Goldstone Equivalence and High Energy Electroweak Physics

TL;DR

This paper introduces a Lorentz-covariant formalism for high-energy electroweak physics that simplifies calculations by making energy power-counting manifest, proving key approximations, and enabling efficient loop computations.

Contribution

It develops a new formalism that ensures well-behaved polarization vectors at high energy, applicable to all orders and gauge choices, facilitating analysis of electroweak processes.

Findings

Proves the validity of the Effective W Approximation.

Derives tree-level splitting functions for collinear emissions.

Demonstrates simplified loop calculations in the Standard Model.

Abstract

The transition between the broken and unbroken phases of massive gauge theories, namely the rearrangement of longitudinal and Goldstone degrees of freedom that occurs at high energy, is not manifestly smooth in the standard formalism. The lack of smoothness concretely shows up as an anomalous growth with energy of the longitudinal polarization vectors, as they emerge in Feynman rules both for real on-shell external particles and for virtual particles from the decomposition of the gauge field propagator. This makes the characterization of Feynman amplitudes in the high-energy limit quite cumbersome, which in turn poses peculiar challenges in the study of Electroweak processes at energies much above the Electroweak scale. We develop a Lorentz-covariant formalism where polarization vectors are well-behaved and, consequently, energy power-counting is manifest at the level of individual Feynman diagrams. This allows us to prove the validity of the Effective W Approximation and, more generally, the factorization of collinear emissions and to compute the corresponding splitting functions at the tree-level order. Our formalism applies at all orders in perturbation theory, for arbitrary gauge groups and generic linear gauge-fixing functionals. It can be used to simplify Standard Model loop calculations by performing the high-energy expansion directly on the Feynman diagrams. This is illustrated by computing the radiative corrections to the decay of the top quark.