Abstract
We propose that all light fermionic degrees of freedom, including the Standard Model (SM) fermions and all possible light beyond-the-standard-model fields, are chiral with respect to some spontaneously broken abelian gauge symmetry. Hypercharge, for example, plays this role for the SM fermions. We introduce a new symmetry, U(1) ν , for all new light fermionic states. Anomaly cancellations mandate the existence of several new fermion fields with nontrivial U(1) ν charges. We develop a concrete model of this type, for which we show that (i) some fermions remain massless after U(1) ν breaking — similar to SM neutrinos — and (ii) accidental global symmetries translate into stable massive particles — similar to SM protons. These ingredients provide a solution to the dark matter and neutrino mass puzzles assuming one also postulates the existence of heavy degrees of freedom that act as “mediators” between the two sectors. The neutrino mass mechanism described here leads to parametrically small Dirac neutrino masses, and the model also requires the existence of at least four Dirac sterile neutrinos. Finally, we describe a general technique to write down chiral-fermions-only models that are at least anomaly-free under a U(1) gauge symmetry.
Highlights
We propose that all light fermionic degrees of freedom, including the Standard Model (SM) fermions and all possible light beyond-the-standard-model fields, are chiral with respect to some spontaneously broken abelian gauge symmetry
We develop a concrete model of this type, for which we show that (i) some fermions remain massless after U(1)ν breaking — similar to SM neutrinos — and (ii) accidental global symmetries translate into stable massive particles — similar to SM protons
We show that the smallness of neutrino masses can be understood if the initially massless Dark Sector (DS) states are charged under the SM lepton number symmetry
Summary
The mediator Lagrangian in eq (3.1) explicitly breaks the U(1)N1 ×U(1)N2 ×U(1)νc ×U(1)l down to U(1)L — what is normally referred to as lepton-number. One needs to revisit the issue with some care since, in the early universe, the sterile neutrinos are kept in thermal equilibrium with the photons via flavor-diagonal Z and Zinteractions, discussed in some detail in the previous section. These interactions determine their relic abundance, as opposed to the standard Dodelson-Widrow mechanism [30], where active-sterile mixing determines the relic abundance of the mostly sterile states (for other possibilities see, for example, [31]). The associated phenomenology and potential current constraints will depend on the mass of the sterile neutrinos but, for most masses, as discussed above, the Ni particles are effectively stable when compared to the time-scales of laboratory experiments and will manifest themselves as missing energy
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