arXiv subjects

Andrej Ficnar

Publications and source records attributed to Andrej Ficnar.

12 recordsLinked to original sources

arXiv2016

Holographic Isotropisation in Gauss-Bonnet Gravity

We study holographic isotropisation of homogeneous, strongly coupled, non-Abelian plasmas in Gauss-Bonnet gravity with a negative cosmological constant. We focus on small values of the Gauss-Bonnet coupling parameter $λ_{GB}$ and linearise the equations of motion around a time-dependent background solution with $λ_{GB}=0$. We numerically solve the linearised equations and show that the entire time evolution of the pressure anisotropy can be well approximated by the linear in $λ_{GB}$ corrections to the quasinormal mode expansion, even in the cases of high anisotropy. We finally show that, quite generally, the time evolution of the pressure anisotropy with the Gauss-Bonnet term is approximately {\it shifted} with respect to the evolution without it, with the sign of the shift being directly related to the sign of the $λ_{GB}$ parameter. This suggests that finite coupling corrections generically {\it increase} the isotropisation time of strongly coupled plasmas.

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arXiv2016

Energy loss, equilibration, and thermodynamics of a baryon rich strongly coupled quark-gluon plasma

Lattice data for the QCD equation of state and the baryon susceptibility near the crossover phase transition (at zero baryon density) are used to determine the input parameters of a 5-dimensional Einstein-Maxwell-Dilaton holographic model that provides a consistent holographic framework to study both equilibrium and out-of-equilibrium properties of a hot and {\it baryon rich} strongly coupled quark-gluon plasma (QGP). We compare our holographic equation of state computed at nonzero baryon chemical potential, $μ_B$, with recent lattice calculations and find quantitative agreement for the pressure and the speed of sound for $μ_B \leq 400$ MeV. This holographic model is used to obtain holographic predictions for the temperature and $μ_B$ dependence of the drag force and the Langevin diffusion coefficients associated with heavy quark jet propagation as well as the jet quenching parameter $\hat{q}$ and the shooting string energy loss of light quarks in the baryon dense plasma. We find that the energy loss of heavy and light quarks generally displays a nontrivial, fast-varying behavior as a function of the temperature near the crossover. Moreover, energy loss is also found to generally increase due to nonzero baryon density effects even though this strongly coupled liquid cannot be described in terms of well defined quasiparticle excitations. Furthermore, to get a glimpse of how thermalization occurs in a hot and baryon dense QGP, we study how the lowest quasinormal mode of an external massless scalar disturbance in the bulk is affected by a nonzero baryon charge. We find that the equilibration time associated with the lowest quasinormal mode decreases in a dense medium.

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arXiv2015

Holographic Three-Jet Events in Strongly Coupled N=4 Yang-Mills Plasma

We analyse classical string configurations with non-trivial transverse dynamics in $AdS_5$-Schwarzschild. These strings develop kink-like structures which, via the gauge/gravity duality, can be interpreted as the propagation of hard gluons produced in association with a quark-antiquark pair in a strongly coupled $\mathcal{N}=4$ SYM plasma. We observe the appearance of two physically distinct regimes of the in-plasma dynamics, depending on whether the medium is able to resolve the transverse structure of the string prior to its total quench. From these studies we extract the medium resolution scale of the strongly coupled SYM plasma, defined as the smallest angular separation between two jets that the medium can resolve, $θ_{\rm res} = \mathcal{C}_{\rm res} ( E /\sqrtλ T)^{-2/3}$, where $\mathcal{C}_{\rm res}= \frac{2^{4/3}}π\frac{Γ(3/4)^2}{Γ(5/4)^2}$. Our analysis constitutes the first study of proxies for three-jet events in a holographic context.

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arXiv2014

Shooting String Holography of Jet Quenching at RHIC and LHC

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.

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arXiv2014

Holographic light quark jet quenching at RHIC and LHC via the shooting strings

A new shooting string holographic model of jet quenching of light quarks in strongly coupled plasmas is presented to overcome the phenomenological incompatibilities of previous falling string holographic scenarios that emerged when confronted with the recent LHC data. This model is based on strings with finite momentum endpoints that start close to the horizon and lose energy as they approach the boundary. This framework is applied to compute the nuclear modification factor RAA of light hadrons at RHIC and LHC, showing that this model improves greatly the comparison with the recent light hadron suppression data. The effects of the Gauss-Bonnet quadratic curvature corrections to the AdS5 geometry further improve the agreement with the data.

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arXiv2013

Finite momentum at string endpoints

We argue that classical strings, both bosonic and supersymmetric, can have finite energy and momentum at their endpoints. We show that in a general curved background, string endpoints must propagate along null geodesics as long as their energy remains finite. Finite endpoint momentum allows strings with a fixed energy to travel a greater distance in an AdS5-Schwarzschild background than has been possible for classical solutions considered previously. We review the relevance to heavy ion phenomenology of the dependence of this distance on energy, and we propose a scheme for determining the instantaneous rate of energy loss.

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arXiv2012

AdS/CFT Energy Loss in Time-Dependent String Configurations

We analyze spacetime momentum currents on a classical string worldsheet, study their generic connection via AdS/CFT correspondence to the instantaneous energy loss of the dual field theory degrees of freedom and suggest a general formula for computing energy loss in a time-dependent string configuration. Applying this formula to the case of falling strings, generally dual to light quarks, reveals that the energy loss does not display a well-pronounced Bragg peak at late times, as previously believed. Finally, we comment on the possible implications of this result to the jet quenching phenomena in heavy ion collisions.

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arXiv2012

Falling Strings and Light Quark Jet Quenching at LHC

We explore phenomenological signatures of light quark jet quenching within the AdS/CFT correspondence. Numerical studies of the instantaneous energy loss of light quarks, modeled as falling strings, suggest a linear path dependence. We propose a phenomenological model for the energy loss and use it to compute the nuclear modification factor $R_{AA}$ for light quarks in an expanding plasma with Glauber initial conditions. The results are compared to the light hadron $R_{AA}$ data at the LHC and, although they show qualitative agreement, the quantitative disagreement we found motivated the exploration of effects from higher order derivative corrections to $AdS_5$ on this observable.

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arXiv2011

Jet Quenching in Non-Conformal Holography

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.

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arXiv2011

Non-conformal Holography of Heavy Quark Quenching

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.

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arXiv2008

alpha'^2-corrections to extremal dyonic black holes in heterotic string theory

We calculate alpha'^2-corrections to the entropy of the 5-dimensional 3-charge and the 4-dimensional 4-charge large extremal black holes using the low energy effective action of the heterotic string theory. In the 4-dimensional case, our results are in agreement with the microscopic statistical entropy both for the BPS and the non-BPS black holes. In the more interesting 5-dimensional case, where the direct microscopic stringy description is still unknown, our results for the BPS black holes are in agreement with the results obtained from the action supplemented with R^2-correction obtained by supersymmetric completion of the gravitational Chern-Simons term. This agreement does not extend to the non-BPS black holes, for which we propose a different expression for the entropy. We show that the new expression is supported by certain alpha'^3-order calculations, and by the arguments based on the AdS/CFT correspondence.

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arXiv2007

Five-dimensional black holes in heterotic string theory

We review recent results on near-horizon static black hole solutions and entropy in R^2-corrected N=2 SUGRA in D=5, focusing on actions connected to heterotic string compactified on K3\times S^1. Comparison with α'-perturbative results, results obtained by using simple Gauss-Bonnet R^2-correction, OSV conjecture and microscopic stringy description (for small black holes) shows that situation in D=5 is, in a sense, even more interesting then in D=4.

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