Speaker
Description
We revisit the domain wall problem for QCD axion models with more than
one quark charged under the Peccei-Quinn symmetry. Symmetry breaking during or
after inflation results in the formation of a domain wall network which would cause cosmic catastrophe if it comes to dominate the Universe. The network may be
made unstable by invoking a ‘tilt’ in the axion potential due to Planck scale
suppressed non-renormalisable operators. Alternatively the random walk of the
axion field during inflation can generate a ‘bias’ favouring one of the degenerate
vacuua, but we find that this mechanism is in practice irrelevant. Consideration
of the axion abundance generated by the decay of
the wall network then requires the Peccei-Quinn scale to be rather low — thus ruling
out e.g. the DFSZ axion with mass below ∼ 60 meV, where most
experimental searches are in fact focussed.
