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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?

enter image description here

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.

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The idea is to introduce a hidden sector field $\phi$ and then to add a term proportional to $H^\dagger H\,\phi^2$ to the Higgs potential: the Higgs-portal. Plus a self-coupling term proportional to $\phi^4$. The problem this is trying to solve is that the Higgs vacuum expectation value is much smaller than the Planck scale and not predicted by the Standard Model. This is an elaboration on an old idea by Coleman and Weinberg who showed that the symmetry-breaking vacuum expectation value of a scalar field and the mass of the resulting particle could emerge from renormalisation even when the Lagrangian does not feature any mass scale. But CW's does not work for the Higgs as it predicts too small a mass.

See arxiv 1301.4224 [hep-ph] for an in-depth presentation, and the references therein for earlier work.

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