The Case of the Disappearing Instanton

TL;DR

This paper classifies how instantons, which are tunneling solutions in quantum field theory, can disappear under small potential changes, revealing abrupt and smooth types with significant physical implications.

Contribution

It provides a unified framework for understanding instanton disappearances, including new classifications and examples like the 6D Einstein-Maxwell theory.

Findings

Abrupt instanton disappearances involve annihilation with higher-action solutions.

Smooth disappearances occur with decay rates approaching zero.

Examples include cases with enhanced symmetry and higher-order catastrophes.

Abstract

Instantons are tunneling solutions that connect two vacua, and under a small change in the potential, instantons sometimes disappear. We classify these disappearances as smooth (decay rate goes to 0 at disappearance) or abrupt (decay rate not equal to 0 at disappearance). Abrupt disappearances mean that a small change in the parameters can produce a drastic change in the physics, as some states become suddenly unreachable. The simplest abrupt disappearances are associated with annihilation by another Euclidean solution with higher action and one more negative mode; higher-order catastrophes can occur in cases of enhanced symmetry. We study a few simple examples, including the 6D Einstein-Maxwell theory, and give a unified account of instanton disappearances.