sterile neutrino

The three known neutrinos are already the shyest particles in the Standard Model, touching the world only through the weak force. A sterile neutrino, if it exists, would be shyer still: a hypothetical fourth kind that ignores even the weak force and feels only gravity. It would be all but undetectable on its own — a ghost among ghosts — betraying its presence only indirectly, by mixing with the ordinary neutrinos and subtly altering how they oscillate.

The idea is not arbitrary. In the Standard Model, ordinary neutrinos appear only in a 'left-handed' form, and many theories that give neutrinos mass naturally introduce 'right-handed' partners that happen to be sterile. Such particles are appealing because, in the seesaw mechanism, very heavy sterile neutrinos could explain why the ordinary ones are so light. A lighter sterile neutrino, on the other hand, has been floated as a possible dark-matter candidate. Experimentally, a sterile neutrino would reveal itself as extra oscillation — neutrinos appearing or disappearing more than the three-flavor picture allows.

There are tantalizing but unconfirmed hints. A few experiments over the years have reported anomalies — neutrino counts a little off from expectation — that a sterile neutrino could explain. Yet other, often more sensitive experiments have found no such effect, and the picture remains muddled and contested. As of today there is no established evidence that sterile neutrinos exist; they are a well-motivated hypothesis under active test, not a confirmed part of nature.