Questions and Answers about Neutrinos
and the Super-Kamiokande Discovery
University of Hawaii professor of physics, Super-Kamiokande collaborator
WHAT ARE NEUTRINOS?
Neutrinos are the least massive elementary particle in the set of building blocks of nature, which include six quarks (down, up, strange, charmed, bottom and top) and leptons. Neutrinos have no charge and are in the family of neutral leptons. They do not feel the strong force that binds quarks into protons and neutrons, and protons and neutrons into nuclei.
There are three kinds, or "flavors" as they are called, of neutrinos electron, muon and tau. There are also three anti-neutrinos of the same flavors. The neutrinos get their names from their charged lepton brethren in order of increasing mass, the electron, muon and tau. Many theoreticians have thought the mass of neutrinos to be zero. The findings of such small values is both a mystery and undoubtedly a clue.
WHERE DO NEUTRINOS COME FROM?
Neutrinos are produced in many circumstances. The ones we are concerned with here are the result of cosmic rays hitting the earth's atmosphere.
The primary cosmic rays make a spray of secondary particles, all traveling close to the same direction and at nearly the speed of light. Some of these secondaries (mostly pi and K mesons, and tertiary muons) decay, resulting in neutrinos.
The charged particles and photons get absorbed in the atmosphere or ground. There are quite a few of these neutrinos, despite being down the decay chain from the incoming cosmic rays‹of the energies we are concerned with, about 100 of the cosmic-ray induced neutrinos from the atmosphere pass through you each second. (Yet there is only a one in 10 chance that one will hit a nucleon in your body during your lifetime.)
HOW ARE NEUTRINOS DETECTED?
Neutrinos generally go right through the earth unscattered, but occasionally one interacts in the Super-Kamiokande detector, typically striking a quark in the nucleus of an oxygen atom within a water molecule and snatching a plus or minus charge to become either a muon or an electron. That charged particle travels some distance in the water. As it moves at high speed, it radiates Cherenkov light, which is detected with the photomultipliers. Muons travel relatively straight and produce a rather clean ring image on the wall. Electrons are distinguished as they scatter and make fuzzier images, which can be recognized with about 98 percent accuracy. On average, Super-Kamiokande catches one atmospheric neutrino every 90 minutes.
WHAT IS THE ATMOSPHERIC NEUTRINO ANOMALY?
Because of the well known nature of neutrino production, we knew that the there should have been twice as many muon neutrinos as electron neutrinos from the atmosphere. Yet, in observations first made more than ten years ago in the IMB and Kamioka detectors and later confirmed, the ratio of muon to electron neutrino interactions was closer to 1:1.
Many explanations have been proposed for this "atmospheric neutrino anomaly." Hypotheses included greater abundance of electron neutrinos (perhaps from some unexpected though seemingly unlikely extraterrestrial source or nucleon decay), a problem with calculations of the neutrino flux or neutrino interaction rates or some problem unique to water detectors. Scientists suspected that oscillations might be the cause, but had no compelling, exclusive arguments. Some experimenters thought the problem would be resolved as an experimental artifact.
HOW WAS THE ANOMALY RESOLVED?
The new Super-Kamiokande data show the anomaly is due specifically to a deficit in the muon neutrinos--more come from "overhead" than come "up" through the earth. Super-Kamiokande analysis shows this can only be the result of the muon neutrino oscillating into another type of neutrino during long flight paths. Neutrinos coming through the earth (13,000 km) travel further and have greater distance in which to change from a muon neutrino to another neutrino and back again, through many cycles at typical energies. Neutrinos coming from overhead (20 km) have not had time to oscillate before reaching the detector. Neutrinos of higher energies oscillate more slowly. The net result is that we see muon neutrinos disappearing in proportion to their flight path and inversely proportional to their energy. This is the hallmark of the hypothesized oscillation phenomenon.
WHAT ARE OSCILLATIONS?
Neutrino oscillations are a peculiar quantum mechanical effect. It's hard to find a good macroscopic analogy as it has to do with the particle-wave duality of fundamental matter.
We only know what a particle is by the way it is produced or interacts; that is how we name it. When a pion decays, it results in a muon and a muon (anti)neutrino; when a neutron decays, it results in a proton, an electron and an electron (anti)neutrino. When a muon is produced by a neutrino we know it was produced by a muon neutrino. And so on.
Another way to know a particle is by weight, as expressed by speed given a certain amount of energy and also as it is attracted by gravity. Usually these identifications are the same for each particle, but muon neutrinos appear to be very mixed up.
If we create a muon neutrino beam at an accelerator and pass it through a kilometer of earth and iron shielding to eliminate all the charged particles, we see muons occasionally produced in a detector, in the right direction and just after the particle beam pulse strikes the production target. Neutrinos are well known particles in this sense, and their interactions have been studied at the particle accelerators, underground and at reactors for more than 30 years.
The strange situation for neutrinos, different from all the other elementary particles, is that the state of the particle which we call the muon neutrino may not be the same as the particle mass state. Neutrinos are a Dr. Jekyll and Mr. Hyde sort of affair. The muon neutrino is apparently composed of two different masses.
The muon neutrino may be composed of half each of two states of slightly different mass that oscillate in and out of phase with each other as they travel along, alternately interacting as a muon neutrino and then making a tau neutrino. Which is observed depends on where the detector intercepts the beam.
We have yet to do this experimentally using beams of neutrinos‹the distance for oscillations (hundreds of miles) has been too long. New experiments are being proposed based upon the information we are finding, however.
WHAT DO OTHER STUDIES SHOW ABOUT NEUTRINO MASS AND
The mass of neutrinos and the possibility of their oscillations has eluded researchers for many years. Accelerator-based experiments and others using reactors and radioactive sources have so far only yielded upper limits on neutrino masses, and no oscillations have been firmly observed.
Many experiments have sought to directly measure neutrino mass, which is very difficult. We know neutrinos are light, far less than the mass of the electron. Indeed, many theoreticians have thought neutrino mass would prove to be zero.
WHAT DOES SUPER-KAMIOKANDE DATA SHOW?
Every proposed alternate hypothesis explaining the atmospheric neutrino anomaly (detector systematics, input physics, alternative physics explanations) has been definitively ruled out. The Super-Kamiokande team spent the last year carefully examining every possible problem in the data that might confound their result; none was found. Only the hypothesis of muon neutrino oscillations fits the data, and it fits very well.
The paper being submitted to Physical Review Letters contains results based upon analysis of 4,700 neutrino interactions collected over 537 days that meets the rules of probability for a correct hypothesis. All systematic parameters lie within expectations; the results are robust insensitive to variations in event selection criteria, data sets, fitting algorithms and multiple independent analyses. In June 2000 the Collabortatoin updated the oscillations analysis with 1144 days of data, with the conclusions becoming statistically even stronger.
WHAT DO THE MUON NEUTRINOS OSCILLATE WITH?
Our data tell us unambiguously that muon neutrinos are oscillating, though we could not at first be sure with what other neutrino state. Various tests, which will be published in late 2000, now inidicate that the oscillating partner of the muon neutrino is NOT a new sterile neutrino, but is entirely consistent with being the tau neutrino. It remains to identify the appearance of the tau neutrino, but preliminary indications are encouraging.
ARE THE NEW FINDINGS CONSISTENT WITH OTHER DATA?
For the very same reason we only obtained these results in 1998, there is not much other data which could directly confront these results. Preliminary results suggest consistency with re-analysis of data from the IMB experiment. Moreover in 1998 and 1999 there appeared strongly supporting evidence now from the Soudan II (Minnesota, USA) and MACRO (Gran Sasso, Italy) experiments. In summary, no data which gainsays the results reported and there is supportive evidence.
Super-Kamiokande research was mostly funded by Japan's Mombusho (Ministry of Education, Science, Sports and Culture) and the U.S. Department of Energy, Division of High Energy Physics. The project was made possible by significant support from the Kamioka Mining and Smelting Company and many other corporations and individuals.
Locally, we particularly acknowledge significant support from the University of Hawaii, particularly the Department of Physics and Astronomy and the dean of the School of Natural Sciences. Many
colleagues in the department, the Institute for Astronomy and the School of Ocean and Earth Sciences at UH have contributed to this work in various supportive ways over many years.