Geometry and unitarity of scalar fields coupled to gravity

TL;DR

This paper develops a geometric, frame-independent formulation of scalar fields coupled to gravity, revealing invariance under field redefinitions and Weyl transformations, and relates the target space curvature to physical cutoff scales.

Contribution

It extends the target space geometry to include the conformal mode, providing a Weyl-invariant framework for scalar-gravity theories.

Findings

The extended geometry is manifestly Weyl-invariant.

The cutoff scale is inversely related to the square root of the extended target space curvature.

The formalism's cutoff matches scattering amplitude calculations.

Abstract

We formulate scalar field theories coupled non-conformally to gravity in a manifestly frame-independent fashion. Physical quantities such as the $S$ matrix should be invariant under field redefinitions, and hence can be represented by the geometry of the target space. This elegant geometric formulation, however, is obscured when considering the coupling to gravity because of the redundancy associated with the Weyl transformation. The well-known example is the Higgs inflation, where the target space of the Higgs fields is flat in the Jordan frame but is curved in the Einstein frame. Furthermore, one can even show that any geometry of O$(N)$ nonlinear $\sigma$ models can be flattened by an appropriate Weyl transformation. In this Letter, we extend the notion of the target space by including the conformal mode of the metric, and show that the extended geometry provides a compact formulation that is manifestly Weyl-transformation/field-redefinition invariant. We identify the cutoff scale with the inverse of square root of the extended target space curvature and confirm that it coincides with that obtained from two-to-two scattering amplitudes based on our formalism.