Geometry of the Scalar Sector

TL;DR

This paper explores the geometric structure of the scalar sector in quantum field theories, analyzing how field redefinitions affect the $S$-matrix and the implications for effective field theories like SMEFT and HEFT.

Contribution

It provides a geometric framework for understanding scalar field transformations, computes one-loop corrections in non-linear formulations, and relates geometric invariants to physical observables.

Findings

The $S$-matrix remains finite in non-linear scalar formulations.

HEFT can be recast as SMEFT if the scalar manifold has a fixed point.

Geometric invariants of the scalar manifold influence experimental observables.

Abstract

The $S$-matrix of a quantum field theory is unchanged by field redefinitions, and so only depends on geometric quantities such as the curvature of field space. Whether the Higgs multiplet transforms linearly or non-linearly under electroweak symmetry is a subtle question since one can make a coordinate change to convert a field that transforms linearly into one that transforms non-linearly. Renormalizability of the Standard Model (SM) does not depend on the choice of scalar fields or whether the scalar fields transform linearly or non-linearly under the gauge group, but only on the geometric requirement that the scalar field manifold ${\mathcal M}$ is flat. We explicitly compute the one-loop correction to scalar scattering in the SM written in non-linear Callan-Coleman-Wess-Zumino (CCWZ) form, where it has an infinite series of higher dimensional operators, and show that the $S$-matrix is finite. Standard Model Effective Field Theory (SMEFT) and Higgs Effective Field Theory (HEFT) have curved ${\mathcal M}$, since they parametrize deviations from the flat SM case. We show that the HEFT Lagrangian can be written in SMEFT form if and only if ${\cal M}$ has a $SU(2)_L \times U(1)_Y$ invariant fixed point. Experimental observables in HEFT depend on local geometric invariants of ${\mathcal M}$ such as sectional curvatures, which are of order $1/\Lambda^2$, where $\Lambda$ is the EFT scale. We give explicit expressions for these quantities in terms of the structure constants for a general $\mathcal G \to \mathcal H$ symmetry breaking pattern. (Full abstract in pdf)