Speaker
Description
The Strong CP Problem is solved elegantly and economically by endowing the Standard Model of Particle Physics with a new complex scalar, and a spontaneously broken, anomalous global Peccei-Quinn (PQ) symmetry whose Goldstone boson is called the axion. Unfortunately, this solution may be spoiled by the global symmetry-breaking effects generically expected to arise in any effective theory of quantum gravity. This is known as the Axion Quality Problem, and it presents a highly non-trivial constraint on axion model-building. In this presentation, we will formulate a novel no-go theorem for any 3+1D model in which PQ symmetry arises residually from the spontaneous breaking of some larger (compact, connected) symmetry group: in a word, one cannot suppress gravitational corrections without also inadvertently suppressing the anomalous axion potential, in which case there is no relative protection to axion quality. As a motivating example, we will consider how this issue manifests in a so-called clockwork model, where the nearest-neighbour interactions of a field-space lattice of complex scalars can be used to exponentially suppress the axion coupling to gravity. However, drawing inspiration from the link between clockwork models and the deconstruction of higher-dimensional theories in curved spacetime, we will explore how the situation is fundamentally different in 4+1D, where the additional anomaly structure provided by the 5D Chern-Simons term may be a way past our 3+1D no-go result.
