Testing Brane World Models with Ultra High Energy Cosmic Rays

TL;DR

This paper investigates how ultra high energy cosmic rays can test brane world models by constraining particle propagation in extra dimensions, ruling out many models unless symmetries protect field confinement at extremely high energies.

Contribution

It analyzes the production and propagation of bulk and KK modes at ultra high energies, providing new constraints on brane world models based on cosmic ray observations.

Findings

Large parts of brane world parameter space are ruled out.

Symmetries are needed to protect field confinement at ultra high energies.

Confirmed the link between small cosmological constant and hierarchy problem.

Abstract

The arrival time coherence of particles in the Ultra High Energy Air Showers where the center of mass energy of the interaction is of the order of $10^{15} eV$, puts strict constraint on the propagation of particles in a hypothetical extra-dimension. We first argue that at such high energies bulk modes and massive KK-modes can be produced abundantly and in many models their phase space volume is larger than confined modes. Then, we study the minimum propagation time in one and two-brane models and show that a large part of the parameter space of these models are ruled out unless the confinement of fields is proteced by symmetries up to energies not accessible even to the high energy tail of Ultar High Energy Cosmic Rays (UHECRs). As a by-product we confirm the result obtained in some previous works about the close relation between a small Cosmological Constant and the hierarchy problem.