Into the conformal window: multi-representation gauge theories

Abstract

We investigate the conformal window of four-dimensional gauge theories with fermionic matter fields in multiple representations. Of particularly relevant examples are the ultra-violet complete models with fermions in two distinct representations considered in the context of composite Higgs and top partial-compositeness. We first discuss various analytical approaches to unveil the lower edge of the conformal window and their extension to the multiple matter representations. In particular, we argue that the scheme-independent series expansion for the anomalous dimension of a fermion bilinear at an infrared fixed point, $\gamma_{\bar{\chi}\chi,\,{\rm IR}}$, combined with the conjectured critical condition, $\gamma_{\bar{\chi}\chi,\,{\rm IR}} = 1$ or equivalently $\gamma_{\bar{\chi}\chi,\,{\rm IR}} (2-\gamma_{\bar{\chi}\chi,\,{\rm IR}})=1$, can be used to determine the boundary of conformal phase transition on fully physical grounds. In illustrative cases of $SU(2)$ and $SU(3)$ theories with $N_R$ Dirac fermions in various representations, we assess our results by comparing to other analytical or lattice results.