June 22, 2010
Scientists Measure Mass Of Neutrino Particles
Scientists have made their most accurate measurement yet of the "ghost particles" known as neutrinos, according to a recent BBC report.
Neutrinos are called ghost particles because they interact so weakly with other forms of matter.
Previous experiments show that neutrinos have a mass, but it was so tiny it was unable to be measured.
Researchers used data from the largest every survey of galaxies and put the mass of a neutrino at no greater than 0.28 electron volts.
The scientists say this is less than a billionth of the mass of a single hydrogen atom.
The study's results are to be published in the journal Physical Review Letters and will be presented at the Weizmann U.K. conference at University College London (UCL) this week.
"Back in 2002, we put an upper limit on the neutrino mass of 1.8 electron volts. So this is an improvement by a factor of six," co-author Professor Ofer Lahav, from UCL, told BBC.
"It is remarkable that the distribution of galaxies on huge scales can tell us about the mass of the tiny neutrinos."
UCL scientists Shaun Thomas, along with Professor Lahav and Dr. Filipe Abdalla, produced the work.
The scientists used the largest ever 3D map of galaxies in the Universe, based on data gathered by the Sloan Digital Sky Survey.
The researchers determined a new upper limit for the neutrino particle by analyzing the distribution of galaxies across the universe.
The matter in the universe naturally forms into "clumps" of galaxy groups and clusters.
Because neutrinos are extremely light, they can move quickly across the Universe. The scientists say this helps smooth out the natural "clumsiness" of matter.
Professor Lahav says this is like ocean waves smoothing out a pile of sand on a beach.
Scientists were able to work out the upper limits of neutrino mass once they analyzed the extent to which this "smoothing-out" of galaxies has taken place.
Professor Lahav says the ghost particles are a minor component of cold dark matter, which is mysterious "stuff" that comprises of about 25% of the Universe and over 80% of matter in the Universe.
"The neutrino is squeezed into that slice [of the Universe] that is dark matter. But it probably accounts for less than one percent of that dark matter," he told BBC.
The neutrino particle is available in three varieties: muon, tau and electron. Physicists caught a neutrino in the act of changing from one type to another during a recent experiment.
Researchers at the Opera experiment in Italy used the findings to provide a missing piece in the puzzle that has challenged scientists for years.
U.S. scientists Ray Davis observed far fewer neutrinos arriving at the Earth from the Sun than moles predicted in the 1960s. Either the models were wrong, or something happened to the neutrinos on their way.
One theorist suggests that chameleon-like oscillatory changes between different types of neutrinos could be responsible for the apparent deficit.
Several experiments have confirmed the oscillation theory after observing the disappearance of muon neutrinos.
However, no observations of the appearance of a tau-neutrino beam have been observed, until the Opera results.
Another project known as Minos reported results that point to a fundamental difference between neutrinos and their anti-matter counterparts, known as "anti-neutrinos."
A beam of muon anti-neutrinos was fired from the Fermilab particle accelerator in Chicago through the Earth to the Soudan underground lab in Minnesota.
The researchers found a relatively large difference in the way neutrinos and anti-neutrinos oscillated between one type and another. This difference could not be explained by the established theory of particle physics, known as the Standard Model.
Image Caption: Messier 51, The Whirlpool Galaxy. The SDSS image of this famous spiral galaxy (interacting with a smaller neighbor at the lower left) occupies about three one-millionths of the total sky area imaged by the SDSS. The SDSS imaging survey detected about 100 million galaxies, most of them much more distant, and thus much smaller and fainter in appearance, than M51. Some of these distant galaxies can be seen as small extended sources on this image, while the sharper, point-like sources are mostly foreground stars in our own Milky Way galaxy. The diameter of M51 is roughly 75,000 light years. Credit: The Sloan Digital Sky Survey.
On the Net:
- Physical Review Letters
- University College London (UCL)
- Sloan Digital Sky Survey
- Opera experiment
- Minos project
- Soudan Lab