They're back at the top of the news again, and who's to say they won't stick around this time? Time to catch up.
A sterile neutrino is a hypothetical neutrino that does not interact via any of the fundamental interactions of the Standard Model except gravity.[note 1] It is a right-handed neutrino or a left-handed anti-neutrino.
Such a particle belongs to a singlet representation with respect to the strong interaction and the weak interaction and has zero weak hypercharge, zero weak isospin and zero electric charge. The left-handed anti-neutrino has a B-L of 1 and an X charge of 5. Sterile neutrinos would still interact via gravity, so if they are heavy enough, they could explain cold dark matter or warm dark matter. In some grand unification theories, such as SO(10), they also interact via gauge interactions which are extremely suppressed at ordinary energies because their gauge boson is extremely massive. They do not appear at all in some other GUTs, such as the Georgi-Glashow model (i.e. all its SU(5) charges or quantum numbers are zero).
Sterile neutrinos may mix with ordinary neutrinos via a Dirac mass[clarification needed]. The sterile neutrinos and ordinary neutrinos may also have Majorana masses. In certain models[which?], both Dirac and Majorana masses are used in a seesaw mechanism, which drives ordinary neutrino masses down and makes the sterile neutrinos much heavier than the Standard Model interacting neutrinos. In some models[which?] the heavy neutrinos can be as heavy as the GUT scale (~1015 GeV). In other models[which?] they could be lighter than the weak gauge bosons W and Z as in the so-called νMSM model where their masses are between GeV and keV. A light (with the mass ~1 eV) sterile neutrino was suggested as a possible explanation of the results of the LSND experiment. On April 11, 2007, researchers at the MiniBooNE experiment at Fermilab announced that they had not found any evidence supporting the existence of such a sterile neutrino. More recent results and analysis have provided some support for the existence of the sterile neutrino.