SLED Phenomenology: Curvature vs. Volume

TL;DR

This paper investigates the viability of the SLED model with finite brane width and dilaton-brane couplings, finding that achieving acceptable 4D curvature or extra dimension size requires fine-tuning or leads to phenomenological issues, challenging the model's feasibility.

Contribution

It provides a comprehensive analysis of the SLED model including finite brane width and SI breaking couplings, revealing inherent limitations and tuning requirements for phenomenological viability.

Findings

Either the 4D curvature or the size of extra dimensions is unacceptably large without fine-tuning.

Certain SI breaking brane-dilaton couplings can avoid tuning but worsen phenomenological problems.

Results recover previous thin-brane limits and address concerns about their validity.

Abstract

We assess the question whether the SLED (Supersymmetric Large Extra Dimensions) model admits phenomenologically viable solutions with 4D maximal symmetry. We take into account a finite brane width and a scale invariance (SI) breaking dilaton-brane coupling, both of which should be included in a realistic setup. Provided that the microscopic size of the brane is not tuned much smaller than the fundamental bulk Planck length, we find that either the 4D curvature or the size of the extra dimensions is unacceptably large. Since this result is independent of the dilaton-brane couplings, it provides the biggest challenge to the SLED program. In addition, to clarify its potential with respect to the cosmological constant problem, we infer the amount of tuning on model parameters required to obtain a sufficiently small 4D curvature. A first answer was recently given in [arXiv:1508.01124], showing that 4D flat solutions are only ensured in the SI case by imposing a tuning relation, even if a brane-localized flux is included. In this companion paper, we find that the tuning can in fact be avoided for certain SI breaking brane-dilaton couplings, but only at the price of worsening the phenomenological problem. Our results are obtained by solving the full coupled Einstein-dilaton system in a completely consistent way. The brane width is implemented using a well-known ring regularization. In passing, we note that for the couplings considered here the results of [arXiv:1508.01124] (which only treated infinitely thin branes) are all consistently recovered in the thin brane limit, and how this can be reconciled with the concerns about their correctness, recently brought up in [arXiv:1509.04201].