Which Right Neutrino is Right?
It's one of the mysteries of the universe: do neutrinos act as their own antiparticles? Force-carrying particles like photons do. Particles that don't carry force have separate antiparticles: the electron has the positron; the b quark has the anti-b quark, and so on.
What about the neutrino? Like electrons and quarks, it's a fermion. Yet, it might very well act as its own antiparticle. In this case, it is called a Majorana neutrino. Knowing whether Majorana neutrinos exist will shed light on how neutrinos get mass, what a right-handed neutrino looks like, and how the universe became dominated by matter. To answer these pressing questions, experiments such as the Stanford-led Enriched Xenon Observatory (EXO) plan to look for the phenomenon.
"It would be incredibly cool if fermions could be their own antiparticles," said SLAC theorist Michael Peskin.
It would also solve quandaries. Neutrinos, formed by a weak interaction, are all left-spinning. But because neutrinos have mass, right-spinning neutrinos must exist too (see image). So what is the right-spinning neutrino? There are two options. One, it could be a new kind of particle that's never been seen, a particle whose existence is neither predicted nor ruled out by the Standard Model. Or two—and this is the scenario Peskin prefers—neutrinos can turn into their own antiparticle. In this case, the neutrino's known antiparticle—the right-spinning antineutrino—is also the right-spinning particle necessitated by neutrinos having mass.
"At this moment, we have no idea experimentally if the answer is case one or case two. We would very much like to find out because this is the most fundamental question about neutrino mass," said Peskin. "Option one introduces a new particle that is not in the Standard Model. Option two violates lepton number. So there is something strange about both options, but we are forced by the discovery of neutrinos mass to have one or the other."
In case one, neutrinos get their mass from the Higgs particle, like all of the other quarks and leptons. But it remains mysterious why neutrino masses are so small. With case two— Majorana neutrinos—the tiny mass of neutrinos is explained, by theorists Peter Minkowski, Murray Gell-Mann, Pierre Ramond, Richard Slansky and Tsutomo Yanagida. These explanations include the possibility of new, very heavy neutrinos, and from their asymmetric decays, a mechanism called leptogenesis. This idea, proposed by Yanagida and Masataka Fukugita, can explain why the universe is made of matter instead of antimatter.
The theories that rest on the narrow shoulders of neutrinos now await results from future experiments.
—Heather Rock Woods, May 10, 2007
Michael Peskin's chalkboard diagram shows why neutrinos with mass cannot be only left-spinning. (Click on image for a close-up of the diagram and an explanation).