Queen's University
Kingston, Ontario

June 9, 1999

First Neutrino detected at the Queen's-led Sudbury Neutrino Observatory

An international team of scientists, research associates and graduate students associated with the Sudbury Neutrino Observatory are celebrating the first detection of neutrinos at this one-of-a-kind underground observatory.

Among its first images are stunning examples of the pools of light formed by the interactions of neutrinos that began life in the sun or in the atmosphere on the opposite side of the Earth.

"This is tremendously exciting," says Art McDonald, Queen's professor and SNO Institute Director. "After all the hard work which has been devoted to the SNO project, to see such clear examples of neutrino interactions within days of finally turning on the full detector was a real triumph for the entire SNO team."

Located 6,800 feet underground, SNO is part of a world-wide effort to understand neutrinos, the basic building blocks of the universe. The results from the SNO experiments are expected to help answer questions about the nature of matter at the smallest scales and provide insight into the structure of the stars and the Universe as a whole.

"This is the most exciting physics experiment of this decade," says George Ewan, emeritus professor at Queen's University and first Canadian spokesman for the SNO project. "It is a dream come true. Now we can do the exciting experiments we started discussing in 1984."

SNO is a collaboration of nearly 100 scientists from 11 universities and laboratories in Canada, the US and the UK.

Background information, images of the SNO detector and a neutrino signal:

SNO website

Carleton University
Ottawa, Canada KIS 5B6

Carleton University a key player in Sudbury Neutrino Observatory success

OTTAWA, June 9, 1999 -- The Sudbury Neutrino Observatory (SNO) announced today that the first neutrinos had been detected at the SNO facility located near Sudbury, Ontario. Carleton University, through its Centre for Research in Particle Physics (CRPP), is one of the five Canadian universities participating in this international mega-project, and a key contributor to the operations and on-going project management.

Scientists at SNO are excited that the detector began picking up neutrinos within days of being turned on. Among its first images are stunning examples of the patterns of light formed by the interactions of neutrinos that began life in the sun or in the atmosphere on the opposite side of the Earth. Dr. David Sinclair of Carleton's CRPP presented examples of these images at the Annual Congress of the Canadian Association of Physicists in Fredericton, New Brunswick yesterday. Some of the images, with a full description, are available on the Web at http://www.sno.phy.queensu.ca.

Located 2000 metres underground in a nickel mine in Sudbury, Ontario, Canada, SNO is shielded from cosmic rays and other sources of unwanted "background." It is designed to detect neutrinos from sources beyond the Earth, in particular from the Sun, which bathes each square centimetre of the Earth's surface with billions of neutrinos a second. SNO was built by a collaboration of nearly 100 scientists from 11 universities and laboratories in Canada, the US and the UK. Carleton's CRPP was responsible for the design, construction (working with Carleton's Science Technology Centre), installation and operation of the water systems for the detector.

The observatory uses heavy water as a detection medium for neutrinos from the sun and other astrophysical sources. The SNO detector sees neutrinos -- tiny ghostlike particles which are fundamental building blocks of matter -- through the faint flashes of light that are produced as they are stopped or scattered in 1000 tonnes of heavy water at the heart of the detector. These systems purify the light and heavy waters to remove radioactive impurities and to monitor the residual impurities. The impurity levels at which they must operate are as low as a few atoms per ton of water -- about 1000 times lower than could have been detected previously. The water systems must also ensure the safety of the large heavy water inventory. Carleton faculty also play an important managerial role in the project. The two Associate Directors of SNO are Dr. Anthony Noble (Operations) and Dr. Sinclair (Science), both from Carleton's CRPP.

Backgrounder

Although neutrinos are the commonest particles in the Universe -- at least as common as the photons of light -- some of their basic properties, such as how much they "weigh", are still not known for sure. This is mainly because neutrinos are not at all easy to study as they interact rarely with other matter and so are very difficult to "see."

SNO has been built to help resolve some of the mysteries that continue to surround neutrinos, in particular the puzzle of why previous experiments do not detect as many neutrinos from the Sun as expected. The results will help to answer questions about the nature of matter at the smallest scales, as well as provide insight into the structure of the stars and the Universe as a whole.

Since only 10-20 solar-neutrino events are expected per day, and careful analysis is required for neutrinos and background events, the Sudbury Neutrino Observatory has a multi-year measurement program planned. It is expected that the results from SNO will contribute unique information on neutrinos and their properties in the near future. The world's only heavy water based neutrino detector, SNO has the potential to make a major contribution to the world-wide research effort on the properties of neutrinos and their role in the universe.

The SNO project has been supported by the federal and provincial governments in Canada (NSERC, Industry Canada, National Research Council of Canada, FEDNOR, Northern Ontario Heritage Fund, Province of Ontario), by the U.S. Department of Energy and the U.K. Particle Physics and Astronomy Research Council. The Observatory is sited in INCO's Creighton mine through the extensive cooperation of INCO Limited. The heavy water from Canada's reserves is on loan from AECL with the cooperation of Ontario Hydro.

Lawrence Berkeley National Laboratory

FIRST NEUTRINOS DETECTED AT SUDBURY NEUTRINO OBSERVATORY

Berkeley, June 9 -- The Sudbury Neutrino Observatory (SNO), a unique type of telescope that operates from deep within a cave in Canada has seen its "first light." The light was created when neutrinos, the ghostlike particles emitted from the sun and supernovae, interacted with SNO's 1000 tons of heavy water.

SNO is a collaboration involving more than 100 scientists from 11 laboratories and universities in the United States, Canada, and the United Kingdom. Its mission is to answer some of the most perplexing questions about neutrinos which are the most common particles in the universe but interact so rarely with other matter that one could pass untouched through a wall of lead stretching from the earth to the moon. It is estimated that the sun bathes each square centimeter of Earth's surface with billions of neutrinos a second and yet scientists don't know how much a single neutrino weighs.

SNO has been built to help resolve some of the mysteries that continue to surround neutrinos, in particular the puzzle of why previous experiments do not detect as many neutrinos from the sun as expected. The results are expected to help answer questions about the nature of matter at the smallest scales, as well as provide insight into the structure of the stars and the universe as a whole.

One of the participating institutes in the SNO collaboration is the Lawrence Berkeley National Laboratory. Said Kevin Lesko, leader of Berkeley Lab's SNO effort, "It is very rewarding after all the work of the entire collaboration to see SNO begin operating and immediately permit the study of neutrinos.

Berkeley Lab scientists and engineers designed and built that panel arrays which house SNO's 9,600 photomultiplier tubes (PMTs), ultra-sensitive light-sensors that detect faint flashes of light called Cerenkov radiation produced when a neutrino is stopped or scattered by heavy water (deuterium oxide or D2O).

Berkeley's PMT array is performing "flawlessly " Lesko says.

Among SNO's first images are stunning examples of the pools of light formed by the interactions of neutrinos that began life in the sun or in the atmosphere on the opposite side of the earth.

Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified research and is managed by the University of California. Visit our website at http://www.lbl.gov.

University of Wisconsin-Madison

March 17, 1999

Detector in polar ice to hunt for neutrinos

The hunt for the cosmic neutrino is on

This winter, after an extensive shakedown period, the Antarctic Muon and Neutrino Detector Array or AMANDA, a novel telescope set kilometers deep in the ice at the South Pole, began its search for the ghost-like cosmic neutrino. The nearly massless particle is rocketed through space, scientists think, by supernovas, black holes, quasars, gamma ray bursts and whirling neutron stars.

Unlike any other astronomical telescope ever built, AMANDA is not a telescope in the conventional sense. It is composed of 422 basketball-sized glass orbs, photomultiplier tubes arranged on cables and sunk deep into the Antarctic ice in concentric rings.

The device looks down through the Earth and is designed to catch the fleeting signals left by cosmic neutrinos, high-energy particles that are believed to emanate from objects deep in space and whose bizarre properties permit them to pass through entire planets without skipping a beat.

If AMANDA successfully detects cosmic neutrinos and traces their paths back to the objects from which they come, it will open a new window to the universe, permitting scientists to study some of the most intriguing phenomena in the cosmos, according to Francis Halzen, a UW-Madison scientist who helped develop the telescope.

"We've spent over a year understanding the idiosyncratic nature of this instrument," says Halzen. "Nobody's ever built anything like this before."

AMANDA was built with extensive support from the National Science Foundation and in collaboration with other institutions in Europe and the United States.

The AMANDA telescope works by detecting the fleeting flashes of blue light created by muons, particles created when neutrinos occasionally collide with other subatomic particles called nucleons. The muon's flash of light creates a bow wave much like that made by a boat in water. In theory, the bow wave will point back to the source from which the neutrino comes.

The deep Antarctic ice is crystal clear and, at great depths, is free of air bubbles and nearly free of other imperfections. It serves as an ideal medium in which to look for the rare signals left by the billions of neutrinos that continuously pass through the Earth.

To detect these signals, AMANDA looks down through the Earth to suspected neutrino sources in the sky of the Northern Hemisphere.

"If something emits a lot of gamma rays, it's a good bet there are a lot of neutrinos there," says Robert Morse, a UW-Madison physicist who has spent years helping oversee the construction of the AMANDA telescope.

Suspected sources include black holes, the remains of supernovas, and neutron stars, planet-sized, burned out husks of stars that spin at amazing speeds. Other potential sources are what scientists call active galactic nuclei, things like quasars and blazers, extremely bright and energetic objects at the centers of distant galaxies.

What all of these objects have in common, says Morse, is that they act like enormous versions of the accelerators scientists build on Earth to study high-energy, subatomic particles. They also are at great distances from Earth.

"The sources are far away. Gamma ray bursts, for instance, could be three to five billion light years away, or maybe even half way to the suspected edge of the universe. So you need a big detector," Morse says.

In conventional forms of astronomy, the photon, the particle that makes up visible light and other parts of the electromagnetic spectrum, is what is sampled by telescopes on remote mountaintops, satellites and radio telescopes. But photons can be deflected and absorbed as they traverse space and encounter interstellar dust and pockets of gas and radiation. The cosmic neutrino, on the other hand, is unhindered by such obstacles.

The tradeoff, says Morse, is that neutrinos are very hard to detect. Moreover, the sun and cosmic rays crashing into the Earth's atmosphere also make neutrinos, creating a soup of high-energy particles. But neutrinos from different sources, whether the sun or from a distant black hole, have defining characteristics that would permit scientists to identify the particles of interest.

"It's like a police line-up," says Morse. "They have to pass the test."

Over the past year, the AMANDA telescope has been tuned and tested and has succeeded in sampling neutrinos, but not the cosmic neutrinos of interest.

"We've gotten the apparatus tuned up to the point that what we're seeing really are neutrinos," Halzen says. "But the majority of the neutrinos we've seen are atmospheric neutrinos. What we have to do now is pick out that one event out of 10 million."

Yet the neutrinos now being sampled by AMANDA are the highest energy neutrinos ever detected, according to Albrecht Karle, a UW-Madison physicist. And the muons they spawn are tracked in the AMANDA detector for distances of up to 400 meters through the crystal clear Antarctic ice.

Constructed at a cost of $7 million over seven years, the AMANDA detector will nearly double in size next year with the addition of seven more strings, each with 48 photomultiplier tubes. The ultimate configuration, says Morse, is a proposed cubic kilometer detector of 80 to 100 strings with as many as 5,000 to 6,000 photomultiplier tubes.

The larger telescope will not only make a bigger target for the elusive cosmic neutrino, but also will make a key diagnostic test, measuring the energy of neutrino particles more precisely. That enhancement would permit a search for neutrino oscillations on a cosmological scale, says Morse.

"Neutrinos can bring us a message of the most violent and cataclysmic processes occurring at the very edge of the universe -- colliding black holes, neutron stars and maybe even colliding galaxies," Morse says. "But it's very difficult to make the measurements. AMANDA, we think, is our best bet to do that."

AMANDA: Facts at a glance

What is AMANDA?

AMANDA stands for Antarctic Muon and Neutrino Detector Array. A sort of telescope, it consists of strings of photomultiplier tubes sunk deep into the Antarctic ice using a hot-water drill. AMANDA is designed to detect the fleeting signatures of light left by cosmic neutrinos as they pass through the polar ice cap.

What are neutrinos?

Neutrinos are particles without electric charge and, as far as scientists know, they have no mass. Neutrinos are hard to find because their interaction with matter is extremely feeble, producing nothing more than a fleeting burst of light. Large, very sensitive detectors like AMANDA have the best chance to find the distant and exotic sources of cosmic neutrinos.

Where do neutrinos come from?

Scientists believe cosmic neutrinos are constantly bombarding the Earth from many distant sources, including:

  • Black holes: Massive black holes in the centers of galaxies swallow up most everything with huge gravitational pull.

  • Other galaxies: Supernovae, which are exploding stars, distant galaxies are believed to emit neutrinos.

  • Quasars: Quasars (quasi-stellar objects) have been enigmatic because they emit prodigious amounts of energy from a very compact source.

    Who cares?

    Scientists are searching for cosmic neutrinos because they promise insight into some of the most violent and enigmatic objects in the universe: black holes, supernovae and quasars. Finding the elusive particle would open a new realm of astronomy.

    Who pays?

    AMANDA is funded primarily by the National Science Foundation. UW-Madison and University of California-Irvine and UC-Berkeley have also made substantial contributions. Other partners in the AMANDA collaboration include Lawrence Berkeley National Laboratory, Stockholm University, University of Uppsala and DESY, the German national high-energy physics laboratory.

    IMAGE CAPTION:

    Frances Halzen, a UW-Madison scientist who helped develop AMANDA, shows the inside of one of the 422 basketball-sized glass orbs, called photomultiplier tubes, used to catch the fleeting signals left by high-energy neutrinos.

    University of Minnesota

    March 15, 1999

    U OF M, FERMILAB TO PROBE NATURE OF NEUTRINOS

    MINNEAPOLIS / ST. PAUL -- Physicists are preparing to go underground to solve one of nature's most baffling mysteries: whether the elusive subatomic particles known as neutrinos have mass. In search of a definitive answer, Fermi National Accelerator Laboratory in Chicago will send a beam of neutrinos underground to a University of Minnesota-run detection facility in Soudan Underground Mine State Park in northern Minnesota. Physicists will look for evidence that neutrinos leaving Fermilab change their character, or "flavor," before arriving at the mine a fraction of a second later; such a change would prove that neutrinos have mass.

    The university has selected the Hugo, Minn., firm Lametti & Sons to excavate the new underground laboratory in Soudan that will house the Minnesota detector for the $146 million neutrino experiment, called MINOS (Main Injector Neutrino Oscillation Search). The experiment, funded by the U.S. Department of Energy, the United Kingdom and the state of Minnesota, is expected to begin gathering data in 2002. Led by principal investigator Stanley Wojcicki (Voy-JIT-ski) of Stanford University, MINOS involves about 200 scientists from 20 institutions in five countries. MINOS is part of Fermilab's NUMI (Neutrinos at the Main Injector) project, headed by physicist Tom Fields.

    Researchers hope that if the mass of neutrinos can be determined, so can their contribution to the total mass of the universe. Physicists estimate that about 80 to 90 percent of the mass in the universe is "dark matter": matter that can't be seen. Of this, neutrinos could account for as much as 10 percent. If so, their combined mass -- and the gravity associated with objects that have mass -- could have played a role in the formation of stars and galaxies throughout the universe. Further, knowing how much, if any, mass is tied up in neutrinos might help physicists develop a Theory of Everything to explain gravity, electromagnetism and the forces operating in the atomic nucleus, all in the same terms.

    "Neutrinos are the lightest particles with mass, assuming they have mass," said University of Minnesota physicist Earl Peterson. "We want to know what the family ties between neutrinos are, just as we already know the family ties between quarks -- the building blocks of protons and neutrons."

    Previous studies of neutrinos coming from the upper atmosphere have hinted that the particles may have mass and change flavor while in motion. But studying the behavior of atmospheric neutrinos is difficult and fraught with uncertainties. Starting with a controlled and well understood population of neutrinos generated by a particle accelerator should make it easier to sort out what's going on, the researchers said.

    "If the results from previous experiments turn out to be correct -- if, indeed, neutrinos have mass -- a new and very exciting area of scientific exploration will open up," said Wojcicki. "All of us are looking forward to being part of this adventure."

    Two things for certain about neutrinos: They have no electric charge, and they are exceedingly small. Therefore, they usually pass through the densest matter without bumping into anything. This makes them very hard to detect. The University of Minnesota detection facility in the old Soudan iron mine in Tower, Minn., will await the beam of neutrinos with about 10 million pounds of steel plates -- a huge, dense target to maximize the chance that neutrinos will hit an atomic nucleus.

    "We'll probably run the beam of neutrinos nine months of the year for four years," said Peterson. "Each pulse will contain trillions of neutrinos. We might get a neutrino interaction, or hit, in about one in a thousand pulses. Each hit will produce a spatial pattern of electrical signals in detectors between the steel plates."

    Neutrinos exist in three flavors: tau, muon and electron. They are produced naturally in the environment -- for example, within the sun. Neutrinos are also produced when very energetic cosmic rays -- nuclei of atoms streaming in from space -- crash into atoms in the atmosphere. The collisions produce sprays of subatomic particles, which decay to leave two muon neutrinos for every electron neutrino. However, experiments detect too few muon neutrinos to correspond with that ratio. This deficit suggests that muon neutrinos change -- or oscillate, as physicists put it -- into other kinds of neutrinos as they travel from the upper atmosphere to detectors on the Earth.

    Similarly, physicists in the MINOS experiment will be on the lookout for missing muon neutrinos. Fermilab will generate a beam of muon neutrinos and direct it through 445 miles of earth and rock to the Soudan mine in Tower, Minn. There, half a mile underground, the massive steel detector will determine whether the muon neutrinos all arrived, or whether some of them changed into other kinds en route. The detection must be performed underground to prevent interference from the millions of particles generated by cosmic rays. The beam from Fermilab will send a pulse of neutrinos every 1.9 seconds. Each pulse will contain 300 trillion (300 million million) neutrinos. The distribution of neutrinos in the universe is about 300 per cubic centimeter.

    Columbia University

    February 16, 1999

    Columbia Physicist Building 800-Ton Vat Of Oil To Observe Elusive Oscillating Neutrino

    A Columbia University physicist is building an underground vat that will hold 800 tons of mineral oil in an attempt to trap the elusive oscillating neutrino.

    Janet Conrad, assistant professor of physics at Columbia, leads an experiment at Fermi National Accelerator Laboratory -- Fermilab, outside Chicago, that will accelerate one kind of neutrino into a powerful beam, then fire that beam into the ultra-pure mineral oil in an effort to witness those neutrinos transform themselves into another kind. Such a transformation, called a "neutrino oscillation," would be proof positive that neutrinos have mass, early indications of which have already turned the physics world upside down.

    Her work also travels outside the laboratory. She has taken research techniques used to observe neutrinos and translated them into classroom experiments for Columbia students, to demonstrate their utility to other fields. Medical research has been profoundly influenced by basic physics research, and some treatments for cancer were developed at particle accelerators, Professor Conrad points out. When neutrinos interact with the nucleus of a molecule of mineral oil, a tiny flash of light, called a "scintillation," is emitted; medical researchers also observe scintillations in studies of radioisotopes. Professor Conrad is designing a series of laboratory experiments to demonstrate basic science techniques to undergraduates and high school students.

    She received the nation's top science award for young researchers at a White House ceremony Feb. 10 for her "original contributions to measuring neutrino mass and connecting the measurement techniques to applications in medicine to inspire undergraduate and K-12 students."

    Neal Lane, director of the White House Office of Science and Technology Policy, presented the third annual Presidential Early Career Awards in Science and Engineering to 20 young researchers from across the nation. The awards are granted by the National Science Foundation and are worth $500,000 over five years. They were established by President Clinton in February 1996.

    Professor Conrad expects to use the funding to help construct the Booster Neutrino Experiment, or BooNE, at Fermilab. She was also the recipient last summer of a Faculty Career Development Award and, in 1996, a Career Advancement Award, both from NSF and both to study neutrinos. "We're extremely grateful to NSF for their continued support of this important basic science, and hope to merit the trust they have placed in us," she said.

    Enrico Fermi coined the term "neutrino"; it means "little neutral one" in Italian. Physicists have since determined that neutrinos come in three varieties, or generations, corresponding to the groupings of subatomic particles in the prevailing physics theory, the Standard Model: electron, tau and muon neutrinos. The BooNE collaboration, which includes about 40 physicists and physics students from 11 institutions, is attempting to show that neutrinos of one generation can change into neutrinos of another.

    "Neutrino oscillations are closely tied to the question of neutrino mass," said Professor Conrad, who is one of two spokespersons, or leaders, of the BooNE collaboration. "If we do see that neutrinos can change types, then quantum mechanics says that they must have mass."

    Since neutrinos are so pervasive and numerous -- any space the size of a clothes closet holds at least a billion of them -- the notion that they have mass has already shaken the physics world. Astrophysicists are revising their calculations of the total mass of the universe and their models of its evolution and eventual fate. Theoreticians are devising new schemes to relate fundamental particles and forces, and even the Standard Model is getting a facelift. There is also hope that finding a neutrino with mass will help resolve the "missing mass" question: the mass of the visible, luminous matter in the universe is nowhere near enough to account for the gravitational attraction astrophysicists observe among stars, galaxies and clumps of galaxies.

    Because neutrinos do not interact very often, physicists who study them need large detectors and an intense neutrino beam to see them. BooNE, approved by Fermilab last spring and set to run in the year 2001, is a spherical underground tank 40 feet in diameter holding 800 tons of ultra-pure mineral oil, similar to the baby oil sold in drugstores. In extremely rare events, a neutrino will interact with a molecule of mineral oil; when it does so, it emits a tiny flash of light. The tank will be lined with approximately 1500 phototubes, photon-sensitive detectors that will observe the photons emitted by neutrino interactions.

    The neutrino beam is generated in the Booster, one of Fermilab's underground particle accelerators. It will accelerate a very pure beam of muon neutrinos and direct them into the vat of mineral oil. If scientists observe a significant number of electron neutrinos in the oil, they will know that oscillations have taken place.

    Dr. Conrad's investigations are not taking place in a vacuum. Several experiments already have offered researchers clues that neutrinos have mass, however slight or imperceptible. Most notably, researchers at the Super- Kamiokande experiment west of Tokyo reported in June that they observed extremely rare interactions between neutrinos and atomic nuclei, from which they inferred the mass of a neutrino, in an underground tank containing 50,000 tons of ultra-pure water. Other indications of neutrino oscillations have come from the Homestake Experiment, a cavern filled with dry-cleaning fluid in the former Homestake gold mine in South Dakota. And the Liquid Scintillator Neutrino Detector, or LSND, experiment at Los Alamos National Laboratory in New Mexico has seen about 50 interactions that may indicate muon neutrino to electron neutrino oscillations.

    To verify that what LSND investigators observed are really oscillations, Professor Conrad and her collaborators need to find many more of these interactions under different experimental conditions. If the effect continues to occur, then it will be clear that the LSND result is really the result of oscillations. The BooNE collaborators will conduct experiments designed to conclusively verify or disprove the LSND result.

    "Three different experiments have now seen indications for oscillations," Professor Conrad said. "But in every case, very few events have been seen, so it is hard to be sure."

    The program of experiments to be run on BooNE was developed by Professor Conrad and by Michael Shaevitz, professor of physics at Columbia. The research group also includes five Columbia graduate students and four undergraduates.

    US Super-Kamiokane Home Page

    PHYSICS NEWS UPDATE

    The American Institute of Physics Bulletin of Physics News
    Number 375 June 5, 1998

    by Phillip F. Schewe and Ben Stein

    NEUTRINO OSCILLATION HAS BEEN DEMONSTRATED at the Super-Kamiokande lab in Japan to a higher degree of certainty than in previous experiments. Neutrinos, weakly interacting elementary particles only detected for the first time in 1956, are thought by some theorists to reside in a kind of schizoid existence; that is, a neutrino would regularly transform (or oscillate) among several alternative neutrino states, each having a slightly different mass. Such a theory would help to explain the apparent shortfall of neutrinos coming from the Sun. The oscillation proposition has been tested using four neutrino sources: the Sun, Earth's atmosphere, reactors, and particle accelerators. Some tests find tentative but ambiguous evidence for oscillation. Today, at the Neutrino 98 conference in Takayama Japan, the Super-Kamiokande collaboration (comprising 100 scientists from 23 institutions in Japan and the US) is announcing the most exacting evidence yet for neutrino oscillation. They study neutrinos made when cosmic rays from outer space strike the upper atmosphere. Some neutrinos, those made overhead above Japan, travel about 20 km or so before entering the underground detector. Other neutrinos, those made in the atmosphere on the far side of the globe, have a travel path of 20,000 km into the detector. In either case, they create, among other things, a high energy electron or muon, which in turn emits a telltale cone of light (Cerenkov radiation) observed by an array of thousands of photodetectors mounted in a tank filled with pure water. Sorting events by electron neutrino or muon neutrino, by high energy or lower energy, and by zenith angle (overhead approach or through the Earth), statistical evidence for oscillation becomes evident. A 1-GeV muon neutrino seems to oscillate every few hundred miles. Four years ago, the same group, using a smaller detector, reported preliminary evidence on the basis of 200 events (Physics Today, Oct 1994). The new report is based on several thousands of events, and provides an approximate mass difference (the test cannot render any neutrino species' mass directly) of about 0.07 eV. Because they are so numerous in the universe, neutrinos, with even a small mass, might play an important role in the formation of galaxies. (See http://www.phys.hawaii.edu:80/~jgl/nuoascstory.html)

    Official Super-Kamiokande Press Release

    Takayama, Japan
    June 5, 1998

    EVIDENCE FOR MASSIVE NEUTRINOS

    We announce today at "Neutrino '98", the international physics conference underway in Takayama, Japan, that the Super-Kamiokande Experiment has found evidence for non-zero neutrino mass. Neutrinos are tiny, electrically neutral, sub-atomic particles. Papers related to the results were submitted to the scientific journals "Physical Review Letters" and "Physics Letters." The experiment yields results that are outside the standard theory of particle physics, which describes the fundamental constituents of matter and their interactions. Until now, there has been no firm evidence that neutrinos possess mass.

    The new evidence is based upon studies of neutrinos which are created when cosmic rays, fast-moving particles from space, bombard the earth's upper atmosphere producing cascades of secondary particles which rain down upon the earth. Most of these neutrinos pass through the entire earth un-scathed. The Super-Kamiokande group uses a large, 50,000 ton tank of highly purified water, located about 1000 meters underground in the Kamioka Mining and Smelting Company Mozumi Mine. Faint flashes of light given off by the neutrino interactions in the tank are detected by more than 13,000 photomultiplier tubes that were manufactured for the experiment by Hamamatsu Corporation.

    By classifying the neutrino interactions according to the type of neutrino involved (electron-neutrino or muon-neutrino) and counting their relative numbers as a function of the distance from their creation point, we conclude that the muon-neutrinos are "oscillating". Oscillation is the changing back and forth of a neutrino's type as it travels through space or matter. This can occur only if the neutrino possesses mass. The Super-Kamiokande result indicates that muon-neutrinos are disappearing into undetected tau-neutrinos or perhaps some other type of neutrino (e.g., sterile-neutrino). The experiment does not determine directly the masses of the neutrinos leading to this effect, but the rate of disappearance suggests that the difference in masses between the oscillating types is very small. The primary result that we are reporting has a statistical significance of more than 5 standard deviations. An independent measurement based on upward-going muons in the detector confirms the result at the level of more than 3 standard deviations.

    The Super-Kamiokande Collaboration includes scientists from 23 institutions in Japan and the United States. Principal funding for the experiment is provided by the Japanese Ministry of Education, Science, Sports, and Culture (Mombusho) while funding for the detector's outer most region is provided by the United States Department of Energy. In addition to advancing our understanding of basic science, the collaboration has established a strong international partnership between the Japanese and American teams.

    Since the beginning of its operation in April, 1996, the Super-Kamiokande experiment has been the most sensitive in the world for monitoring neutrinos from various sources. In our studies, we have found interesting results in the measurements of electron-neutrinos coming from the sun. The number detected is about 35% of the number predicted by the well established theoretical model of the sun's neutrino producing processes. In addition, we obtained an indication that the observed energy spectrum of those neutrinos is deformed from the the predicted one. Super-Kamiokande's observation of too few electron-neutrinos coming from the direction of the sun also may be interpreted as due to oscillations. We are continuing to study this exciting possibility.

    Reflecting on the significance of the new finding, we note that massive neutrinos must now be incorporated into the theoretical models of the structure of matter and that astrophysists concerned with finding the 'missing or dark matter' in the universe, must now consider the neutrino as a serious candidate.

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