The Radion in the Karch-Randall Braneworld

TL;DR

This paper reveals that the radion mode remains a physical excitation in the Karch-Randall braneworld, measuring the distance between the brane and the AdS boundary, contrary to previous expectations for single brane models.

Contribution

It demonstrates that the radion in the Karch-Randall model always remains a physical mode, even in single brane scenarios, and clarifies its role in measuring the brane-boundary distance.

Findings

The radion is a physical mode in the Karch-Randall model.

The radion measures the distance between the brane and the AdS boundary.

Contrasts with the expectation that the radion decouples in single brane models.

Abstract

In a braneworld context, the radion is a massless mode coupling to the trace of the matter stress tensor. Since the radion also governs the separation between branes, it is expected to decouple from the physical spectrum in single brane scenarios, such as the one-brane Randall-Sundrum model. However, contrary to expectations, we demonstrate that the Karch-Randall radion always remains as a physical excitation, even in the single brane case. Here, the radion measures the distance not between branes, but rather between the brane and the anti-de Sitter boundary on the other side of the bulk.