Messengers from imaginary time: instantons and topological non-perturbative physics — from quantum mechanics to QCD and experimental searches

Tomas Sykora examines topological physics that perturbation theory cannot capture. Complex field configurations in path integrals include solitons, monopoles, domain walls, cosmic strings, sphalerons, baryon junctions and instantons; non-perturbative effects are central to confinement and hadron masses and spins. Starting with tunnelling in a double-well potential and imaginary time, the talk follows instantons into QCD, covering the theta vacuum, strong CP and axions, chiral symmetry breaking, topology and resurgence. It also considers whether a gluonic Y-shaped baryon junction carries baryon number instead of the valence quarks. The experimental discussion reviews instanton searches at HERA and the LHC, junction searches at RHIC, and prospective Electron-Ion Collider tests. No confirmed signal is claimed.