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Email: international@du.ac.ir

Damghan University

University Blvd, Damghan, IR

Leila Shahkarami

Assistant Professor of Physics

DOI: 10.1140/epjc/s10052-018-6318-7

The one-loop partition function of the f(R, Rμ νRμ ν) gravity theory is obtained around AdS 4 background. After a suitable choice of the gauge condition and computation of the ghost determinant, we obtain the one-loop partition function of the theory. The traced heat kernel over the thermal quotient of the AdS 4 space is also computed and the thermal partition function is obtained for this theory. We then consider quantum corrections to the thermodynamical quantities in some special cases. © 2018, The Author(s).

PUBLISHER: Springer New York LLC

DOI: 10.1103/PhysRevD.97.046013

The Schwinger effect in the presence of instantons is considered in this paper. Using AdS/CFT correspondence in the near horizon limit of the D3+D(-1)-brane background, we calculate the total potential of a quark-antiquark pair in an external electric field. It is shown that instantons tend to suppress the pair creation effect and increase the critical electric field above which the pairs are produced freely without any suppression. Interestingly, no other critical electric field, common for all confining field theories, is observed here at finite temperature. However, as expected we find such a critical electric field at zero temperature. The pair production rate evaluated by the calculation of the expectation value of the circular Wilson loop also confirms this result. © 2018 authors. Published by the American Physical Society.

PUBLISHER: American Physical Society

DOI: 10.1016/j.physletb.2017.08.016

Using the gauge/gravity duality, we investigate the evolution of an out-of-equilibrium strongly-coupled plasma from the viewpoint of the two-point function of scalar gauge-invariant operators with large conformal dimension. This system is out of equilibrium due to the presence of anisotropy and/or a massive scalar field. Considering various functions for the initial anisotropy and scalar field, we conclude that the effect of the anisotropy on the evolution of the two-point function is considerably more than the effect of the scalar field. We also show that the ordering of the equilibration time of the one-point function for the non-probe scalar field and the correlation function between two points with a fixed separation can be reversed by changing the initial configuration of the plasma, when the system is out of the equilibrium due to the presence of at least two different sources like our problem. In addition, we find the equilibration time of the two-point function to be linearly increasing with respect to the separation of the two points with a fixed slope, regardless of the initial configuration that we start with. Finally we observe that, for larger separations the geodesic connecting two points on the boundary crosses the event horizon after it has reached its final equilibrium value, meaning that the two-point function can probe behind the event horizon. © 2017

PUBLISHER: Elsevier B.V.

DOI: 10.1103/PhysRevD.94.046008

Via gauge-gravity duality, relaxation of far-from-equilibrium initial states in a strongly coupled gauge theory has been investigated. In the system we consider in this paper there are two ways where the state under study can deviate from its equilibrium: anisotropic pressure and time-dependent expectation value of a scalar operator with Δ=3. In the gravity theory, this system corresponds to Einstein's general relativity with a nontrivial metric, including the anisotropy function, coupled to a massive scalar matter field. We study the effect of different initial configurations for the scalar field and anisotropy function on physical processes such as thermalization, i.e., time evolution of an event horizon; equilibration of the expectation value of a scalar operator; and isotropization. We also discuss time ordering of these time-scales. © 2016 American Physical Society.

PUBLISHER: American Physical Society

DOI: 10.1103/PhysRevD.93.124044

The effective action of the recently proposed vector Galileon theory is considered. Using the background field method, we obtain the one-loop correction to the propagator of the Proca field from vector Galileon self-interactions. Contrary to the so-called scalar Galileon interactions, the two-point function of the vector field gets renormalized at the one-loop level, indicating that there is no nonrenormalization theorem in the vector Galileon theory. Using dimensional regularization, we remove the divergences and obtain the counterterms of the theory. The finite term is analytically calculated, which modifies the propagator and the mass term and generates some new terms also. © 2016 American Physical Society.

PUBLISHER: American Physical Society

DOI: 10.1103/PhysRevD.93.086005

Using holography we study the evolution of the Wilson loop of a quark-antiquark pair in a dynamical strongly coupled plasma. The time-dependent plasma, whose dynamics is originated from the energy injection, is dual to the anti-de Sitter-Vaidya background. The quark-antiquark pair is represented by the endpoints of a string stretched from the boundary to the bulk. The evolution of the system is studied by evaluating the expectation value of the Wilson loop, throughout the process. Our results show that the evolution of the Wilson loop depends on the speed of the energy injection as well as the final temperature of the plasma. For high enough temperatures and rapid energy injection, it starts oscillating around its equilibrium value, immediately after the injection. © 2016 American Physical Society.

PUBLISHER: American Physical Society

DOI: 10.1103/PhysRevD.91.106008

The dynamics of a probe D7-brane in an asymptotically anti-de Sitter-Vaidya background has been investigated in the presence of an external magnetic field. Holographically, this is dual to the dynamical meson melting in the N=2 super Yang-Mills theory. If the final temperature of the system is large enough, the probe D7-brane will dynamically cross the horizon (the black hole embedding). By turning on the external magnetic field and raising it sufficiently, the final embedding of the corresponding D7-brane changes to a Minkowski embedding. On the field theory side, this means that the mesons which melt due to the raise in the temperature will form bound states again by applying an external magnetic field. We also show that the evolution of the system to its final equilibrium state is postponed due to the presence of the magnetic field. © 2015 American Physical Society.

PUBLISHER: American Physical Society