Physics of θ-Vacua and Inevitability of Axions in Standard Model and Gravity

by Giorgi (Gia) Dvali (Ludwig Maximilian University of Munich)

Tuesday 7 Jul 2026, 16:00 → 17:00 Europe/Berlin

ENC-D308

In this talk we discuss how the vacuum structure of the known gauge interactions of nature implies the existence of three different species of axion-type pseudo-scalar particles, one per each gauge sector: QCD, electroweak and gravity. Each species of axion is designated for cancelling the corresponding CP-violating θ-angle dynamically. Although the axion of QCD has been widely discussed as the remedy for the naturalness strong-CP puzzle, only recently it has been understood to represent a consistency requirement. Similarly to QCD, the electroweak sector gives rise to a new axion-like particle, a so-called ηw-meson, which is sourced by the B+L-anomaly and gets its mass from the topological susceptibility of the electroweak vacuum. The third species of axion is delivered by gravity which, due to Eguchi-Hanson instantons, possesses the CP-violating θ-vacuum. Strikingly, this implies the existence of a “gravi-axion”, a pseudo-Goldstone coupled via the gravitational chiral anomaly of spin-3/2 fermions. Due to this, the gravitational θ-vacuum demands the existence of local supersymmetry. We discus some phenomenological and cosmological implications of the presented topics.