Interactions of Charged Spin-2 Fields

TL;DR

This paper investigates the possibility of constructing ghost-free theories of charged spin-2 fields, demonstrating that while the Federbush theory is ghost-free, extending it to a full gravitational theory with U(1) symmetry reintroduces ghosts.

Contribution

The authors show that the Federbush theory with a dynamical U(1) field is ghost-free and explore gravitational extensions, revealing fundamental obstacles due to U(1) symmetry constraints.

Findings

Federbush theory is ghost-free with a dynamical U(1) field.

U(1) symmetry prevents consistent non-linear gravitational extensions.

Modified kinetic terms reintroduce the Boulware-Deser ghost.

Abstract

In light of recent progress in ghost-free theories of massive gravity and multi-gravity, we reconsider the problem of constructing a ghost-free theory of an interacting spin-2 field charged under a U(1) gauge symmetry. Our starting point is the theory originally proposed by Federbush, which is essentially Fierz-Pauli generalized to include a minimal coupling to a U(1) gauge field. We show the Federbush theory with a dynamical U(1) field is in fact ghost-free and can be treated as a healthy effective field theory to describe a massive charged spin-2 particle. It can even potentially have healthy dynamics above its strong-coupling scale. We then construct candidate gravitational extensions to the Federbush theory both by using Dimensional Deconstruction, and by constructing a general non-linear completion. However, we find that the U(1) symmetry forces us to modify the form of the Einstein-Hilbert kinetic term. By performing a constraint analysis directly in the first-order form, we show that these modified kinetic terms inevitably reintroduce the Boulware-Deser ghost. As a by-product of our analysis, we present a new proof for ghost-freedom of bi-gravity in 2+1 dimensions (also known as Zwei-Dreibein gravity). We also give a complementary algebraic argument that the Einstein-Hilbert kinetic term is incompatible with a U(1) symmetry, for a finite number of gravitons.