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Bleeding Edge


by Cyrus Ance
Aug 14,2003



I have mentioned neutrinos in two columns and it is time to tackle them. Their history goes back a long way. Beta radiation is when a neutron in a nucleus of an atom turns into a proton emitting a negatively charged particle, that we now know to be an electron, but which were originally called beta particles. In the 1910's careful studies of atoms that underwent beta decays revealed that the process did not seem to conserve energy or momentum. There was more energy in the initial atom than in the final atom plus the beta particle and the beta particle carried away momentum that was not balanced by the recoiling final atom. Two explanation were advanced. One was that the force responsible for the beta decay of the proton, called the Weak Nuclear Force or today simply the Weak Force, does not conserve energy and momentum. This is not a very plausible suggestion. We have come to understand that conservation of momentum is related to translational invariance, physical laws do not depend on where you test them, and conservation of energy is related to time invariance, physical laws do not depend on when you test them. A future column will discuss the related question of whether physical constants vary with time.

The other suggestion was that something unseen was carrying away the energy and momentum that is needed to balance the scales. Enrico Fermi, the father of the atom bomb, further suggested that the unseen thing had to be almost massless, have zero charge, and only interact via the Weak Force. He suggested that unseen thing was a particle that he called the neutrino, "little neutral guy" in Italian. Due to the theoretical prejudice against violating the conservation of momentum and energy the neutrino hypothesis was favored, but sobering calculations of just how weakly neutrinos interact with ordinary matter, a typical neutrino from a beta decay is not likely to interact with anything as it passes through the earth, made it difficult to imagine observing neutrinos. World War II put a damper on research aimed at understanding the neutrino and the Weak Force due not only of the difficulty of the experiments, but also because almost all of the experts were working on atom bomb projects.

Fred Rienes realizes that an atomic explosion would be a copious source of neutrinos and in 1951 suggests to Fermi that such an experiment should be mounted. Rienes meets Clyde Cowan in 1952 and he suggests that an atomic reactor is the right place to do such an experiment. Their first try at Hanford, Washington is suggestive, but not definitive. Next in 1953 they try at Savannah River, South Carolina and succeed. Their experiment observed inverse beta decay where an incoming neutrino turns a proton into a neutron and an anti-electron, positron. The outgoing positron and neutron interact with nearby atoms producing light and the simultaneous detection of the two light signals is the signature of the original neutrino. Rienes was awarded a long over due Nobel prize in 1995 for this observation.

The next big step was to notice that neutrinos come in multiple flavors. In the early 1960's experiments at Brookhaven Lab on Long Island observed the difference between the electron neutrino and the muon neutrino. Muons are heavy versions of electrons which belong to the second generation of fundamental particles. Definitive experiments at CERN in the late 1980's and early 1990's showed that there are three and only three types of neutrinos and thus three generations of fundamental particles. I would like to write a column about why there are exactly three generations, but no one knows and there are no plausible explanations.

Neutrinos can be used to learn about the universe. The sun is a giant nuclear reactor and copiously produces neutrinos. The number of neutrinos coming from the sun should be related to the energy output from the sun. Experiments were mounted to measure the rate of neutrinos produced by the sun. The most famous was begun in 1969 by Ray Davies in the Homestake mine. From its first result it observed that that there about one third as many neutrinos as expected from the sun. Three explanations were advanced: the experiment could have been wrong (it is a very difficult experiment); there is a problem in our understanding of the sun; or there is a problem in our understanding of neutrinos. Later experiments confirmed the Homestake result and many other observations confirmed that we understand the sun at a much better level than the 50% or so discrepancy in the solar rate. By the mid 1990's all the evidence pointed towards neutrinos as the source of the problem. Neutrinos have also been observed from a supernova explosion in one of the Milky Way's companion galaxies.

Other experiments in the mid-1990's confirmed the solar neutrino deficit and in a spectacular result in 2000 the SNO Experiment, a giant vat of heavy water in the Sudbury mine in Canada, observed that the total number of neutrinos coming from the sun agrees with our expectation but that the neutrinos were changing their generation as they traveled from the sun to the earth. The other experiments had only been looking at electron type neutrinos. Ray Davies was awarded the Nobel prize in 2003. The "mixing" of neutrino from different generations implies that neutrinos have mass, and for a short time it was thought that they could be the dark matter of the universe. Results from WMAP in 2003 make it unlikely that neutrinos are the dark matter that we see in galaxies.

Other experiments had pointed the way on neutrino mixing. Super-Kamiokande, a giant underground vat of water in Japan, in 1998 had indicated that in cosmic ray showers fewer muon neutrinos were showing up while they were seeing the expected number of electron neutrinos. They were further chasing this down with a beam of neutrinos when in November 2000 many of the phototubes looking at the water vat were destroyed in a chain reaction set off by one of the tubes imploding. The remains are not pretty. In late 2002 Kamland, an experiment using power reactors in Japan, has also observed fewer than the expected number of neutrinos putting the exclamation point on the neutrino mixing observation.

Where are we today? Neutrinos having mass and mixing makes them much more interesting than their being massless as was thought up until the mid- to late-1990's. Exactly what their masses are remains an open question. Do they exhibit a range of masses that spans a range of 1000 as their fellow elementary particles? Is there a pattern to their mixing? If so it appears to be quite different than the one for the quarks. Does something more fundamental give rise to either of these patterns? Do neutrinos have lots of CP violation and thus be responsible for the matter-anti-matter asymmetry of the universe? It is still not completely clear that neutrinos have different anti-matter partners or if they are their own anti-partner. A stack of experiments are either in their early stages or being planned to try to expand our understanding of the very elusive neutrinos.

The history of the neutrino up until 1999 is well described at the Neutrino History site. Follow the links for more information and a list of past, present, and future neutrino experiments.

Roleplaying Ideas

All of these were inspired by actual science papers or talks that have appeared in the last few years.

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