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[escepticos] Neutrinos: El secreto está en la masa



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     EMBARGOED FOR RELEASE: 5 June 1998 at 05:00:00 ET US
     
     Contact: John Learned
     jgl en uhheph.phys.hawaii.edu
     808 956-2964
     University of Hawaii
     
     Mass And Oscillations Discovered For Elusive Neutrino 
     
     A team of Japanese and American physicists have produced evidence
     of mass and oscillations in neutrinos, elementary particles that
     individually have the smallest mass yet collectively may account
     for much of the mass of the universe. In a paper to be presented at
     the Neutrino 98 Conference in Japan on June 5 and submitted to the
     leading physics journal, the scientists present evidence that the
     ghostly elementary particles called neutrinos do possess mass and
     that they alternately change their identities in time as they
     travel. The results come from the first two years of data from
     Super-Kamiokande, a $100 million experiment in a
     12.5-million-gallon, stainless steel-lined cavity carved out
     beneath the Japanese alps, filled with ultra pure water and
     observed by 13,000 large area light detectors.
     
     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.
     
     FOLLOWING: Fact Sheet, Q&A, Timeline. Graphics, Photo available 808
     956-8856.
     
                      Neutrino Discovery--A Fact Sheet
                                      
     THE DETECTOR
     
     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.
     
     DATA COLLECTION
     
     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.
     
     THE CLAIM
     
     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.
     
                        History Of Neutrino Research
                                      
     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.
     
                Neutrinos And The Super-Kamiokande Discovery
                                      
     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.
     
     ACKNOWLEDGMENTS
     
     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.
     
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Víctor R. Ruiz                rvr en idecnet.com
    Agrupación Astronómica de Gran Canaria
   Sociedad de Meteoros y Cometas de España
 Asociación de Variabilistas de España - AVE
info.astro  http://www.astrored.org/infoastro
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