arXiv subjects

Amin Akhavan

Publications and source records attributed to Amin Akhavan.

13 recordsLinked to original sources

arXiv2025

Applying Constraints from Outside the System

We investigate the role of external constraints in quantum field theory using the path integral formalism. We begin by reviewing the quantization of constrained systems and extend the analysis to cases where constraints are added to the action via auxiliary fields. These constraints involve both the degrees of freedom and their time derivatives. Using the resulting framework, we derive the generating functional for matter vector fields constrained to the spin-one state. However, for systems with singular actions, preserving the classical form of constraints at the quantum level requires conditions, and we provide examples of sufficient conditions. We also present an example where the classical form of constraints is not preserved, involving a singular action with an antisymmetric tensor field leading to a strength-field interpretation.

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arXiv2023

The Constrained State of Minimum Energy and the Effective Equation of Motion

We define the state of minimum energy while the expectation values of the field operators and their time derivatives in a determined moment in such a state are constrained. As an axiom, we consider such a state as the background of the quantum field theory. As an example, we consider the scalar field with {\lambda}/4!{\Phi}4 interaction. To the third order of perturbation, we obtain the equation of motion of the dynamic expectation value of the scalar field in the defined state.

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arXiv2021

The minimum energy state

We define the minimum energy state while the expectation value of the field, evolves in time. We obtain the relation between the n-point functions in such a state, and the external field for all the moments. We obtain an equation of motion and the renormalization counterterms for the external field in the first order of interaction.

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arXiv2020

An Infalling Observer and Behind the Horizon Cutoff

Using Papadodimas and Raju construction of operators describing the interior of a black hole, we present a general relation between partition functions of operators describing inside and outside the black hole horizon. In particular for an eternal black hole the partition function of the interior modes may be given in terms those partition functions associated with the modes of left and right exteriors. By making use of this relation we observe that setting a finite UV cutoff will enforce us to have a cutoff behind the horizon whose value is fixed by the UV cutoff. The resultant cutoff is in agreement with what obtained in the context of holographic complexity.

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arXiv2019

On the Role of Counterterms in Holographic Complexity

We consider the Complexity=Action (CA) proposal in Einstein gravity and investigate new counterterms which are able to remove all the UV divergences of holographic complexity. We first show that the two different methods for regularizing the gravitational on-shell action proposed in Ref. \cite{Carmi:2016wjl} are completely equivalent, provided that one considers the Gibbons-Hawking-York term as well as new counterterms inspired from holographic renormalization on timelike boundaries of the WDW patch. Next, we introduce new counterterms on the null boundaries of the WDW patch for four and five dimensional asymptotically AdS spacetimes which are able to remove all the UV divergences of the on-shell action. Moreover, they are covariant and do not change the equations of motion. At the end, by applying the null counterterms, we calculate the holographic complexity of an AdS-Schwarzschild black hole as well as the complexity of formation. We show that the null counterterms do not change the complexity of formation.

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arXiv2018

Complexity and Behind the Horizon Cut Off

Motivated by $T{\overline T}$ deformation of a conformal field theory we compute holographic complexity for a black brane solution with a cut off using "complexity=action" proposal. In order to have a late time behavior consistent with Lloyd's bound one is forced to have a cut off behind the horizon whose value is fixed by the boundary cut off. Using this result we compute holographic complexity for two dimensional AdS solutions where we get expected late times linear growth. It is in contrast with the naively computation which is done without assuming the cut off where the complexity approaches a constant at the late time.

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arXiv2017

New Bi-Gravities

We show that the problem of ghosts in critical gravity and its higher dimensional extensions can be resolved by giving dynamics to the symmetric rank two auxiliary field existing in the action of these theories. These New Bi-Gravities, at linear level around the AdS vacuum, are free of Boulware-Deser ghost, kinetic ghost and tachyonic instability within the particular range of parameters. Moreover, we show that the energy and entropy of AdS-Schwarzschild black hole solutions of these new models are positive in the same range of parameters. This may be the sign that these new models are also free of ghost instabilities at the non-linear level.

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arXiv2016

New Bi-Gravity from New Massive Garvity

Using the action of three dimensional New Massive Gravity (NMG) we construct a new bi-gravity in three dimensions. This can be done by promoting the rank two auxiliary field appearing in the expression of NMG's action into a dynamical field. We show that small fluctuations around the AdS vacuum of the model are non-tachyonic and ghost free within certain range of the parameters of the model. We study central charges of the dual field theory and observe that in this range they are positive too. This suggests that the proposed model might be a consistent three dimensional bi-gravity.

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arXiv2015

Gauge Symmetry Breaking in Gravity and Auxiliary Effective Action

At first, we consider the path integral method for the covariant symmetry breaking in gravity. We replace the scalar fields, instead of the degrees of freedom which have been removed by gauge fixing constraints. Finally the specific ghost degree of freedom, remains excluded via one of the constraints. Secondly, we define an auxiliary effective action. Entering an auxiliary field, we will have a new dynamic field separated from the fundamental field.

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arXiv2014

The Scalar Field Effective Action for The Spontaneous Symmetry Breaking in Gravity

We calculate the quantum effective action for a scalar field which has been recently used for a specific kind of symmetry breaking in gravity. Our study consists of calculating the 1-loop path integral of canonical momentum and determining the renormalization conditions. We will also discuss on the new renormalization conditions to redefine the new degrees of freedom corresponding to a massive vector field.

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arXiv2011

P-Wave Holographic Insulator/Superconductor Phase Transition

Using a five dimensional AdS soliton in an Einstein-Yang-Mills theory with SU(2) gauge group we study p-wave holographic insulator/superconductor phase transition. To explore the phase structure of the model we consider the system in the probe limit as well as fully back reacted solutions. We will also study zero temperature limit of the p-wave holographic superconductor in four dimensions.

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arXiv2009

Fermions in non-relativistic AdS/CFT correspondence

We extend the non-relativistic AdS/CFT correspondence to the fermionic fields. In particular we study the two point function of a fermionic operator in non-relativistic CFTs by making use of a massive fermion propagating in geometries with Schrodinger group isometry. Although the boundary of the geometries with Schrodinger group isometry differ from that in AdS geometries where the dictionary of AdS/CFT is established, using the general procedure of AdS/CFT correspondence, we see that the resultant two point function has the expected form for fermionic operators in non-relativistic CFTs, though a non-trivial regularization may be needed.

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arXiv2008

Non-relativistic CFT and Semi-classical Strings

We study different features of 3D non-relativistic CFT using gravity description. As the corresponding gravity solution can be embedded into the type IIB string theory, we study semi-classical closed/open strings in this background. In particular we consider folded rotating and circular pulsating closed strings where we find the anomalous dimension of the dual operators as a function of their quantum numbers. We also consider moving open strings in this background which can be used to compute the drag force. In particular we find that for slowly moving particles, the energy is lost exponentially and the characteristic time is given in terms of the temperature, while for fast moving particles the energy loss goes as inverse of the time and the characteristic time is independent of the temperature.

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