Neutrinos

What are they?

Neutrinos are members of the Standard Model, belonging to a class of particles called leptons. For a long time scientists believed neutrinos were massless and moved at the speed of light. However, physicists have found increasing evidence that these tiny particles in fact have mass, although one much less than that of the electron. Right now we only know the upper limits on what the mass could be and the mass differences between flavors of neutrinos, although there are many current experiments designed to probe this question. The difficulty lies in the fact that neutrinos are extremely non-interacting and therefore troublesome to detect. Scientists know with certainty that they are chargeless and have a spin angular momentum of 1/2. Every measurement made of the elements of the standard model (including neutrinos) has shown that they have no internal structure; indeed, that is why they are called fundamental particles. In addition, neutrinos seem to be stable.

Leptons come in pairs; each neutrino has a charged partner. The electron neutrino is paired with the electron, the muon neutrino with the muon and the tau neutrino with the tau. They are created only in pairs due to lepton number conservation, a fundamental principle that dictates the type and quantity of lepton must be conserved in any event.

A little history:

Neutrinos didn't emerge onto the particle physics scene until 1930, when Wolfgang Pauli invented the neutrino to "save" conservation of energy, which was under threat from observations of beta decay in radioactive materials. Scientists such as Henri Bequerel and Marie and Pierre Curie performed the first studies into radiation starting in 1898. In the years that followed radiation was classified into 3 categories: alpha, beta and gamma. In studying beta radiation, scientists discovered a disturbing phenomenon. It seemed that when a nucleus underwent beta decay, which consisted of the emission by a neutron of an electron to create a proton, conservation of energy was violated. There was a missing amount of energy that could not be accounted for by their measurements or calculations. In 1930 Pauli made his hypothesis, saying:

Beta decay

I have hit upon a desperate remedy to save the "exchange theorem" of statistics and the law of conservation of energy. Namely, the possibility that there could exist in the nuclei electrically neutral particles, that I wish to call neutrons, which have spin 1/2 and obey the exclusion principle and which further differ from light quanta in that they do not travel with the velocity of light. The mass of the neutrons should be of the same order of magnitude as the electron mass and in any event not larger than 0.01 proton masses. The continuous beta spectrum would then become understandable by the assumption that in beta decay a neutron is emitted in addition to the electron such that the sum of the energies of the neutron and the electron is constant...

It was not until 1933 that Pauli admitted the possibility of a zero mass neutrino (the discovery of the neutron in 1932 by James Chadwick forced him to change the hypothesized particle's name to neutrino). Today we know that neutrinos have some unknown mass and that they move close to the speed of light. The first detection of neutrinos occurred in 1956 by Clyde Cowan and Fredrick Reines who found a convenient source of neutrinos--nuclear power plants. Power is created in nuclear plants when atoms undergo nuclear fission, a process of which the neutrino is a byproduct. Cowan and Reines employed a 400-L tank of cadmium chloride as their target. The neutrinos struck a proton inside the target, producing a positron and a neutron. That positron encountered an electron; the two annihilated each other, producing two gamma rays (or photons). The neutron was absorbed by a cadmium chloride atom, producing a photon at a 15-microsecond delay from the emission from the positron. Using this knowledge of the photon emission, Cowan and Reines were able to detect the electron neutrino.

Leon Lederman, Mel Schwartz, and Jack Steinberger followed with the detection of the muon neutrino in 1962. They fired a GeV beam of protons through a target creating pions, which decayed into muons and muon neutrinos. Thick shielding halted the muons but the neutrinos continued until they entered a detector where they produced muons, decaying into electrons and a photon that were observed in the spark chambers.

Enter DONUT. The tau neutrino remained in hiding for many years until researchers overcame two major obstacles. First, the tau lepton has an extremely short lifetime--only 300 femtoseconds. Because neutrinos are detected by tracking their charged lepton partners, the lepton partner must be relatively easy to track. However, since the tau has such a short lifetime (even at relativistic speeds) it is difficult to detect. Secondly, tau neutrino production is very rare. Out of the 1013 neutrinos produced only 103 neutrino interactions, 4 of which were identified as tau neutrinos. Finally, in 2000, the DONUT team announced that they had the first direct evidence of the tau neutrino.

Where do they come from?

The picture above is of Super Kamiokande,
a solar and atmospheric neutrino detector in Japan.

Neutrinos come from several sources. The majority of neutrinos were created during the first few fractions of a second after the big bang, approximately 15 billion years ago, when the universe was comprised of elementary particles. These neutrinos are very low energy; they are so low, in fact, that we cannot detect them. These, along with microwave radiation, constitute the cosmic background radiation that permeates the entire universe, creating a picture of the events immediately following the big bang. Other neutrinos are produced in stars such as our own sun. In its core four protons combine with two electrons to form a helium nucleus and two electron neutrinos. Also, as we have mentioned before, beta decay is another instance of neutrino production. Cosmic rays, coming from various stellar phenomena and bombarding Earth perpetually, are partially comprised of neutrinos. Lastly, there are man-made sources such as physics laboratories where we create them by smashing high energy particles into fixed or moving targets.

Why are we interested in them?

Neutrinos are a fundamental part of nature and we know relatively little about them. There are many important questions being asked by scientists around the world. What are the neutrino masses? We now believe neutrinos oscillate between different flavors, but how do they do so and for how long? What implications do oscillations have on the standard model? Do neutrinos constitute dark matter? And there are many others which scientists ask so that they can better understand the world we live in.


Last updated: 6/29/01 comments