| Popis: |
We discuss the construction of the two-flavour axion-pion effective Lagrangian at the next-to-leading order (NLO) in chiral perturbation theory and present two phenomenological applications: the scattering $a\pi \to \pi\pi$, relevant for axion thermalization in the early Universe, and the decay rate of a GeV-scale axion-like particle via the channel $a \to \pi\pi\pi$. In both cases we assess, through the NLO computation, the range of validity of the effective field theory and show that the chiral expansion breaks down earlier than what previously assumed. These results call for alternative non-perturbative approaches in order to extend the chiral description of axion-pion interactions. |
