4D gravity localized in non Z_2-symmetric thick branes

TL;DR

This paper compares 4D gravity localization on non Z_2-symmetric thick branes in Riemannian and Weyl geometries, analyzing graviton modes and their spectra, and finds that Weyl geometries do not inherently mimic quantum behavior.

Contribution

It provides a comparative analysis of gravity localization on thick branes in Riemannian and Weyl geometries, highlighting differences in curvature behavior and spectrum properties.

Findings

A single bound zero mode indicates a stable 4D graviton.

The spectrum of KK states is continuous and gapless.

Weyl geometries do not inherently mimic quantum behavior in this setup.

Abstract

We present a comparative analysis of localization of 4D gravity on a non Z_2-symmetric scalar thick brane in both a 5-dimensional Riemannian space time and a pure geometric Weyl integrable manifold. This work was mainly motivated by the hypothesis which claims that Weyl geometries mimic quantum behaviour classically. We start by obtaining a classical 4-dimensional Poincare invariant thick brane solution which does not respect Z_2-symmetry along the (non-)compact extra dimension. The scalar energy density of our field configuration represents several series of thick branes with positive and negative energy densities centered at y_0. The only qualitative difference we have encountered when comparing both frames is that the scalar curvature of the Riemannian manifold turns out to be singular for the found solution, whereas its Weylian counterpart presents a regular behaviour. By studying the transverse traceless modes of the fluctuations of the classical backgrounds, we recast their equations into a Schroedinger's equation form with a volcano potential of finite bottom (in both frames). By solving the Schroedinger equation for the massless zero mode m^2=0 we obtain a single bound state which represents a stable 4-dimensional graviton in both frames. We also get a continuum gapless spectrum of KK states with positive m^2>0 that are suppressed at y_0, turning into continuum plane wave modes as "y" approaches spatial infinity. We show that for the considered solution to our setup, the potential is always bounded and cannot adopt the form of a well with infinite walls; thus, we do not get a discrete spectrum of KK states, and we conclude that the claim that Weylian structures mimic, classically, quantum behaviour does not constitute a generic feature of these geometric manifolds.