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[escepticos] Neutrinos: El secreto está en la masa
Hola:
Aquí les pongo la nota de prensa original.
· · · · · · ·
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
http://ccdis.dis.ulpgc.es:8086/AAGC/aagc.html
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