I am reading about the prospective decay of Higgs into Majorana fermions (possibly sterile neutrinos). In the image the branching ratio of Higgs to a pair of Majorana fermions is given for different Higgs Portal Scales. What is a Higgs Portal Scale?
Higgs branching fraction into Majorana fermions $\chi$ resulting from the partial width of Eq. (30), as a function of the Higgs portal scale M and the mass of the fermion $ m_\chi$. We fix the coupling $\kappa$ to be equal to 1.