues of the thermal Wilson loop, in both the fundamental and adjoint representations. We show that the classical potential of the fundamental loop can be parametrized within a simple model which includes a Vandermonde potential and terms linear and quadratic in the Polyakov loop. We discuss how parametrizations for the other cases can be obtained from this model.

hep-lat↗

arXiv2013

Magnetic screening in high-energy heavy-ion collisions

We show that classical chromomagnetic fields produced coherently in the initial stage of a heavy-ion collision exhibit screening. From the two-point field strength correlator we determine the magnetic mass for SU(2) to be m ~ 5 times the saturation scale. Magnetic screening leads to an intuitive understanding of the area law scaling of spatial Wilson loops observed previously. The presence of screening effects in the initial state provides a basis for defining kinetic processes in the early stage of heavy-ion collisions, with electric and magnetic masses of the same order.

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arXiv2013

The initial state of heavy-ion collisions

We present a brief review of recent theoretical developments and related phenomenological approaches for understanding the initial state of heavy-ion collisions, with emphasis on the Color Glass Condensate formalism.

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arXiv2012

Review of Recent Developments in the CGC

I review recent developments in the CGC approach to high-energy collisions. The focus is on topics related to the Quark Matter conference, specifically on predictions for the p+Pb run at the LHC; as an added bonus some of these predictions are confronted with first data taken during the p+Pb machine performance test at the LHC performed in september 2012. I also highlight recent work related to particle production fluctuations, suppression of nuclear modification factors in p+A from rcBK evolution, (partial) NLO corrections in forward production, and photon-hadron as well as di-hadron correlations.

hep-ph↗

arXiv2012

Gluon saturation in $pA$ collisions at the LHC: KLN model predictions for hadron multiplicities

The upcoming p+Pb run at the LHC will probe the nuclear gluon distribution at very small Bjorken x (from $x \sim 10^{-4}$ at mid-rapidity down to $x \sim 10^{-6}$ in the proton fragmentation region) and will allow to test approaches based on parton saturation. Here, we present the predictions of the KLN model for hadron multiplicities and multiplicity distributions in p+Pb collisions at a center-of-mass energy of 4.4 TeV. We also compare the model to the existing pp, dA and AA data from RHIC and LHC.

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arXiv2012

CGC predictions for p+Pb collisions at the LHC

We present predictions for total multiplicities and single inclusive particle production in proton-lead collisions at the LHC. The main dynamical input in our calculations is the use of solutions of the running coupling BK equation tested in e+p data. We use a Monte-Carlo treatment of the nuclear geometry and either $k_t$-factorization or the hybrid formalisms to describe particle production in the central and forward rapidity regions, respectively.

hep-ph↗

arXiv2012

KNO scaling from a nearly Gaussian action for small-x gluons

Transverse momentum integrated multiplicities in the central region of pp collisions at LHC energies satisfy Koba-Nielsen-Olesen scaling. We attempt to relate this finding to multiplicity distributions of soft gluons. KNO scaling emerges if the effective theory describing color charge fluctuations at a scale on the order of the saturation momentum is approximately Gaussian. From an evolution equation for quantum corrections which includes both saturation as well as fluctuations we find that evolution with the QCD β-function satisfies KNO scaling while fixed-coupling evolution does not. Thus, non-linear saturation effects and running-coupling evolution are both required in order to reproduce geometric scaling of the DIS cross section and KNO scaling of virtual dipoles in a hadron wave function.

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arXiv2012

Eigenvalue repulsion in an effective theory of SU(2) Wilson lines in three dimensions

We perform simulations of an effective theory of SU(2) Wilson lines in three dimensions. We include a non-perturbative "fuzzy-bag" contribution which is added to the one-loop perturbative potential for the Wilson line. We confirm that, at moderately weak coupling, this leads to eigenvalue repulsion in a finite region above the deconfining phase transition which shrinks in the extreme weak-coupling limit. A non-trivial Z(N) symmetric vacuum arises in the confined phase.

hep-lat↗

arXiv2012

Effective matrix model for deconfinement in pure gauge theories

We construct matrix models for the deconfining phase transition in SU(N) gauge theories, without dynamical quarks, at a nonzero temperature T. We generalize models with zero and one free parameter to study a model with two free parameters: besides perturbative terms ~T^4, we introduce terms ~T^2 and ~T^0. The two N-dependent parameters are determined by fitting to data from numerical simulations on the lattice for the pressure, including the latent heat. Good agreement is found for the pressure in the semi-quark gluon plasma (QGP), which is the region from Tc, the critical temperature, to about ~4 Tc. Above ~1.2 Tc, the pressure is a sum of a perturbative term, ~ +T^4, and a simple non-perturbative term, essentially just a constant times ~ -Tc^2 T^2. For the pressure, the details of the matrix model only enter within a very narrow window, from Tc to ~1.2 Tc, whose width does not change significantly with N. Without further adjustment, the model also agrees well with lattice data for the 't Hooft loop. This is notable, because in contrast to the pressure, the 't Hooft loop is sensitive to the details of the matrix model over the entire semi-QGP. For the (renormalized) Polyakov loop, though, our results disagree sharply with those from the lattice. Matrix models provide a natural and generic explanation for why the deconfining phase transition in SU(N) gauge theories is of first order not just for three, but also for four or more colors. Lastly, we consider gauge theories where there is no strict order parameter for deconfinement, such as for a G(2) gauge group. To agree with lattice measurements, in the G(2) matrix model it is essential to add terms which generate complete eigenvalue repulsion in the confining phase.

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