Discrete graviton spectrum from super-exponential cup potentials and their application to braneworld physics

TL;DR

This paper introduces a new braneworld model with a super-exponential warp factor that localizes gravity, exhibits a discrete spectrum of massive tensor modes, and predicts Planck-scale bound KK excitations with negligible phenomenological effects.

Contribution

It provides an explicit solution with a novel discrete KK spectrum arising from a cup potential, contrasting with previous models with continuous spectra.

Findings

The tensor perturbations obey a Schrödinger-like equation with a cup potential.

The KK spectrum is discrete and bound to the brane, with quantized masses.

The massive KK modes have Planck-scale masses, making them phenomenologically unreachable.

Abstract

Super-exponential warp factors have been proposed in the context of two-brane models to solve the gauge hierarchy problem of scales by using a compactification scale of the same order of the fundamental Planck scale, completely eliminating the hierarchy between the electroweak and the Planck scales. However, most of the so far studied families of braneworlds with super-exponential warp factors do not localize 4D gravity when one of the branes is sent to infinity. Recently a braneworld model generated by a canonical scalar field $\phi$ minimally coupled to 5D gravity with a bulk cosmological constant was shown to localize 4D gravity and to be stable under linear tensorial and scalar perturbations. Here we present an explicit new solution for the latter braneworld configuration and study the dynamics of its tensorial perturbations: we find that they obey a Schr\"odinger-like equation with a well potential which possesses exponentially increasing walls (we call it the {\it cup potential}), yielding a {\it novel discrete spectrum} for the massive Kaluza-Klein (KK) tensorial excitations, despite the non-compact nature of the fifth dimension, and in contrast to the braneworld models proposed so far, where the mass spectrum of the KK modes is continuous, or at most mixed. Finally, the corrections to Newton's law coming from these massive KK modes are computed. Their novelty arises from the fact that these {\it KK excitations possess quantized masses and are bound to the brane}, however, they are all of the Planck mass order, leading to unreachable energy scales from the phenomenological viewpoint.