Anomalous solutions to the strong CP problem

TL;DR

This paper introduces a novel mechanism for addressing the strong CP problem using a Z2 symmetry and an anomalous U(1) symmetry, leading to models predicting new TeV-scale colored particles.

Contribution

It proposes two new models employing discrete and anomalous symmetries to solve the strong CP problem with distinctive IR manifestations and testable predictions.

Findings

The models predict new colored particles at the TeV scale.

The first model uses massless bifundamental quarks and the η' boson to relax the theta angle.

The second model employs a dynamically generated mass term to cancel the theta angle.

Abstract

We present a new mechanism for solving the strong CP problem using a Z2 discrete symmetry and an anomalous U(1) symmetry. A Z2 symmetry is used so that two gauge groups have the same theta angle. An anomalous U(1) symmetry makes the difference between the two theta angles physical and the sum unphysical. Two models are presented where the anomalous symmetry manifests itself in the IR in different ways. In the first model there are massless bifundamental quarks, a solution reminiscent of the massless up quark solution. In the IR of this model, the $\eta'$ boson relaxes the QCD theta angle to the difference between the two theta angles - in this case zero. In the second model, the anomalous U(1) symmetry is realized in the IR as a dynamically generated mass term that has exactly the phase needed to cancel the theta angle. Both of these models make the extremely concrete prediction that there exist new colored particles at the TeV scale.