Abstract: 

Dark confining sectors are phenomenologically rich: for instance a confining phase transition can be used to trigger baryogenesis, and some heavy axion models identify the dark \eta′ particle as an axion candidate. By modelling the dark confining phase transition with spontaneous symmetry breaking of chiral symmetry, previous studies have found that a Gravitational Wave (GW) signal could be visible at future experiments. The mesonic degrees of freedom are often described with the Linear Sigma Model potential during the transition, where an explicit U(1)_A breaking term is used to generate the dark \eta’ mass.

The \eta′ meson gains its mass directly from the U(1)_A anomaly; commonly this is argued to be generated by instanton effects. Recent arguments however, that results in QCD-like theories can be obtained from exact results in Anomaly Mediated Supersymmetry Breaking, find that the \eta′ mass cannot always be generated by instanton effects. This also follows considering the limit of a large number of colours. Interestingly, the modified explicit U(1)_A breaking term proposed following this argument also explicitly breaks chiral symmetry. With the mesonic EFT, we can only describe a secondary phase transition between non-symmetric vacua. We will discuss such a phase transition in this talk, including possible GW spectra were this phase transition first-order.