UV Sensitivity of the Axion Mass from Instantons in Partially Broken Gauge Groups

Understand

We examine the contribution of small instantons to the axion mass in various UV completions of QCD.

• We show that the reason behind the potential dominance of such contributions is the non-trivial embedding of QCD into the UV theory.
• The effects from instantons in the partially broken gauge group appear as "fractional instanton" corrections in the effective theory.
• These will exhibit unusual dependences on the various scales in the problem whenever the index of embedding is non-trivial.