Brane World Models Need Low String Scale

TL;DR

Large extra dimension models aim to lower the fundamental Planck scale to the electroweak scale but face unitarity issues, suggesting the need for string theory effects at accessible energies, with experimental bounds constraining these models.

Contribution

This paper demonstrates the unitarity breakdown in large extra dimension models and argues for the necessity of a low string scale to resolve this, highlighting potential experimental signatures.

Findings

Unitarity breaks down at about 75% of the effective Planck scale.

String theory can potentially fix the unitarity problem.

Experimental bounds constrain the string and Planck scales.

Abstract

Models with large extra dimensions offer the possibility of the Planck scale being of order the electroweak scale, thus alleviating the gauge hierarchy problem. We show that these models suffer from a breakdown of unitarity at around three quarters of the low effective Planck scale. An obvious candidate to fix the unitarity problem is string theory. We therefore argue that it is necessary for the string scale to appear below the effective Planck scale and that the first signature of such models would be string resonances. We further translate experimental bounds on the string scale into bounds on the effective Planck scale.