arXiv subjects

Hajar Ebrahim

Publications and source records attributed to Hajar Ebrahim.

12 recordsLinked to original sources

arXiv2023

Holographic entanglement entropy and mutual information in deformed field theories at finite temperature

In this paper we have studied the effect of deformation and temperature on holographic entanglement entropy and mutual information between two subsystems in a deformed field theory at finite temperature. The $T{\overline{T}}$ deformation operator in field theory introduces a cut-off in the dual gravity background. We define different regimes of parameters and calculate the entanglement entropy and mutual information analytically. We observe that temperature and deformation parameter have similar effects on entanglement entropy and mutual information. They both lead to decrease in mutual information or somehow the correlation between the two subsystems. Interestingly we observe the emergence of correction terms in entanglement entropy and mutual information that are universal and do not depend on the scale of the non-local operator.

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arXiv2020

Holographic Mutual Information and Critical Exponents of the Strongly Coupled Plasma

This note contains discussions on the entanglement entropy and mutual information of a strongly coupled field theory with a critical point which has a holographic dual. We investigate analytically, in the specific regimes of parameters, how these non-local operators behave near the critical point. Interestingly, we observe that although the mutual information is constant at the critical point, its slope shows a power-law divergence in the vicinity of the critical point. We show that the leading behavior of mutual information at and near the critical point could yield a set of critical exponents if we regard it as an order parameter. Our result for this set of static critical exponents is (1/2,1/2,1/2,2) which is identical to the one calculated via the thermodynamic quantities. Hence it suggests that beside the numerous merits of mutual information, this quantity also captures the critical behavior of the underlying field theory and it could be used as a proper measure to probe the phase structure associated with the strongly coupled systems.

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arXiv2018

Dynamically probing strongly-coupled field theories with critical point

The dependence of the rescaled equilibration time on different parameters of the field theories with a holographic dual has been investigated in this paper. We consider field theories with nonzero chemical potential at finite temperature which are dual to asymptotically AdS charged black holes. We examine a dynamical probe scalar operator where its dynamics is due to a time-dependent source, quantum quench, or out-of-equilibrium initial condition in field theory with fixed or varying temperature and chemical potential. We observe that the behavior of the scalar operator equilibration time with respect to temperature or chemical potential can not be predicted merely by field theory parameters and depends on how fast the energy is injected into the system. It is shown that in field theories with critical point the rescaled equilibration time is shorter for thermodynamically stable systems. We also observe that the rescaled equilibration time as one approaches the critical point enhances and acquires an infinite slope though its value remains finite. We show that for fast quenches, even though the system is far from equilibrium, the dynamical critical exponent is the same as the one reported for quasi-normal modes in the same background. However for slow quenches the dynamical critical exponent picks up a different value.

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arXiv2014

Chiral Symmetry Breaking: To Probe Anisotropy and Magnetic Field in QGP

We discuss the (spontaneous) chiral symmetry breaking in a strongly coupled anisotropic quark-gluon plasma (QGP) in the presence of the magnetic field, using holography. The physical quantities related to the chiral symmetry breaking (m;B_c) distinguish between the effects of the anisotropy and magnetic field on the plasma. Anisotropy affects the system similar to the temperature and for its larger values heavier quarks can live in the QGP without getting condensed. Raising the anisotropy in the system will also increase the value of the critical magnetic field, B_c, at which the spontaneous chiral symmetry breaking happens. Both of these growths are even more when the magnetic field is applied perpendicular to the anisotropy direction. Such behaviour persists in the high temperature limit where the temperature is kept fixed. However, when the entropy density is held fixed, as one increases the anisotropy, lighter mesons melt when the magnetic field is applied along the anisotropy direction, in contrast to when the magnetic field is perpendicular to the anisotropy direction.

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arXiv2013

Thermalization in External Magnetic Field

In the AdS/CFT framework meson thermalization in the presence of a constant external magnetic field in a strongly coupled gauge theory has been studied. In the gravitational description the thermalization of mesons corresponds to the horizon formation on the flavour D7-brane which is embedded in the AdS_5 x S^5 background in the probe limit. The apparent horizon forms due to the time-dependent change in the baryon number chemical potential, the injection of baryons in the gauge theory. We will numerically show that the thermalization happens even faster in the presence of the magnetic field on the probe brane. We observe that this reduction in the thermalization time sustains up to a specific value of the magnetic field.

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arXiv2012

Meson Thermalization in Various Dimensions

In gauge/gravity duality framework the thermalization of mesons in strongly coupled (p+1)-dimensional gauge theories is studied for a general Dp-Dq system, q>=p, using the flavour Dq-brane as a probe. Thermalization corresponds to the horizon formation on the flavour Dq-brane. We calculate the thermalization time-scale due to a time-dependent change in the baryon number chemical potential, baryon injection in the field theory. We observe that for such a general system it has a universal behaviour depending only on the t'Hooft coupling constant and the two parameters which describe how we inject baryons into the system. We show that this universal behaviour is independent of the details of the theory whether it is conformal and/or supersymmetric.

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arXiv2011

Instantaneous Thermalization in Holographic Plasmas

Thin-shell AdS-Vaidya spacetimes can be considered as holographic models of the thermalization process in strongly-coupled conformal field theories following a rapid injection of energy from an external source. While the expected thermalization time is the inverse temperature, Bhattacharyya and Minwalla have pointed out that bulk causality implies that expectation values of local field-theory observables actually take on their thermal values immediately following the injection. In this paper we study two-point functions, for which the causality argument does not apply. Specifically, we study the Brownian motion of a "quark" represented by a string stretching from the boundary to the horizon of an AdS_3-Vaidya spacetime. Surprisingly, we find that the two-point function also thermalizes instantly. Since Brownian motion is a 1/N effect, our result shows that, at least in certain cases, the rapid thermalization property of holographic plasmas persists beyond leading order in 1/N.

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arXiv2008

On 5D Small Black Holes

Using higher order corrections we argue that five dimensional N=2 and N=4 small black holes exhibit supersymmetry enhancement in near horizon geometry leading to eight and sixteen supercharges, respectively. Using this enhancement at supergravity level we can identify the global supergroup of the near horizon geometry. In particular we show how this supergroup distinguishes between small and large black holes in N=2 case.

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arXiv2006

New Attractors, Entropy Function and Black Hole Partition Function

By making use of the entropy function formalism we study the generalized attractor equations in the four dimensional N=2 supergravity in presence of higher order corrections. This result might be used to understand a possible ensemble one could associate to an extremal black hole.

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arXiv2006

Semiclassical Strings Probing NS5 Brane Wrapped on S^5

We study little string theory on R^1 x S^5, defined by a theory which lives on type IIA N NS5 branes wrapped on S^5, using its supergravity dual. In particular we study semiclassical rotating closed strings in this background. We also consider Penrose limit of this background that leads to a plane wave on which string theory is exactly solvable.

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arXiv2006

Non-Supersymmetric Attractors and Entropy Function

We study the entropy of non-supersymmetric extremal black holes which exhibit attractor mechanism by making use of the entropy function. This method, being simple, can be used to calculate corrections to the entropy due to higher order corrections to the action. In particular we apply this method for five dimensional non-supersymmetric extremal black hole which carries two magnetic charges and find the R^2 corrections to the entropy. Using the behavior of the action evaluated for the extremal black hole near the horizon, we also present a simple expression for C-function corrected by higher order corrections.

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arXiv2005

Semiclassical String Solutions on 1/2 BPS Geometries

We study semiclassical string solutions on the 1/2 BPS geometry of type IIB string theory characterized by concentric rings on the boundary plane. We consider both folded rotating strings carrying nonzero R-charge and circular pulsating strings. We find that unlike rotating strings, as far as circular pulsating strings are concerned, the dynamics remains qualitatively unchanged when the concentric rings replace AdS_5\times S^5. Using the gravity dual we have also studied the Wilson loop of the corresponding gauge theory. The result is qualitatively the same as that in AdS_5\times S^5 in the global coordinates where the corresponding gauge theory is defined on S^3\times R. We show that there is a correction to 1/L leading order behavior of the potential between external objects.

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