arXiv subjects

K. Farakos

Publications and source records attributed to K. Farakos.

At least 37 records · Page 2Linked to original sources

arXiv2007

Neutrino oscillations in a stochastic model for space-time foam

We study decoherence models for flavour oscillations in four-dimensional stochastically fluctuating space times and discuss briefly the sensitivity of current neutrino experiments to such models. We pay emphasis on demonstrating the model dependence of the associated decoherence-induced damping coefficients in front of the oscillatory terms in the respective transition probabilities between flavours. Within the context of specific models of foam, involving point-like D-branes and leading to decoherence-induced damping which is inversely proportional to the neutrino energies, we also argue that future limits on the relevant decoherence parameters coming from TeV astrophysical neutrinos, to be observed in ICE-CUBE, are not far from theoretically expected values with Planck mass suppression. Ultra high energy neutrinos from Gamma Ray Bursts at cosmological distances can also exhibit in principle sensitivity to such effects.

hep-ph↗

arXiv2007

Graviton localization and Newton's law for brane models with a non-minimally coupled bulk scalar field

Brane world models with a non-minimally coupled bulk scalar field have been studied recently. In this paper we consider metric fluctuations around an arbitrary gravity-scalar background solution, and we show that the corresponding spectrum includes a localized zero mode which strongly depends on the profile of the background scalar field. For a special class of solutions, with a warp factor of the RS form, we solve the linearized Einstein equations, for a point-like mass source on the brane, by using the brane bending formalism. We see that general relativity on the brane is recovered only if we impose restrictions on the parameter space of the models under consideration.

hep-th↗

arXiv2007

Gauss-Bonnet gravity, brane world models, and non-minimal coupling

We study the case of brane world models with an additional Gauss-Bonnet term in the presence of a bulk scalar field which interacts non-minimally with gravity, via a possible interaction term of the form $-1/2 ξR ϕ^2$. The Einstein equations and the junction conditions on the brane are formulated, in the case of the bulk scalar field. Static solutions of this model are obtained by solving numerically the Einstein equations with the appropriate boundary conditions on the brane. Finally, we present graphically and comment these solutions for several values of the free parameters of the model.

hep-th↗

arXiv2006

Brane world scenario in the presence of a non-minimally coupled bulk scalar field

We present our recent work on brane world models with a non-minimally coupled scalar field. In [9] we examined the stability of these models against scalar field perturbations and we discussed possible physical implications, while in [10] we developed a numerical approach for the solution of the Einstein equations with the non-minimally coupled scalar field.

hep-th↗

arXiv2006

The 4-D Layer Phase as a Gauge Field Localization: Extensive Study of the 5-D Anisotropic U(1) Gauge Model on the Lattice

We study a 4+1 dimensional pure Abelian Gauge model on the lattice with two anisotropic couplings independent of each other and of the coordinates. A first exploration of the phase diagram using mean field approximation and monte carlo techniques has demonstrated the existence of a new phase, the so called Layer phase, in which the forces in the 4-D subspace are Coulomb-like while in the transverse direction (fifth dimension) the force is confining. This allows the possibility of a gauge field localization scheme. In this work the use of bigger lattice volumes and higher statistics confirms the existence of the Layer phase and furthermore clarifies the issue of the phase transitions' order. We show that the Layer phase is separated from the strongly coupled phase by a weak first order phase transition. Also we provide evidence that the Layer phase is separated by the five-dimensional Coulomb phase with a second order phase transition and we give a first estimation of the critical exponents.

hep-lat↗

arXiv2006

Second Randall-Sundrum brane world scenario with a nonminimally coupled bulk scalar field

In our previous work of Ref. [5] we studied the stability of the RS2-model with a nonminimally coupled bulk scalar field $ϕ$, and we found that in appropriate regions of $ξ$ the standard RS2-vacuum becomes unstable. The question that arises is whether there exist other new static stable solutions where the system can relax. In this work, by solving numerically the Einstein equations with the appropriate boundary conditions on the brane, we find that depending on the value of the nonminimal coupling $ξ$, this model possesses three classes of new static solutions with different characteristics. We also examine what happens when the fine tuning of the RS2-model is violated, and we obtain that these three classes of solutions are preserved in appropriate regions of the parameter space of the problem. The stability properties and possible physical implications of these new solutions are discussed in the main part of this paper. Especially in the case where $ξ=ξ_c$ ($ξ_c$ is the five dimensional conformal coupling) and the fine tuning is violated, we obtain a physically interesting static stable solution.

hep-th↗

arXiv2005

Gravity-induced instability and gauge field localization

The spectrum of a massless bulk scalar field Φ, with a possible interaction term of the form -ξR Φ^{2}, is investigated in the case of RS-geometry [1]. We show that the zero mode for ξ=0, turns into a tachyon mode, in the case of a nonzero negative value of ξ(ξ<0). As we see, the existence of the tachyon mode destabilizes the Φ=0 vacuum, against a new stable vacuum with nonzero Φnear the brane, and zero in the bulk. By using this result, we can construct a simple model for the gauge field localization, according to the philosophy of Dvali and Shifman (Higgs phase on the brane, confinement in the bulk).

hep-th↗

arXiv2005

Extra-Dimensions effects on the fermion-induced quantum energy in the presence of a constant magnetic field

We consider a U(1) gauge field theory with fermion fields (or with scalar fields) that live in a space with $δ$ extra compact dimensions, and we compute the fermion-induced quantum energy in the presence of a constant magnetic field, which is directed towards the x_3 axis. Our motivation is to study the effect of extra dimensions on the asymptotic behavior of the quantum energy in the strong field limit (eB>>M^{2}), where M=1/R. We see that the weak logarithmic growth of the quantum energy for four dimensions, is modified by a rapid power growth in the case of the extra dimensions.

hep-th↗

arXiv2004

Anomalous magnetic moment in parity-conserving QED3

In this article we derive the anomalous magnetic moment of fermions in (2+1)-dimensional parity-conserving QED3, in the presence of an externally applied constant magnetic field. We use a spectral representation of the photon propagator to avoid infrared divergences. We also discuss the scaling with the magnetic field intensity in the case of strong external fields, where there is dynamical mass generation for fermions induced by the magnetic field itself (magnetic catalysis). The results of this paper may be of relevance to the physics of high-temperature superconductors.

hep-ph↗

arXiv2004

Effective potential analysis for 5D SU(2) gauge theories at finite temperature and radius

We calculate the one loop effective potential for a 5D SU(2) gauge field theory at finite temperature $T=1/β$ and radius R=1/M. This calculation is performed, for the first time, in the case of background fields with two constant components $A^{3}_{y}$ (directed towards the compact extra dimension with radius R) and $A^{3}_τ$ (directed towards the compact Euclidean time with radius $β$). This model possesses two discrete symmetries known as Z_{M}(2) and Z_{T}(2). The corresponding phase diagram is presented in Ref. 4. However the arguments which lead to this diagram are mainly qualitative. We present a detailed analysis, from our point of view, for this phase diagram, and we support our arguments performing lattice simulations for a simple phenomenological model with two scalar fields interacting through the previously calculated potential.

hep-ph↗

arXiv2004

Layered Higgs Phase as a Possible Field Localisation on a Brane

So far it has been found by using lattice techniques that in the anisotropic five--dimensional Abelian Higgs model, a layered Higgs phase exists in addition to the expected five--dimensional one. The exploration of the phase diagram has shown that the two Higgs phases are separated by a phase transition from the confining phase. This transition is known to be first order. In this paper we explore the possibility of finding a second order transition point in the critical line which separates the first order phase transition from the crossover region. This is shown to be the case only for the four--dimensional Higgs layered phase whilst the phase transition to the five--dimensional broken phase remains first order. The layered phase serves as the possible realisation of four--dimensional spacetime dynamics which is embedded in a five--dimensional spacetime. These results are due to gauge and scalar field localisation by confining interactions along the extra fifth direction.

hep-ph↗

arXiv2003

Finite temperature Z(N) phase transition with Kaluza-Klein gauge fields

If SU(N) gauge fields live in a world with a circular extra dimension, coupling there only to adjointly charged matter, the system possesses a global Z(N) symmetry. If the radius is small enough such that dimensional reduction takes place, this symmetry is spontaneously broken. It turns out that its fate at high temperatures is not easily decided with straightforward perturbation theory. Utilising non-perturbative lattice simulations, we demonstrate here that the symmetry does get restored at a certain temperature T_c, both for a 3+1 and a 4+1 dimensional world (the latter with a finite cutoff). To avoid a cosmological domain wall problem, such models would thus be allowed only if the reheating temperature after inflation is below T_c. We also comment on the robustness of this phenomenon with respect to small modifications of the model.

hep-ph↗

arXiv2001

The Phase Diagram for the anisotropic SU(2) Adjoint Higgs Model in 5D: Lattice Evidence for Layered Structure

We explore, by Monte Carlo and Mean Field methods, the five--dimensional SU(2) adjoint Higgs model. We allow for the possibility of different couplings along one direction, describing the so--called anisotropic model. This study is motivated by the possibility of the appearance of four--dimensional layered dynamics. Actually, our results lead to the conclusion that the establishment of a layered phase in four dimensions described by U(1) symmetry is possible, the extra dimension being confined due to the SU(2) gauge symmetry. The five-dimensional adjoint Higgs model turns out to have a layered phase, in contradistinction with what is known about the pure SU(2) model.

hep-lat↗

arXiv2001

Multi-Layer Structure in the Strongly Five Dimensional Abelian Higgs Model

We explore the phase diagram of the 5--D anisotropic Abelian Higgs model by Monte Carlo simulations. In particular, we study the transition between the confining phase and the four dimensional layered Higgs phase. We find that, in a certain region of the lattice parameter space, this transition can be first order and that each layer moves into the Higgs phase independently of the others (decoupling of layers).

hep-lat↗

arXiv2001

Branes in the 5D Abelian Higgs Model

We find 3-brane Higgs and Coulomb phases in the 5D Abelian Higgs Model and determine the transition surfaces that separate them from the usual bulk phases.

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arXiv2001

Spatially Anisotropic Four-Dimensional Gauge Interactions, Planar Fermions and Magnetic Catalysis

We consider magnetic catalysis in a field-theoretic system of (3+1)-dimensional Dirac fermions with anisotropic kinetic term. By placing the system in a strong external magnetic field, we examine magnetically-induced fermion mass generation. When the coupling anisotropy is strong, in which case the fermions effectively localize on the plane, we find a significant enhancement of the induced mass gap compared to the isotropic four-dimensional case of quantum electrodynamics. As expected on purely dimensional grounds, the mass and critical temperature scale with the square root of the magnetic field. This phenomenon might be related to recent experimental findings on magnetically-induced gaps at the nodes of d-wave superconducting gaps in high-temperature cuprates.

hep-ph↗

arXiv2001

Multi--Layer Structure in the Strongly Coupled 5D Abelian Higgs Model

We explore the phase diagram of the five-dimensional anisotropic Abelian Higgs model by Monte Carlo simulations. In particular, we study the transition between the confining phase and the four dimensional layered Higgs phase. We find that, in a certain region of the lattice parameter space, this transition can be first order and that each layer moves into the Higgs phase independently of the others (decoupling of layers). As the Higgs couplings vary, we find, using mean field techniques, that this transition may probably become second order.

hep-lat↗

arXiv2001

QED$_3$ with Dynamical Fermions in an External Magnetic Field

In this paper, we present results of numerical lattice simulations of two-flavor QED in three space-time dimensions. First, we provide evidence that chiral symmetry is spontaneously broken in the chiral and continuum limit. Next we discuss the role of an external magnetic field $B$ on the dynamically generated fermion mass. We investigate the $B$-dependence of the condensate through calculations with dynamical fermions using the non-compact formulation of the gauge field, and compare the results with those of a comparable study using the quenched approximation.

hep-lat↗