Instanton effects in Euclidean vacuum, real time production and in the light front wave functions

TL;DR

This paper reviews the role of instantons in non-Abelian gauge theories, highlighting their effects on vacuum structure, hadronic phenomena, and phase transitions, with implications for high-energy experiments and hadronic spectroscopy.

Contribution

It presents new insights into instanton-antiinstanton configurations, their connection to sphaleron transitions, and their influence on hadronic properties and phase transitions in QCD.

Findings

Instanton-antiinstanton streamlines describe sphaleron transitions in high energy collisions.

Instanton effects influence hadronic spectroscopy and form factors.

Advances in semiclassical theory shed light on deconfinement and chiral phase transitions.

Abstract

Nontrivial topological structures of non-Abelian gauge fields were discovered in the 1970's. Instanton solutions, describing vacuum tunneling through topological barriers, have fermionic zero modes which are at the origin of $^\prime$t Hooft effective Lagrangian. In the 1980's instanton ensembles have been used to explain chiral symmetry breaking. In the 1990's a large set of numerical simulations were performed deriving Euclidean correlation functions. The special role of scalar diquarks in nucleons, and color superconductivity in dense quark matter have been elucidated. In these lectures, we discuss further developments of physics related to gauge topology. We show that the instanton-antiinstanton "streamline" configurations describe "sphaleron transitions" in high energy collisions, which result in production of hadronic clusters with nontrivial topological/chiral charges. (They are not yet observed, but discussions of dedicated experiments at LHC and RHIC are ongoing.) Another new direction is instanton effects in hadronic spectroscopy, both in the rest frame and on the light front. We will discuss their role in central and spin-dependent potentials, formfactors and antiquark nuclear "sea". Finally, we summarize the advances in the semiclassical theory of deconfinement, and chiral phase transitions at finite temperature, in QCD and in some of its "deformed" versions.