Brane SUSY Breaking and the Gravitino Mass

TL;DR

This paper investigates how gravitino degrees of freedom behave in non-trivial supergravity backgrounds, revealing intermediate states between massless and massive, with implications for models like brane SUSY breaking.

Contribution

It clarifies the degrees of freedom and propagation characteristics of gravitinos in de Sitter-like backgrounds, especially in models with brane SUSY breaking.

Findings

Gravitino can have intermediate degrees of freedom in non-trivial backgrounds.

In certain models, gravitino behaves as massless despite having more degrees of freedom.

The background geometry influences the gravitino's effective mass and degrees of freedom.

Abstract

Supergravity models with spontaneously broken supersymmetry have been widely investigated over the years, together with some notable non-linear limits. Although in these models the gravitino becomes naturally massive absorbing the degrees of freedom of a Nambu-Goldstone fermion, there are cases in which the naive counting of degrees of freedom does not apply, in particular because of the absence of explicit gravitino mass terms in unitary gauge. The corresponding models require non-trivial de Sitter-like backgrounds, and it becomes of interest to clarify the fate of their Nambu-Goldstone modes. We elaborate on the fact that these non-trivial backgrounds can accommodate, consistently, gravitino fields carrying a number of degrees of freedom that is intermediate between those of massless and massive fields in a flat spacetime. For instance, in a simple supergravity model of this type with de Sitter background, the overall degrees of freedom of gravitino are as many as for a massive spin-3/2 field in flat spacetime, while the gravitino remains massless in the sense that it undergoes null-cone propagation in the stereographic picture. On the other hand, in the ten-dimensional USp(32) Type I Sugimoto model with "brane SUSY breaking", which requires a more complicated background, the degrees of freedom of gravitino are half as many of those of a massive one, and yet it somehow behaves again as a massless one.