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Neutrino Oscillations Confirmed
mfg writes "The Sudbury Neutrino Observatory has found evidence that neutrinos can change type between the Sun
and Earth. See the
BBC news story for more details."
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In the "standard model" of particle physics, there are sixteen "elementary" particles, and their anti-particles. Three of these particles are the neutrinos, which come in three different flavors. It's long been known that their masses must be very small, and it's been thought that neutrinos might have zero mass, like the photon. However, neutrino oscillation implies that there is a mass *difference* between neutrino flavors, and a mass difference means that they can't all be zero. Thus this means that neutrinos have mass, and that's a very important theoretical issue!
Fermlab is in the process of building an X million dollar project to send neutrinos 735km to minnesota to see if they oscilatte during the trip... Kinda pointless now. The project is called NuMI, its kinda interesting, they were going to send neutrinos through the ground to an old mine- check out the NuMI web site.
For the people who have no idea what neutrinos oscillating is about - try here. It gives a good overview, made so someone like me could even understand it.
Experiment confirms Sun theories
The SNO was constructed to solve a mystery
By Dr David Whitehouse
BBC News Online science editor
Neutrinos - some of nature's most elusive sub-atomic particles - do change their properties as they travel through space.
We are much more certain now that we have really shown that solar neutrinos change type
Prof Dave Wark, University of Sussex New evidence confirms last year's indication that one type of neutrino emerging from the Sun's core does switch to another type en route to the Earth.
This explains the so-called solar neutrino mystery, which has had scientists puzzled for 30 years - why so few of the particles expected to emerge from the nuclear furnace in our star can actually be detected.
The new data mean the reactions put forward by physicists to describe how the Sun works are correct.
The data were obtained from the underground Sudbury Neutrino Observatory (SNO) in Canada.
Going underground
Neutrinos are ghostly particles with no electric charge and very little mass. They are known to exist in three types related to three different charged particles - the electron and its lesser-known relatives, the muon and the tau.
Electron-neutrinos are created in the thermonuclear reactions at the solar core. Because these reactions are understood, it has been possible to estimate the number of electron-neutrinos that should emerge from our star.
But it has baffled scientists for decades as to why just a third of this expected number could actually be detected.
Using the underground Sudbury neutrino detector, an international group of researchers has been able to determine that the observed number of electron-neutrinos is only a fraction of the total number emitted from the Sun - clear evidence that the particles change type en route to Earth.
SNO Project Director, Dr Art McDonald, of Queen's University, Canada, said the number of electron-neutrinos detected combined with the numbers of other types picked up at Sudbury gave a total that was consistent with scientists' understanding of the nuclear reactions occurring at the Sun's core.
All types
The Sudbury Neutrino Observatory is a unique neutrino telescope, the size of a 10-storey building, two kilometres underground, down a mine in Ontario.
The SNO detector consists of 1,000 tonnes of ultrapure heavy water, enclosed in a 12-metre-diameter acrylic-plastic vessel, which in turn is surrounded by ultrapure ordinary water in a giant 22-metre-diameter by 34-metre-high cavity.
The observatory detects about one neutrino per hour
Outside the acrylic vessel is a 17-metre-diameter geodesic sphere containing 9,600 light sensors or photomultiplier tubes, which detect tiny flashes of light emitted as neutrinos are stopped or scattered in the heavy water.
At a detection rate of about one neutrino per hour, many days of operation are required to provide sufficient data for a complete analysis.
Because SNO uses "heavy" water - the hydrogen atom in the water molecule has an extra neutron - it is able to detect not only electron-neutrinos through one type of reaction, but also all three known neutrino types through a different reaction.
Very accurate
Dr Andre Hamer, of the Los Alamos National Laboratory, US, said: "In order to make these measurements, we had to restrict the radioactivity in the detector to minute levels and determine the background effects very accurately to show clearly that we are observing neutrinos from the Sun."
The research not only improves our understanding of the Sun but of the elusive neutrinos as well.
The latest results, entirely from the SNO detector, (and which have been submitted to Physical Review Letters) are said to be 99.999% accurate.
Dr MacDonald said: "The SNO team is really excited because these measurements enable neutrino properties such as mass to be specified with much greater certainty for fundamental theories of elementary particles."
Mass differences
This announcement is confirmation of indications released in June 2001 that suggested that it was highly likely that neutrinos changed type on their way from the Sun.
However those conclusions were always tentative because they were based on comparisons of results from SNO with those from a different experiment, the Super-Kamiokande detector in Japan.
Professor Dave Wark, of the University of Sussex and the Rutherford Appleton Laboratory, UK, commented: "Whenever a scientific conclusion relies on two experiments, and on the theory connecting them, it is twice as hard to be certain that you understand what is going on.
"We are therefore much more certain now that we have really shown that solar neutrinos change type."
Professor Hamish Robertson of the University of Washington, US, added: "There's absolutely no question the neutrino type changes and now we know quite precisely the mass differences between these particles."
As an ex-member of SNO (my name (N. Tagg) is on the papers) as well as a current member of MINOS (the experiment you're reffering to at Fermilab) I can say that this is simply not true; the experiments are complimentary, not exclusionary.
In fact, there is a large quantity of work going on in this field. Current experiments include KamLAND, Borexino, Opera, NuMI-MINOS, Super-Kamiokande (when they finish their repairs in a year or so), K2K (KEK to Super-K), MiniBOONE the new JHF facility, plus a bunch more I'm forgettting.
There are several reasons for all this activity. First, there are at least two different types of oscillaitions. (The naive and over-simplified theory is that there is nu-electron to nu-mu oscillation, and nu-mu to nu-tau oscillation, the first of which is seen by SNO, the second of which is seen by atmosphereic neutrinos and by the beam experiments). There may be a third mode, which implies a new variety of neutrino (nicknamed 'sterile' for various reasons).
In addition, we're looking to prove that our theory about the oscillations is correct; that they really oscillate in the way we think they do (i.e. change back and forth between flavours on a given time scale that is dependent on energy and suchlike). We want to know the exact parameters in the theory, so the theorists have some hard numbers to much on to make better overarching theories. And, there's always the possibility that something entirely new will crop up in these studies.
(A note on that last: modern neutrino detectors were born out of eariler attempts to build proton decay experiments... but the neutrinos kept getting in the way! On the 'don't beat 'em, join 'em' approach, people started looking at the neutrinos themselves with more interest.)
--Nathaniel, prowling his favourite topic.
No. This is probably the single most common misconception about physical science; but a misconception it is.
A physical "law" is not a "theory that has been proven". The word "law", in physical science, is used to describe relations between independently observable properties of systems that have been detected through experimentation or observation. Thus we have Newton's Law of Gravitation, which relates an external observable property of an object (the force upon it) to intrinsic but observable properties of that object (its mass, the masses of other objects, and the distances between them); this is a physical law even though, strictly speaking, it isn't true (as we now know that it provides only an approximation, which holds reasonably well over certain domains of length and mass scale).
The fact is that theories are never proven to be true in science. A theory can be falsified, but can never be proven true. This is because no matter how much evidence you have collected in favor of a theory, it is always imaginable that tomorrow, someone will observe some phenomenon that contradicts it. We have tons and tons of evidence supporting conservation of momemtum in systems isolated from external forces; but no matter how much evidence we have, it is logically impossible for me to guarantee that tomorrow someone won't do a robust experiment that shows violation of conservation of momentum. I'll bet all the money in the world that won't happen, I'm confident it won't happen; but I cannot logically assert with 100% confidence that it cannot happen. You can never say with logical certainty what will happen in an experiment until you do the experiment; and because of this, scientific theories are not proven true. Instead of being "proven to be true," scientific theories are "supported by the weight of accumulated evidence"; it is the degree to which that accumulates evidence is convincing that determines the statue of the theory it supports.