arXiv subjects

Anastasios Taliotis

Publications and source records attributed to Anastasios Taliotis.

At least 19 recordsLinked to original sources

arXiv2019

A story of balls, randomness and PDEs

Several differential equations usually appearing in mathematical physics are solved through a power series expansion, which reduces in solving difference equations. In this paper a probability problem is presented whose solution follows a completely reversed but systematic approach. Hence, this work is about illustrating how complex probability problems could be tackled with the more powerful techniques of a better studied and well understood field, that of differential equations. The problem is defined as follows: Inside a box containing r red and w white balls random removals occur. The balls are removed successively according to the three following rules. Rule I: If a white ball is chosen it is immediately discarded. If a red ball is chosen, it is placed back into the box and a new ball is randomly chosen. The second ball is then removed irrespective of the color. Rule II: Once one ball is removed, the game continues from Rule I. Rule III: The game ends once all the red balls are removed. The question posed is the determination of the probability that k white balls remain where k = 0, 1, 2, ..., w. Ending the game once all the white balls are removed, a second question is the determination of the probability that k red balls remain where k = 0, 1, 2, ..., r. While inductive solutions are possible, the current approach demonstrates a different and algorithmic route. In particular, the law of total probability yields a recursion that is transformed into a linear inhomogeneous 2D PDE, with suitable boundary conditions. The PDE solutions, which are found analytically, provide the generating functionals of the required probabilities as a function of r, w and k. Using the functionals, the probability formulas for any r, w and k are finally obtained in a closed form. Reproducing existing results of the literature this method is quite generic and adaptable to a large class of problems.

math.PR↗

arXiv2015

Holographic Hall conductivities from dyonic backgrounds

We develop a general framework for computing the holographic 2-point functions and the corresponding conductivities in asymptotically locally AdS backgrounds with an electric charge density, a constant magentic field, and possibly non-trivial scalar profiles, for a broad class of Einstein-Maxwell-Axion-Dilaton theories, including certain Chern-Simons terms. Holographic renormalization is carried out for any theory in this class and the computation of the renormalized AC conductivities at zero spatial momentum is reduced to solving a single decoupled first order Riccati equation. Moreover, we develop a first order fake supergravity formulalism for dyonic renormalization group flows in four dimensions, allowing us to construct analytically infinite families of such backgrounds by specifying a superpotential at will. These RG flows interpolate between AdS$_4$ in the UV and a hyperscaling violating Lifshitz geometry in the IR with exponents $1<z<3$ and $\theta=z+1$. For $1<z<2$ the spectrum of fluctuations is gapped and discrete. Our hope and intention is that this analysis can serve as a manual for computing the holographic 1- and 2-point functions and the corresponding transport coefficients in any dyonic background, both in the context of AdS/CMT and AdS/QCD.

hep-th↗

arXiv2014

A holographic model for the fractional quantum Hall effect

Experimental data for fractional quantum Hall systems can to a large extent be explained by assuming the existence of a modular symmetry group commuting with the renormalization group flow and hence mapping different phases of two-dimensional electron gases into each other. Based on this insight, we construct a phenomenological holographic model which captures many features of the fractional quantum Hall effect. Using an SL(2,Z)-invariant Einstein-Maxwell-axio-dilaton theory capturing the important modular transformation properties of quantum Hall physics, we find dyonic diatonic black hole solutions which are gapped and have a Hall conductivity equal to the filling fraction, as expected for quantum Hall states. We also provide several technical results on the general behavior of the gauge field fluctuations around these dyonic dilatonic black hole solutions: We specify a sufficient criterion for IR normalizability of the fluctuations, demonstrate the preservation of the gap under the SL(2,Z) action, and prove that the singularity of the fluctuation problem in the presence of a magnetic field is an accessory singularity. We finish with a preliminary investigation of the possible IR scaling solutions of our model and some speculations on how they could be important for the observed universality of quantum Hall transitions.

hep-th↗

arXiv2013

Riccati equations for holographic 2-point functions

Any second order homogeneous linear ordinary differential equation can be transformed into a first order non-linear Riccati equation. We argue that the Riccati form of the linearized fluctuation equations that determine the holographic 2-point functions simplifies considerably the numerical computation of such 2-point functions and of the corresponding transport coefficients by computing directly the response functions, eliminating the arbitrary source from the start. Moreover, it provides a neat criterion for the infrared regularity of the fluctuations. In particular, it is shown that the infrared regularity conditions for scalar and tensor fluctuations coincide, and hence they are either both regular or both singular. We demonstrate our numerical recipe based on the Riccati equations by computing the holographic 2-point functions for the stress tensor and a scalar operator in a number of asymptotically anti de Sitter backgrounds of bottom up scalar-gravity models. Analytical results are obtained for the 2-point function of the transverse traceless part of the stress tensor in two confining geometries, including a geometry that belongs to the class of IHQCD. We find that in this background the spin-2 spectrum is linear and, as expected, the position space 2-point function decays exponentially at large distances at a rate proportional to the confinement scale.

hep-th↗

arXiv2013

Gravitational collapse and thermalization in the hard wall model

We study a simple example of holographic thermalization in a confining field theory: the homogeneous injection of energy in the hard wall model. Working in an amplitude expansion, we find black brane formation for sufficiently fast energy injection and a scattering wave solution for sufficiently slow injection. We comment on our expectations for more sophisticated holographic QCD models.

hep-th↗

arXiv2013

Bremsstrahlung and black hole production from collisions of ultra-boosted particles at non-zero impact parameter

The collision of two massless, gravitationally interacting, point-like massless particles, boosted to the speed of light, colliding with an impact parameter b is being investigated. The collision takes place in four space-time flat dimensional background. A perturbative scheme is employed and the corrections to the energy momentum tensor and to the metric are computed and closed form formulas are provided. This includes the back-reaction on the metric after the collision. Including such corrections suggests that the tracelessness of the initial stress tensors of the colliding particles is preserved during and after the collision. The necessity for introducing an impact parameter in the perturbative treatment is highlighted and the breaking of the underlying perturbative approach at b = 0 is motivated. In addition, the energy radiated in the form of gravitational bremsstrahlung radiation is discussed while an example from gravitational-waves collision is being studied.

hep-th↗

arXiv2012

Extra dimensions, black holes and fireballs at the LHC

The collision of two gravitationally interacting, ultra-relativistic, extended sources is being examined. This investigation classifies the transverse distributions that are collided for fixed collision energy, according to whether one or two (a small and a large) apparent horizons may or may not be formed in a flat background in 4 dimensions. The study extends to the thermodynamical properties of the objects that are created, which exhibit a universal behavior in their entropy, and, suggests the elimination of the possibility in observing black holes (BHs) at the LHC in the absence of extra dimensions. On the other hand, including extra dimensions, and assuming that the matter is localized (dense) enough in those directions, opens new avenues in creating BHs at energies of the order of TeV. The investigation is carried further to $AdS_5$ backgrounds and makes connections with the implications for the quark-gluon plasma (QGP) formation in heavy ion collisions. In particular, classes of the geometries found suggest that a BH is formed if and only if the (central collision) energy is sufficiently large compared to the transverse scale of the corresponding gauge theory side stress-tensor. This implies that when the scattering in the gravity description is mapped onto a heavy ion collision problem yields a result, which is in accordance with the current intuition and data: QGP is formed only at high enough energies compared to $\Lambda_{QCD}$, even for central processes. Incorporating weak coupling physics and in particular the Color Glass Condensate (CGC) model, a satisfactory fitting with the RHIC and the LHC data for multiplicities may be established.

hep-th↗

arXiv2012

Black Topologies Production in Extra Dimensions

The configuration resulting after a collision of gravitational sources in a higher dimensional space with extra dimensions is investigated. Evidence is found that as the energy increases, there is a phase transition in the topology of the black object that is being formed: from the Black Hole to the Black String topology. An intuitive mechanism for the way the transition takes place is being proposed. The transition occurs at a finite value of the energy where an upper and a lower bound is found. Furthermore, at low energies the compact dimension behaves as an extended one while at high energies the extra dimension seems to decouple. Finally, the implications about the Gregory- Laflamme instability, the implications to the accelerators as well as holographic implications are being discussed.

hep-th↗

arXiv2011

Multiplicities from black-hole formation in heavy-ion collisions

The formation of trapped surfaces in the head-on collision of shock waves in conformal and non-conformal backgrounds is investigated. The backgrounds in- clude all interesting confining and non-confining backgrounds that may be relevant for QCD. Several transverse profiles of the shocks are investigated including distributions that fall-off as powers or exponentials. Different ways of cutting-off the UV contributions (that are expected to be perturbative in QCD) are explored. Under some plausible simplifying assumptions our estimates are converted into predictions for multiplicities for heavy-ion collisions at RHIC and LHC.

hep-ph↗

arXiv2011

Mini-Black-Hole production at RHIC and LHC

We argue that heavy-ion collisions provide the best testing ground for mini-black hole physics as $M_P\simeq 4 GeV$ for the gravity dual of YM and give concrete evidence for a new extra dimension, that is visible only to the strong interactions. We analyse the process of production evolution and decay of the mini-black-holes by using recent results on gravity duals of YM. There are several novelties compared with the traditional story of black hole evaporation, including Bjorken scaling instead of sphericity, evaporation via bubble nucleation instead of the Hawking mechanism and lepton-poor final states. Multiplicities are estimated using shock-wave scattering techniques. It is argued that high-multiplicity/high energy pp collisions will also show similar characteristics of mini-black-hole production and decay.

hep-ph↗

arXiv2010

$q{\bar q}$ Potential at Finite T and Weak Coupling in ${\cal N}=4$

We compute the potential between a $q{\bar q}$ singlet for ${\cal N}=4$ $SUSY$ with gauge group $SU(N)$ at finite temperature $T$, large distances $rT \gg 1$ and weak coupling $g$. As a first step, we only consider at the electric modes and we compute the Debye mass $m_D$ where we find that each of the $8N$ bosonic degrees of freedom contributes to $m_D^2$ with $\frac{1}{6}g^2 T^2$ while each of the $8N$ fermionic degrees of freedom contributes with $\frac{1}{12}g^2 T^2$ yielding to $m_D^2=2Ng^2T^2$. Then, motivated by results obtained in the literature from both, a weak coupling approach for $QCD$ and a large coupling investigation of ${\cal N}=4$ $SUSY$ through $AdS/CFT$, we attempt to include magnetic mode corrections.

hep-th↗

arXiv2010

Evolving Geometries in General Relativity

The problem of collisions of shockwaves in gravity is well known and has been studied extensively in the literature. Recently, the interest in this area has been revived trough the anti-de-Sitter space/Conformal Field Theory correspondence (AdS/CFT) with the difference that in this case the background geometry is Anti de Sitter in five dimensions. In a recent project that we have completed in the context of AdS/CFT, we have gained insight in the problem of shockwaves and our goal in this work is to apply the technique we have developed there in the case of ordinary gravity. In the current project, each of the shockwaves correspond to a point-like Stress-Energy tensor that moves with the speed of light while the collision is asymmetric and involves an impact parameter (b). Our method is to expand the metric $(g_{\mu \nu})$ in the background of flat space-time in the presence of the two shockwaves and compute corrections that satisfy causal boundary conditions taking into account back-reactions of the Stress-Energy tensor of the two point-like particles. Our solution respects causality as expected but this casual dependence takes place in an intuitive way. In particular, $g_{\mu \nu}$ at any given point $\vec{r}$ on the transverse plane at fixed $\tau$ evolves according from whether the propagation from the center of each of the shockwaves or from both shockwaves has enough proper time ($\tau$) to reach the point under consideration or not. Simultaneously around the center of each shockwave, the future metric develops a $\delta$-function profile with radius $\tau$; therefore this profile expands outwards from the centers (of the shockwaves) with the speed of light. Finally, we discuss the case of the zero impact parameter collision which results to the violation of conservation and we argue that this might be a signal for the formation of a black hole.

hep-th↗

arXiv2010

Heavy Ion Collisions with Transverse Dynamics from Evolving AdS Geometries

Currently there exists no known way to construct the Stress-Energy Tensor $(T_{\mu \nu})$ of the produced medium in heavy ion collisions at strong coupling from purely theoretical grounds. In this paper, some steps are taken in that direction. In particular, the evolution of $T_{\mu \nu}$ at strong coupling and at high energies is being studied for early proper times $(\tau)$. This is achieved in the context of the AdS/CFT duality by constructing the evolution of the dual geometry in an AdS$_5$ background. Improving the earlier works in the literature, the two incident nuclei have an impact parameter $b$ and a non-trivial transverse profile. The nuclear matter is modeled by two shock waves corresponding to a non-zero five dimensional bulk Stress-Energy Tensor $J_{MN}$. An analytic formula for $T_{\mu \nu}$ at small $\tau$ is derived and is used in order to calculate the momentum anisotropy and spatial eccentricity of the medium produced in the collision as a function of the ratio $\frac{\tau}{b}$. The result for eccentricity at intermediate $\frac{\tau}{b}$ agrees qualitatively with the results obtained in the context of perturbation theory and by using hydrodynamic simulations. Finally, the problem of the negative energy density and its natural connection to the eikonal approximation is discussed.

hep-th↗

arXiv2009

Deep Inelastic Scattering from the AdS/CFT correspondence

We calculate the cross section of an ultra relativistic nucleus scattering on a qq^(bar) pair at large coupling in N=4 SUSY gauge theory. We study the problem in the context of the AdS/CFT correspondence. The nucleus is modeled as a gravitational shockwave in an AdS_5 background moving along the light cone. The dipole qq^(bar) is represented by a Wilson loop moving in the opposite direction. Due to the correspondence, calculating the scattering amplitude of the Wilson loop with the nucleus reduces to calculating the extreme value of the Nambu-Goto action for an open string. Its two end points are attached to the qq^(bar) respectively and it hangs in an AdS_5 shockwave spacetime. Six solutions are found two of which are physically meaningful. Both solutions predict that the saturation scale Q_s at high enough energies becomes energy independent; in particular it behaves as Q_s A^{1/3} where A is the atomic number. One solution predicts pomeron intercept alpha_p=2. However, there is a parameter window of r (dipole size) and s (c.m. energy) where it violates the black disk limit. On the other hand, the other solution respects this limit and corresponds to pomeron intercept alpha_p=1.5. We conjecture that this is the right value for gauge theories at strong coupling.

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↗

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↗