The Neutrino's Past and Future

by Mike Perricone

For nearly 70 years, the neutrino has been the hole in the donut for particle physicists. They could sink their teeth into tasty stuff all around it, but then what? Was the hole still there, or wasn't it?

The latest answer, fittingly, has come from the experiment called Direct Observation of Nu Tau--DONUT. In confirming the predictions for this third-generation neutrino, DONUT has filled a critical hole in the Standard Modelóat least, in the Standard Model as understood today.

Frederick Reines, the neutrino's first observer in 1956 with Clyde Cowan, described this fleeting, baffling subatomic particle as ìthe most tiny quantity of reality ever imagined by a human being.î

The first to imagine it, Wolfgang Pauli, actually doubted that anyone would ever see a neutrino. Pauli found a hole in an equation, and proposed the existence of a new particle to fill itóa particle with no charge and little or no mass, to explain what appeared to be a perplexing example of nonconservation of energy.

When a neutron transformed into a proton and an electron, Pauli saw that some energy and momentum seemed to vanish. The sum of energy and momentum after the decay event did not equal the initial total energy and momentum, threatening the conservation principle that is central to physics: if you know where to look for all the parts, you'll find that what you have in the end is always exactly what you had in the first place.

The new particle was a 'desperate remedy' to this daunting problem, Pauli wrote in 1930. His mysterious, virtually invisible particle accompanied the electron in beta decaysóthus making up for the missing energy and momentum, balancing the equations and saving the principle of conservation.

Unfortunately, Pauli's terminology wasn't as good as his theory. He confusingly called his particle the "neutron."

Enrico Fermi affixed the term "neutrino" (loosely, "little neutral one"), distinguishing it from the neutron of the atomic nucleus. His 1934 theory of beta-radioactivity pointed to the weak nuclear force in creating such instabilities within the atomic nucleus.

The neutrino, Fermi said, had no mass at all--and John Updike said the same in his 1960 poem, "Cosmic Gall."
("Neutrinos, they are very small.
They have no charge and have no mass.
And do not interact at all...")

But the neutrino has grown in complexity under the watchful eyes of experimenters and theorists, and the next generation of experiments on the neutrino time line has no scarcity of holes to fill.