A team of Japanese and American physicists have produced evidence of mass
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One of the three kinds of neutrinos, the muon flavor, has been found to disappear and reappear as it travels hundreds of kilometers through the earth. The energy and flight distance, from neutrino production in the atmosphere by cosmic radiation to the underground instrument, provide a measure of the difference between neutrino masses. This mass, while the smallest yet observed for elementary particles, is still sufficient that the relic neutrinos made in staggering numbers at the time of the Big Bang account for much of the mass of the universe.
These new results could prove to be the key to finding the holy grail of physics, the unified theory, observes University of Hawaii Professor of Physics and Astronomy John Learned, one of the authors. Neutrinos cannot now be neglected in the bookkeeping of the mass of the universe. One only gets such great data once or twice in a professional lifetime, maybe never. The Super-Kamiokande Collaboration will make a major statement June 5 at the Neutrino 98 Conference in Takayama, Japan. (See the XVIII International Conference on Neutrino Astrophysics and Astrophysics web site at www-sk.icrr.u-tokyo.ac.jp.
A paper is being submitted at the time of this release to Physical Review Letters, the premier journal of physics. The collaboration is led by University of Tokyos Institute for Cosmic Ray Research and includes six U.S. groups (Boston University; University of California, Irvine; University of Hawaii; Louisiana State University; State University of New York at Stony Brook; and the University of Washington) and eight from Japan (Gifu University, High Energy Research Organization (KEK), Kobe University, Niigata University, Osaka University, Tohoku University, Tokai University and Tokyo Institute of Technology) as well as other collaborators from both countries.
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The Super-Kamiokande detector is a 50,000-ton double-layered tank of ultra pure water observed by 11,146 photomultiplier tubes, each 20 inches in diameter. The equivalent of an acre of photocathode, it is the largest light detection area ever assembled by more than a factor of ten. Located in a specially carved out cavity in an old zinc mine 2,000 feet under Mount Ikena near Kamioka in the Japanese alps, the detector is reached by driving through a 2 km-long tunnel. The underground site also includes a huge reverse osmosis water filtration system, calibration electron accelerator, five trailers of electronics, the main control room, preparation areas, etc.
The Super-Kamiokande project has been collecting data since April 1, 1996. This discovery is based on data collected through January 15, 1998. Energetic charged elementary particles traveling at close to the vacuum speed of light (300,000 km per second) exceed the speed of light in water. This results in the optical equivalent of a sonic boom, Cherenkov radiation, in which a flash is emitted in a 42-degree half-angle cone trailing the particle. This nanosecond directional burst of blue light is detected with photomulitpliers. Its pattern, timing and intensity allow physicists to determine the particles direction, energy and identity.
Data are acquired at a high rate (about 100 triggers per second), partially processed and sent via fiber optics to the laboratory outside the mine, where they are archived and filtered into different analysis streams. Most of the results discussed in the current paper are deduced from the cases (two-thirds of the time) when a neutrino produces either a single electron or a single muon. These interactions are recorded in the inner 22.5 kilotons of water about 5.5 times per day.
Super-Kamiokande Collaboration claims the discovery of neutrino mass and oscillations. The claim is based upon atmospheric neutrino data, which resolves an anomaly uncovered in 1985 and confirmed and elaborated by subsequent experiments. In its analysis of the present data base, the team observed a deficit of muon neutrinos coming from great distances and at lower energies; the functional behavior of this deficit indicates that muon neutrinos oscillate, thus they have mass.
IMPLICATIONS OF THESE FINDINGS
Oscillations require neutrinos to have mass. Finding non-zero neutrino mass is big news for elementary particle physics, requiring revision of the Standard Model, which has fit all elementary particle data to date, but sets neutrino masses at zero.
The Super-Kamiokande team hopes the insight gained from the peculiar mixing observed between neutrinos spurs progress toward a unified theory that explains the generations or flavors and predicts particle masses. The team also infers that the total mass of neutrinos in the universe must be significant--at a minimum amounting to a significant fraction (10 - 100 percent) of the baryonic mass of the universe and perhaps representing the dominant mass in the universe.
In any event, neutrinos cannot now be neglected in the bookkeeping of the mass of the universe. Indeed, some theoretical calculations indicate that neutrinos may have played a crucial role in the production of an excess of matter over anti-matter, and are thus intimately linked to our very existence.
Clearly this is the single most important finding about neutrinos since their discovery. Some experts call this result the single most important result of the decade in elementary particle physics.
THE PHYSICS TEAM
The collaboration team includes about 100 physicists. from Japan and the United States. The lead Japan group is from the University of Tokyos Institute for Cosmic Ray Research, whose director, Professor Yoji Totsuka, is spokesman for the collaboration. Other Japanese institutions are Gifu University, the High Energy Research Organization (KEK), Kobe University, Niigata University, Osaka University, Tohoku University, Tokai University and Tokyo Institute of Technology. Major U.S. collaborators are from Boston University; University of California, Irvine; University of Hawaii; Louisiana State University; State University of New York at Stony Brook; and University of Washington. Other collaborators are from Brookhaven National Laboratory; California State University, Dominguez Hills; Los Alamos National Laboratory; University of Maryland and George Mason University. U.S. team coordinators are Professors Hank Sobel, UC Irvine (head of the old Reines neutrino group), and Jim Stone of Boston University. U.S. collaborators include many veterans from the IMB experiment.
1930--Pauli hypothesizes the existence of neutrinos to account for the beta decay energy conservation crisis.
1946--Sakata proposes the pi-mu scheme with a neutrino to accompany muon. (There is a long story about the confusion of mu for pi etc. Sakata and Inoue were the first to straighten it out and get the spins right and he essentially wrote down the correct decay scheme completely: pi -> munumu, mu -> enuenumu and noticed that both numu and nue are light, and neutral spin 1/2, and suggested that they might be different.
1956--Fred Reines and Clyde Cowan discover neutrinos using a nuclear reactor. (Reines later wins the Nobel Prize for this and other work.)
1957-62--Theoretical physicists speculate that neutrinos oscillate: Pontecorvo (sterile) and Sakata et al (flavor)
1961--Muon neutrinos are discovered at Brookhaven National Laboratories and it is confirmed that they are different from nues.
1965--The first natural neutrinos are observed by Reines and company in a gold mine in South Africa, setting first astrophysical limits.
1968--Ray Davis and colleagues begin first radiochemical solar neutrino experiment using cleaning fluid in the Homestake Mine in North Dakota, which results in the observed deficit known as the solar neutrino problem.
1976--The tau particle is discovered by Marty Perl at SLAC in Stanford, Calif. Analysis of tau decay modes suggests that nutau is neither nue nor numu. First experimental evidence of quarks.
1976--Designs for a new generation neutrino detectors made at Hawaii workshop, subsequently leading to IMB, HPW and Kamioka detectors.
1980s--The IMB, the first massive underground nucleon decay search instrument and neutrino detector is built in a 2,000-foot-deep Morton Salt mine near Cleveland, Ohio. The Kamioka experiment is built in a zinc mine in Japan.
1985--The atmospheric neutrino anomaly is observed by IMB and Kamioka.
1986--Kamioka group makes first directional counting observation of solar neutrinos and confirms deficit.
1987--The Kamioka and IMB experiments detect burst of neutrinos from Supernova 1987A, heralding the birth of neutrino astronomy and setting many limits on neutrino properties, such as mass.
1991-92--SAGE (in Russia) and GALLEX (in Italy) confirm the solar neutrino deficit in radiochemical experiments.
1995--Discovery of the top quark at Fermilab, completing list of six quarks.
1996--Super-Kamiokande, the largest ever detector, begins searching for neutrino interactions on April 1 at the site of the Kamioka experiment with a Japan-U.S. team of scientists.
1998--After analyzing more than 500 days of data, the Super-Kamiokande team reports finding oscillations and, thus, mass in muon neutrinos.
A Q&A prepared by John Learned, University of Hawaii professor of physics and Super-Kamiokande collaborator for the complete scientific text, see www.phys.hawaii.edu:80/~jgl/ or www-sk.icrr.u-tokyo.ac.jp.
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 neutrinoselectron, muon and tau. There are also three anti-neutrinos of the same flavors. The neutrinos get their names from their charged lepton brethrenin 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 raysof 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 (20,000 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. Its 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 neutrinosthe 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 OSCILLATIONS?
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 robustinsensitive to variations in event selection criteria, data sets, fitting algorithms and multiple independent analyses.
WHAT DO THE MUON NEUTRINOS OSCILLATE WITH?
Our data tell us unambiguously that muon neutrinos are oscillating, though we cannot be sure with what other neutrino state. We cannot yet resolve whether the muon neutrinos oscillate into tau neutrinos or a new sterile neutrino. There are hints both directions. We should be able to make the distinction definitively with Super-Kamiokande within the next year.
ARE THE NEW FINDINGS CONSISTENT WITH OTHER DATA?
For the very same reason we are just finding these results, there is not much other data which directly confronts these results. Preliminary results suggest consistency with re-analysis of data from the IMB experiment. In summary, no data which gainsays the results reported; there is some supportive evidence.
Super-Kamiokande research was mostly funded by Japans 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.