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'Neutrino oscillation': The OPERA experiment likely seen the first tau-neutrino
May 31st, 2010 in Physics / General Physics
'Neutrino oscillation': The OPERA experiment likely seen the first tau-neutrinoEnlarge
(PhysOrg.com) -- After seven years since the start of construction of the OPERA experiment and three years of operation in the underground Gran Sasso Laboratory of the Italian National Institute of Nuclear Physics (INFN), one of the many billions of muon-neutrinos produced at the CERN accelerator complex (CNGS) has likely "transformed" into a tau-neutrino that has been observed by the OPERA apparatus.
This is an extremely important result. The observation of a few more of these tau-neutrino events over a large number of conventional muon-neutrino interactions will represent the long awaited proof of the direct conversion of one type of neutrino into another: the so called "neutrino oscillation" mechanism.
The disappearance of the initial neutrino flavour has already been observed by several experiments in the last 15 years, but the "direct appearance" is still the outstanding missing tile of the puzzle, and the OPERA experiment is unique worldwide for this purpose. Neutrino oscillation is today the only indication of new, fascinating physics beyond the so-called Standard Model of particles and interactions, opening the possibility of unexpected implications in cosmology, astrophysics and particle physics.
The experiment was inaugurated in 2006, when the first "normal" muonneutrinos were detected after a trip of 730 km from CERN, covered in about 2.4 milliseconds, at the speed of light. After then, a careful and tireless search started to find the tiny and very special signal induced by a tau-neutrino.
OPERA accomplishes its neutrino detection task with its "heart" made of more than 150,000 small units called "bricks" (for a total mass of 1,250 tons) each of them equivalent to a sophisticated photographic camera. Thanks to these bricks, made of a sandwich of lead plates and special photographic films, the OPERA researchers can detect all details of the "neutrino events" by accurately measuring the elementary particles produced in the interaction of the neutrino with the brick.
OPERA has been designed, realized and being conducted by a large team of researchers from all over the world: Belgium, Croatia, France, Germany, Israel, Italy, Japan, Korea, Russia, Switzerland, Tunisia and Turkey.
The computer display of the first tau-neutrino candidate event is shown above. One can see a detail of the region around the point of interaction of the neutrino (coming from the left of the figure) producing several particles identified by their tracks in the brick. The detection of the track with a "kink" is the likely signature of a tau-neutrino interaction, with a probability of about 98%. The picture describes a volume of only a few cubic millimetres, but rich of valuable information for the OPERA physicists.
The OPERA Collaboration presently includes about 170 researchers from 33 institutions and 12 countries.
More information: The OPERA experiment - http://operaweb.lngs.infn.it/spip.php?rubrique39
Please see below an AFP story
Physicists solve mystery of missing neutrinos
Scientists in Europe announced Monday they had likely solved the case of the missing neutrinos, one of the enduring mysteries in the subatomic universe of particle physics.
If confirmed in subsequent experiments, the findings challenge core precepts of the so-called Standard Model of physics, and could have major implications for our understanding of matter in the universe, the researchers said.
For decades physicists had observed that fewer neutrinos -- electrically neutral particles that travel close to the speed of light -- arrived at Earth from the Sun than solar models predicted.
That meant one of two things: either the models were wrong, or something was happening to the neutrinos along the way.
At least one variety called a muon-neutrino was actually seen to disappear, lending credence to a Nobel-winning 1969 hypothesis that the miniscule particles were shape-shifting into a new and unseen form.
Now scientists at Italy's National Institute for Nuclear Physics have for the first time observed -- with 98 percent certainty -- what they change into during a process called neutrino oscillation: another type of particle known as tau.
"This will be the long-awaited proof of this process. It was a missing piece of the puzzle," said Antonio Ereditato, a researcher at the Institute and spokesman for the OPERA group that carried out the study.
"If true, it means that new physics will be required to explain this fact," he said by phone.
Under the prevailing Standard Model, neutrinos cannot have mass. But the new experiments prove that they do.
One implication is the existence of other, as yet unobserved types of neutrinos that could help clarify the nature of Dark Matter, which is believed to make up about 25 percent of the universe.
"Whatever exists in the infinitely small always has repercussions in the infinitely big," Ereditato said.
"A model which could explain why the neutrino is so small without vanishing will have profound implications for the understanding of our universe -- how it was, how it evolved, and how it will eventually die."
The transformation of the neutrino occurred during a programmed journey from Geneva to the Gran Sasso Laboratory near L'Aquila in central Italy.
The European Organization for Nuclear Research (CERN) provided a laser-like beam composed of billions upon billions of muon neutrinos that took only 2.4 milliseconds to make the 730-kilometer (453-mile) trip.
The rarity of neutrino oscillation, coupled with the fact that the particles interact only weakly with matter, bedeviled the scientists.
Unlike charged particles, neutrinos are not sensitive to the electromagnetic field normally used by physicists to bend the trajectory of particle beams.
They can also pass through matter, and thus keep the same direction of motion from their inception.
It took nearly four years from the time the beam was switched on to witness the muon-to-tau metamorphosis.
Provided by Gran Sasso Laboratory
"'Neutrino oscillation': The OPERA experiment likely seen the first tau-neutrino." May 31st, 2010. www.physorg.com/news194544551.html
Robert Karl Stonjek