arXiv subjects

Leila Shahkarami

Publications and source records attributed to Leila Shahkarami.

17 recordsLinked to original sources

arXiv2026

Thermodynamic scaling and string dynamics in rotating holographic QCD

We investigate the effect of finite angular velocity on the phase structure and Schwinger pair production in a holographic QCD model based on the Einstein--Maxwell--dilaton framework. Rotation is introduced through a boost construction that generates a stationary rotating geometry from the corresponding static background. We first study the thermodynamic phase diagram using the grand potential together with several response functions and show that all thermodynamic phase boundaries satisfy an exact scaling relation under rotation, implying that the rotating thermodynamic sector is completely determined by the corresponding static solution through a simple boost transformation. We then investigate confinement through the effective string tension and find that, consistent with the thermodynamic observables, rotation decreases the string confinement--deconfinement transition temperature and chemical potential. The corresponding phase boundary, however, no longer obeys the thermodynamic scaling relation. Consequently, the region in the phase diagram that is thermodynamically confined but string deconfined grows with angular velocity. We further study the Schwinger effect using the potential analysis approach. Rotation lowers both the confining and catastrophic critical electric fields and suppresses the height and width of the potential barrier, thereby enhancing pair production in both confined and deconfined phases. In the deconfined phase, rotation also generates a worldsheet horizon that limits the radial extent of connected string solutions and reduces both the maximum quark--antiquark separation and the deepest turning point of the energetically favored string. These results demonstrate that, while rotation acts trivially in the thermodynamic sector through an exact scaling law, it produces genuinely new effects in the string sector.

hep-th↗

arXiv2025

Schwinger effect in dynamical holographic QCD with a supercritical region

We study the phase structure of QCD matter using a dynamical Einstein--Maxwell--Dilaton holographic model, using both thermodynamic and dynamical observables. Depending on the warp factor, the model admits either a standard confinement/deconfinement transition or a first-order \textit{specious confinement}/deconfinement transition ending at a critical end point (CEP), giving rise to a rich phase diagram with a supercritical region. We probe this structure using both thermodynamic (heat capacity) and dynamical (squared speed of sound, IR wall) observables. We show that the loci of maxima in the heat capacity and minima in the sound speed define two distinct crossover lines emanating from the CEP and extending into the supercritical region, each tracing a different separation between confined-like and deconfined-like matter. As a dynamical probe, the persistence of the IR wall introduces another separation line, not emanating from the CEP, and reveals a triangular region, before the CEP, where the system is thermodynamically confined but dynamically deconfined. We further study the Schwinger effect as a nonperturbative probe of vacuum instability, determining the critical and threshold electric fields in both confined and deconfined phases. In the specious confined phase and in the confined-like phase beyond the critical end point, these fields depend on temperature and chemical potential, unlike in the standard confined phase, and we are able to trace their behavior for the first time including the supercritical region. Our results highlight the complementarity of thermodynamic and dynamical probes in mapping the QCD phase diagram and, in particular, establish the Schwinger threshold and critical fields as sensitive diagnostics of confinement not only in the known phase transitions but also in the supercritical regime.

hep-th↗

arXiv2023

Long-term Oscillations and Universal Behavior in Pulsed Electric Fields

We thoroughly analyze the response of the zero-temperature N=2 super Yang-Mills theory to time-dependent electric field quenches via holography. We specially focus on transient pulse-like configurations for the electric field, characterized by some model parameters, such as the maximum value of the electric field $E_0$ and the ramping time $δ_t$ which determines the time interval for switching the electric field on and off. We also compare some of the results with those of tanh-like quenches. The term tanh-like quench is used for a quench that rises from zero to a final finite value during a finite amount of time. Our numerical solutions demonstrate that when the system is subjected to pulse-like electric field quenches, the emerged electric current as a response goes through three stages as time passes. After excitation and rapidly-damping oscillatory stages, it experiences a long-lasting periodic oscillatory region. In fact the effect of the electric pulse remains in the system much longer than the duration of the presence of the electric field itself. It is extremely interesting that, as confirmed by power spectrum diagrams, these oscillations have a unique obvious frequency which is independent of the details of the electric pulse function and its parameters. Moreover, we observe a universal behavior in the adiabatic limit, when the ramping time tends to infinity. In this limit, the early and late time dynamics of the response electric current does not depend on the time dependence of the electric field. In particular, we see that for both pulse-like and tanh-like quenches, the maximum value at the first peak of the oscillations approaches the static value of the current induced by the presence of a static electric field $E_0$. However, the fast quench behavior differs extremely for different kinds of quench functions.

hep-th↗

arXiv2022

Probing inside a charged hairy black hole in massive gravity

In this paper, we investigate the internal structure of a charged hairy black hole solution in the non-linear massive gravity. We first consider the impact of various configurations of massive gravity on the condensate operator and then probe the black hole interior dynamics. Like a standard holographic superconductor system, just below the critical temperature, the interior evolves through several distinct epochs, including a collapse of the Einstein-Rosen bridge, Josephson oscillations of the scalar field, and finally a Kasner (or Kasner inversion) cosmology. However, for the large massive gravity parameter, we see distinguishing features for the interior dynamics. In this regime, at a given temperature, the Einstein-Rosen bridge collapse and subsequent Josephson oscillations epochs completely disappear from the interior dynamics and the final Kasner cosmology epoch starts exactly after the would-be inner horizon and the system does not experience the Kasner inversion epoch.

hep-th↗

arXiv2022

Charged Black Holes in Einsteinian Quartic Gravity

In this paper, we studied Einsteinian quartic gravity minimally coupled to electrodynamics in four dimensions. First, by variation action, we obtain the field equations, and by integration, we obtain a nonlinear third-order differential equation for the metric function and as well as the electromagnetic potential. Then, in the context of the Maxwell field, we discussed the conditions under which the charged black hole exists. Then, we have demonstrated the thermodynamics and stability of the solution for the case of positive coupling of quartic theory. Finally, we showed that the charged black hole solutions of EQG (unlike GR and like ECG) have no inner horizon and do not conform to the extremal bound of GR. Also, the uniqueness of BH solutions in this theory does not work anymore.

gr-qc↗

arXiv2021

Magnetized Einstein-Maxwell-dilaton model under external electric fields

We employ an analytic solution of a magnetized Einstein-Maxwell-dilaton gravity system whose parameters have been determined so that its holographic dual has the most similarity to a confining QCD-like theory influenced by a background magnetic field. Analyzing the total potential of a quark-antiquark pair in an external electric field, we are able to investigate the effect of the electric field on the different phases of the background which are the thermal AdS and the black hole phases. This is helpful for better understanding the confining character and also the phase transitions of the system. We find out that the field theory dual to the black hole solution is always deconfined. However, although the thermal AdS phase describes the confining phase in general, for the quark pairs parallel to $B$ (longitudinal case) and $B>B_{\mathrm{critical}}$ the response of the system to the electric field mimics the deconfinement. We moreover consider the effect of the magnetic field and the chemical potential on the Schwinger effect. We observe that when we are in the black hole phase with sufficiently small values of $μ$ or in the thermal AdS phase, and for both longitudinal and transverse cases, the magnetic field increase leads to the enhancement of the Schwinger effect, which can be termed as the inverse magnetic catalysis. This is deduced both from the decrease of the critical electric fields and from the decreasing the height and width of the total potential barrier that the quarks are facing with. However, by increasing $μ$ to high enough values, the inverse magnetic catalysis turns into magnetic catalysis, as can also be observed from the diagram of the Hawking-Page phase transition temperature versus $B$ for the background geometry itself.

hep-th↗

arXiv2020

Effect of electromagnetic fields on deformed AdS_5 models

The response of a QCD-like gauge theory, holographically dual to a deformed $\mathrm{AdS}_5$ model, to constant electromagnetic fields is thoroughly investigated. The calculations in this paper are performed for three different cases, i.e., with only a quadratic correction, with only a logarithmic correction, and with both quadratic and logarithmic corrections, for which the parameters are chosen as the ones found in \cite{quadlog} by fitting to experimental and lattice results. The critical electric fields of the system are found by analyzing its total potential. Comparing the total potential for the three cases, we observe that the quarks can be liberated easier in quadratic and then logarithmic case, for a given electric field. Then, by calculating the expectation value of a circular Wilson loop, the pair production rate is evaluated while a constant electric field as well as a constant magnetic field are present. The aforementioned result obtained from the potential analysis is also confirmed here when no magnetic fields are present. We moreover find that the presence of a magnetic field perpendicular to the direction of the electric field suppresses the rate of producing the quark pairs and accordingly increases the critical electric field below which the Schwinger effect does not occur. Interestingly, the presence of a parallel magnetic field alone does not change the response of the system to the external electric field, although it enhances the creation rate when a perpendicular magnetic field is also present.

hep-th↗

arXiv2019

Confining D-Instanton Background in an External Electric Field

Using holography, we discuss the effects of an external static electric field on the D3/D-instanton theory at zero-temperature, which is a quasi-confining theory, with confined quarks and deconfined gluons. We introduce the quarks to the theory by embedding a probe D7-brane in the gravity side, and turn on an appropriate $U(1)$ gauge field on the flavor brane to describe the electric field. Studying the embedding of the D7-brane for different values of the electric field, instanton density and quark masses, we thoroughly explore the possible phases of the system. We find two critical points in our considerations. We show that beside the usual critical electric field present in deconfined theories, there exists another critical field, with smaller value, below which no quark pairs even the ones with zero mass are produced and thus the electric current is zero in this (insulator) phase. At the same point, the chiral symmetry, spontaneously broken due to the gluon condensate, is restored which shows a first order phase transition. Finally, we obtain the full decay rate calculating the imaginary part of the DBI action of the probe brane and find that it becomes nonzero only when the critical value of the electric field is reached.

hep-th↗

arXiv2018

Modified gravity one-loop partition function

The one-loop partition function of the $f(R,R_{μν}R^{μν})$ gravity theory is obtained around AdS$_4$ background. After 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 AdS$_4$ space is also computed and the thermal partition function is obtained for this theory. We have then consider quantum corrections to the thermodynamical quantities in some special cases.

gr-qc↗

arXiv2018

Holographic Schwinger Effect in a D-Instanton Background

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.

hep-th↗

arXiv2015

One-loop corrections to vector Galileon theory

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 non-renormalization 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.

hep-th↗

arXiv2014

Vacuum Polarization and Casimir Energy of a Dirac Field Induced by a Scalar Potential in One Spatial Dimension

We investigate the vacuum polarization and the Casimir energy of a Dirac field coupled to a scalar potential in one spatial dimension. Both of these effects have a common cause which is the distortion of the spectrum due to the coupling with the background field. Choosing the potential to be a symmetrical square-well, the problem becomes exactly solvable and we can find the whole spectrum of the system, analytically. We show that the total number of states and the total density remain unchanged as compared with the free case, as one expects. Furthermore, since the positive- and negative-energy eigenstates of the fermion are fermion-number conjugates of each other and there is no zero-energy bound state, the total density and the total number of negative and positive states remain unchanged, separately. Therefore, the vacuum polarization in this model is zero for any choice of the parameters of the potential. It is important to note that although the vacuum polarization is zero due to the symmetries of the model, the Casimir energy of the system is not zero in general. In the graph of the Casimir energy as a function of the depth of the well there is a maximum approximately when the bound energy levels change direction and move back towards their continuum of origin. The Casimir energy for a fixed value of the depth is a linear function of the width and is always positive. Moreover, the Casimir energy density (the energy density of all the negative-energy states) and the energy density of all the positive-energy states are exactly the mirror images of each other. Finally, computing the total energy of a valence fermion present in the lowest fermionic bound state, taking into account the Casimir energy, we find that the lowest bound state is almost always unstable for the scalar potential.

hep-th↗

arXiv2014

An Investigation of the Casimir Energy for a Fermion Coupled to the Sine-Gordon Soliton with Parity Decomposition

We consider a fermion chirally coupled to a prescribed pseudoscalar field in the form of the soliton of the sine-Gordon model and calculate and investigate the Casimir energy and all of the relevant quantities for each parity channel, separately. We present and use a simple prescription to construct the simultaneous eigenstates of the Hamiltonian and parity in the continua from the scattering states. We also use a prescription we had introduced earlier to calculate unique expressions for the phase shifts and check their consistency with both the weak and strong forms of the Levinson theorem. In the graphs of the total and parity decomposed Casimir energies as a function of the parameters of the pseudoscalar field distinctive deformations appear whenever a fermionic bound state energy level with definite parity crosses the line of zero energy. However, the latter graphs reveal some properties of the system which cannot be seen from the graph of the total Casimir energy. Finally we consider a system consisting of a valence fermion in the ground state and find that the most energetically favorable configuration is the one with a soliton of winding number one, and this conclusion does not hold for each parity, separately.

hep-th↗

arXiv2013

Exact Solutions of a Fermion-Soliton System in Two Dimensions

We investigate a coupled system of a Dirac particle and a pseudoscalar field in the form of a soliton in (1+1) dimensions and find some of its exact solutions numerically. We solve the coupled set of equations self-consistently and non-perturbatively by the use of a numerical method and obtain the bound states of the fermion and the shape of the soliton. That is the shape of the static soliton in this problem is not prescribed and is determined by the equations themselves. This work goes beyond the perturbation theory in which the back reaction of the fermion on soliton is its first order correction. We compare our results to those of an exactly solvable model in which the soliton is prescribed. We show that, as expected, the total energy of our system is lower than the prescribed one. We also compute non-perturbatively the vacuum polarization of the fermion induced by the presence of the soliton and display the results. Moreover, we compute the soliton mass as a function of the boson and fermion masses and find that the results are consistent with Skyrme's phenomenological conjecture. Finally, we show that for fixed values of the parameters, the shape of the soliton obtained from our exact solutions depends slightly on the fermionic state to which it is coupled. However, the exact shape of the soliton is always very close to the isolated kink.

hep-th↗

arXiv2012

Non-adiabatic Non-cyclic Generalization of the Berry Phase for a Spin-1/2 Particle in a Rotating Magnetic Field

In this paper we define a non-dynamical phase for a spin-1/2 particle in a rotating magnetic field in the non-adiabatic non-cyclic case, and this phase can be considered as a generalized Berry phase. We show that this phase reduces to the geometric Berry phase, in the adiabatic limit, up to a factor independent of the parameters of the system. We could add an arbitrary phase to the eigenstates of the Hamiltonian due to the gauge freedom. Then, we fix this arbitrary phase by comparing our Berry phase in the adiabatic limit with the Berry's result for the same system. Also, in the extreme non-adiabatic limit our Berry phase vanishes, modulo $2π$, as expected. Although, our Berry phase is in general complex, it becomes real in the expected cases: the adiabatic limit, the extreme non-adiabatic limit, and the points at which the state of the system returns to its initial form, up to a phase factor. Therefore, this phase can be considered as a generalization of the Berry phase. Moreover, we investigate the relation between the value of the generalized Berry phase, the period of the states and the period of the Hamiltonian.

quant-ph↗

arXiv2012

Casimir Energy for a Coupled Fermion-Soliton System

In this paper we compute the Casimir energy for a coupled fermion-pseudoscalar field system. In the model considered in this paper the pseudoscalar field is \textit{static} and \textit{prescribed} with two adjustable parameters. These parameters determine the values of the field at infinity ($\pm θ_0$) and its scale of variation ($μ$). One can build up a field configuration with arbitrary topological charge by changing $θ_0$, and interpolate between the extreme adiabatic and non-adiabatic regimes by changing $μ$. This system is exactly solvable and therefore we compute the Casimir energy exactly and unambiguously by using an energy density subtraction scheme. We show that in general the Casimir energy goes to zero in the extreme adiabatic limit, and in the extreme non-adiabatic limit when the asymptotic values of the pseudoscalar field properly correspond to a configuration with an arbitrary topological charge. Moreover, in general the Casimir energy is always positive and on the average an increasing function of $θ_0$ and always has local maxima when there is a zero mode, showing that these configurations are energetically unfavorable. We also compute and display the energy densities associated with the spectral deficiencies in both of the continua, and those of the bound states. We show that the energy densities associated with the distortion of the spectrum of the states with $E>0$ and $E<0$ are mirror images of each other. We also compute and display the Casimir energy density. Finally we compute the energy of a system consisting of a soliton and a valance electron and show that the Casimir energy of the system is comparable with the binding energy.

hep-th↗

arXiv2012

Casimir Energy for a Coupled Fermion-Kink System and its stability

We compute the Casimir energy for a system consisting of a fermion and a pseudoscalar field in the form of a prescribed kink. This model is not exactly solvable and we use the phase shift method to compute the Casimir energy. We use the relaxation method to find the bound states and the Runge-Kutta-Fehlberg method to obtain the scattering wavefunctions of the fermion in the whole interval of $x$. The resulting phase shifts are consistent with the weak and strong forms of the Levinson theorem. Then, we compute and plot the Casimir energy as a function of the parameters of the pseudoscalar field, i.e. the slope of $ϕ(x)$ at x=0 ($μ$) and the value of $ϕ(x)$ at infinity ($θ_0$). In the graph of the Casimir energy as a function of $μ$ there is a sharp maximum occurring when the fermion bound state energy crosses the line of E=0. Furthermore, this graph shows that the Casimir energy goes to zero for $μ\rightarrow 0$, and also for $μ\rightarrow \infty$ when $θ_0$ is an integer multiple of $π$. Moreover, the graph of the Casimir energy as a function of $θ_0$ shows that this energy is on the average an increasing function of $θ_0$ and has a cusp whenever there is a zero fermionic mode. We finally compute the total energy of a system consisting of a valence fermion in the ground state. Most importantly, we show that this energy (the sum of the Casimir energy and the energy of the fermion) is minimum when the background field has winding number one, independent of the details of the background profile. Throughout the paper we compare our results with those of a simple exactly solvable model, where a piece-wise linear profile approximates the kink. We find that the kink is an almost reflectionless barrier for the fermions, within the context of our model.

hep-th↗