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.

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.

Modified gravity one-loop partition function

The one-loop partition function of the $f(R,R_{mu u}R^{mu u})$ 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.

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.

Massive Jackiw-Rebbi Model

In this paper we analyze a generalized Jackiw-Rebbi (J-R) model in which a massive fermion is coupled to the kink of the $lambdaphi^4$ model as a prescribed background field. We solve this massive J-R model exactly and analytically and obtain the whole spectrum of the fermion, including the bound and continuum states. The mass term of the fermion makes the potential of the decoupled second order Schrodinger-like equations asymmetric in a way that their asymptotic values at two spatial infinities are different. Therefore, we encounter the unusual problem in which two kinds of continuum states are possible for the fermion: reflecting and scattering states. We then show the energies of all the states as a function of the parameters of the kink, i.e. its value at spatial infinity ($theta_0$) and its slope at $x=0$ ($mu$). The graph of the energies as a function of $theta_0$, where the bound state energies and the two kinds of continuum states are depicted, shows peculiar features including an energy gap in the form of a triangle where no bound states exist. That is the zero mode exists only for $theta_0$ larger than a critical value $(theta_0^{textrm{c}})$. This is in sharp contrast to the usual (massless) J-R model where the zero mode and hence the fermion number $pm1/2$ for the ground state is ever present. This also makes the origin of the zero mode very clear: It is formed from the union of the two threshold bound states at $theta_0^{textrm{c}}$, which is zero in the massless J-R model.

A Complete Spectral Analysis of the Jackiw-Rebbi Model, Including its Zero Mode

In this paper we present a complete and exact spectral analysis of the $(1+1)$-dimensional model that Jackiw and Rebbi considered to show that the half-integral fermion numbers are possible due to the presence of an isolated self charge conjugate zero mode. The model possesses the charge and particle conjugation symmetries. These symmetries mandate the reflection symmetry of the spectrum about the line $E=0$. We obtain the bound state energies and wave functions of the fermion in this model using two different methods, analytically and exactly, for every arbitrary choice of the parameters of the kink, i.e. its value at spatial infinity ($theta_0$) and its scale of variations ($mu$). Then, we plot the bound state energies of the fermion as a function of $theta_0$. This graph enables us to consider a process of building up the kink from the trivial vacuum. We can then determine the origin and evolution of the bound state energy levels during this process. We see that the model has a dynamical mass generation process at the first quantized level and the zero-energy fermionic mode responsible for the fractional fermion number, is always present during the construction of the kink and its origin is very peculiar, indeed. We also observe that, as expected, none of the energy levels crosses each other. Moreover, we obtain analytically the continuum scattering wave functions of the fermion and then calculate the phase shifts of these wave functions. Using the information contained in the graphs of the phase shifts and the bound states, we show that our phase shifts are consistent with the weak and strong forms of the Levinson theorem. Finally, using the weak form of the Levinson theorem, we confirm that the number of the zero-energy fermionic modes is exactly one.

One-loop quantum correction to the mass of the supersymmetric Kink in (1 + 1) dimensions using the exact spectra and the phase shifts

We compute the quantum correction to the mass of the kink at the one-loop level in (1+1) dimensions with minimal supersymmetry. In this paper we discuss this issue from the Casimir energy perspective using phase shifts along with the mode number cut-off regularization method. Exact solutions and in particular an exact expression for the phase shifts are already available for the bosonic sector. In this paper we derive analogous exact results for the fermionic sector. Most importantly, we derive a unique and exact expression for the fermionic phase shift, using the exact solutions for the continuum parts of the spectrum and a prescription we had introduced earlier. We use the strong and weak forms of the Levinson theorem merely for checking the consistency of our phase shifts and results, and not as an integral part of our procedure. Moreover, we find that the properties of the fermionic spectrum, including bound and continuum states, are independent of the magnitude of the Yukawa coupling constant $lambda$, and that the dynamical mass generation occurs at the tree level. These are all due to SUSY and are in sharp contrast to analogous models without SUSY, such as the Jackiw-Rebbi model, where $lambda$ is a free parameter. We use the renormalized perturbation theory and find the counterterm which is consistent with supersymmetry. We show that this procedure is sufficient to obtain the accepted value for the one-loop quantum correction to the mass of the SUSY kink which is $-frac{m}{2pi}$.

A Renormalization Program for Systems with Non-Perturbative Conditions

In this paper we introduce an alternative renormalization program for systems with non-perturbative conditions. The non-perturbative conditions that we concentrate on in this paper are confined to be either the presence of non-trivial boundary conditions or non-perturbative background fields. We show that these non-perturbative conditions have profound effects on all physical properties of the system and our renormalization program is consistent with these conditions. We formulate the general renormalization program in the configuration space. The differences between the free space renormalization program and ours manifest themselves in the counter-terms as well, which we shall elucidate. The general expressions that we obtain for the counter-terms reduce to the standard results in the free space cases. We show that the differences between these divergent counter-terms are extremely small. Moreover we argue that the position dependences induced on the parameters of the renormalized Lagrangian via the loop corrections, however small, are direct and natural consequences of the non-perturbative position dependent conditions imposed on the system.