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New evidence of neutrino oscillations.

Of the fundamental particles of matter, neutrinos are among the most elusive, though theorists postulate postulate: see axiom.  that these particles are plentiful in the universe. Originally thought to have no mass, neutrinos barely interact with other forms of matter, making them extremely difficult to detect.

Now, a 2-year experiment at the Los Alamos (N.M.) National Laboratory has produced evidence supporting the idea that neutrinos actually have a mass, albeit a small one. This finding has important implications for cosmology, because it allows neutrinos to account for much of the mass in the universe.

D. Hywel White of Los Alamos and his coworkers at 11 other institutions announced their preliminary findings last week and plan to submit them to PHYSICAL REVIEW LETTERS Physical Review Letters is one of the most prestigious journals in physics.[1] Since 1958, it has been published by the American Physical Society as an outgrowth of The Physical Review. .

According to the standard model of particle physics, neutrinos come in three varieties: the electron neutrino, muon neutrino, and tau neutrino, along with their antimatter antimatter: see antiparticle.

Substance composed of elementary particles having the mass and electric charge of ordinary matter (such as electrons and protons) but for which the charge and related magnetic properties are opposite in sign.

The theory says that these particles have no mass. If they do have a mass, however, neutrinos of one type would be able to transform themselves into neutrinos of another type through a process known as oscillation.

The Los Alamos experiment involved the liquid scintillator scin·til·la·tor
A substance that glows when hit by high-energy particles or photons.
 neutrino detector (LSND LSND Liquid Scintillator Neutrino Detector (Los Alamos) ), which is particularly sensitive to certain neutrino neutrino (ntrē`nō) [Ital.,=little neutral (particle)], elementary particle with no electric charge and a very small mass emitted during the decay of certain other particles.  transformations -- if they occur. The researchers used a particle accelerator to fire high-energy protons into water to create pions. These pions decay into muons, muon neutrinos, muon muon (my`ŏn), elementary particle heavier than an electron but lighter than other particles having nonzero rest mass.  antineutrinos, and electron neutrinos, but not electron antineutrinos.

The neutrinos collide with atomic nuclei in the detector, which consists of a vat of mineral oil surrounded by an array of photodetectors. The collisions create electrons and other charged particles, which leave detectable trails of light in the liquid.

In two runs, the second completed last November, the team detected 29 events indicating the presence of electron antineutrinos, which normally would be absent. One explanation is that they were created from other types of neutrinos; this suggests that neutrinos have a mass.

Considering limits set by the LSND's sensitivity, the neutrino's mass appears to be at least 0.5 electronvolt Electronvolt

A unit of energy used for convenience in atomic systems. Specifically, it is the change in energy of an electron, or of any particle having a charge numerically equal to that of an electron, when it is moved through a difference of potential of 1
. By comparison, an electron has a mass of about 51 1,000 electronvolts. Though small, such a neutrino mass, multiplied by the number of neutrinos in the cosmos, could contribute significantly to the universe's total mass.

At the same time, the LSND data don't necessarily rule out alternative explanations for the presence of electron antineutrinos. "With such an important result, we need more data to establish this hypothesis firmly," White says. The researchers plan to run another LSND experiment in August.
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Title Annotation:in experiments with a particle accelerator, researchers discovered evidence that neutrino particles may have mass
Author:Peterson, Ivars
Publication:Science News
Article Type:Brief Article
Date:Feb 11, 1995
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