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1. General Seminars
Physics of θ-Vacua and Inevitability of Axions in Standard Model and Gravity.
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Europe/Rome
Aula Salvini (Laboratori Nazionali di Frascati)
Aula Salvini
Laboratori Nazionali di Frascati
Description
In this talk we first review why the CP-violating θ-vacuum is a built-in feature of QCD and how the mass of the η′-meson provides a direct experimental evidence for it. We then explain how the axion protects CP dynamically and define the axion-quality measure. This discussion, in particular, will make it transparent that the η′-meson is nothing but a poor-quality axion. Next, we introduce the so-called gauge formulation of axion in which the axion emerges as intrinsic part of QCD gauge redundancy without the need of any anomalous global symmetry. By the power of gauge symmetry, the gauge axion has an exact quality, as it is protected agains arbitrary continuous deformations of the theory. Next, we show that in a generic gauge theory the elimination of the θ vacua by an anomalous symmetry is correlated with the existence of a pseudo-scalar particle that non-linearly realizes that symmetry. Applied to the electroweak sector, this implies the existence of a new 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. We then discuss how gravity opens up a whole new perspective on the axion, promoting it into a consistency issue. Namely, once coupled to gravity, every topologically non-trivial gauge sector must be accompanied by the axion of exact quality. This requirement must be fulfilled by gravity itself 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. Thus, the gravitational θ-vacuum demands the existence of local supersymmetry. In conclusion, Standard Model is predicted to be accompanied by three axions, one per each gauge sector: QCD, electroweak and gravity. We discus some phenomenological and cosmological implications of the presented topics.
