Solution to 30-Year Neutrino Mystery Poses New Questions

June 21, 2001

Writer: Jerene Parkman
UA physics department communications

After a more than 30-year search, physicists from the United States, Canada and the United Kingdom this week announced concrete evidence that about two-thirds of the solar neutrinos change into other types, or flavors, on their way from the sun to the Earth, and that these neutrinos have mass.

For the past 30 years scientists have been trying to detect how many of these small particles, which are one of the fundamental building blocks of the universe, have been reaching the Earth. But something seemed to be amiss with their theories when the measuring devices used showed far fewer neutrinos making the journey to Earth than expected.

The discoveries, announced by the Sudbury Neutrino Observatory in Sudbury, Ontario, mean big things for both small and large-scale physics. The findings showed that the neutrinos oscillate, or change, from one flavor to another. In order for neutrinos to oscillate, they must have mass.

The "flavors" of neutrinos include electron, muon and tau neutrinos. They are named based on the subatomic particles that they are usually associated with. The solar neutrinos began as the electron type and then changed into other flavors as they traveled to the Earth.

Some physicists have wondered if the universe would collapse if neutrinos had enough mass, but the findings show that the mass of neutrinos is far short of the amount that would be needed for such results to occur.

In addition to neutrinos produced by processes that occur inside stars like our sun, cosmic neutrinos are believed to be produced at the birth of the universe according to the Big-Bang model, said UA physics Professor Ina Sarcevic, who studies neutrinos.

"Neutrinos are also produced by cosmic rays penetrating the Earth's atmosphere. It is plausible that neutrinos are also produced in astrophysical objects such as the Active Galactic Nuclei and gamma ray bursts," she added.

This discovery leads to a problem with the Standard Model of Physics, which has been a given in the science world -- until now. The Standard Model will have to be significantly modified because it currently allows for a massless neutrino.

At the same time, it was thought that "dark matter" could possibly be explained by a neutrino with mass. However, it appears that scientists will have to search for dark matter elsewhere due to the findings that confirmed earlier research that the particles can only account for a small part of the universe's invisible mass.

Because neutrinos are so small, with a mass of about one ten-millionth that of an electron -- and no charge -- it has been very difficult to study these particles. They can pass through anything, including the densest of materials, such as lead. And not just a thin shield of lead, but lead one light-year thick - about six trillion miles - unimpeded.

The sun and other stars emit neutrinos constantly by fusing hydrogen into helium, and yet Earth and other cosmic bodies are virtually transparent to them. Three trillion neutrinos pass through every square centimeter every second, but only one in 10 billion ever interacts with matter.

Sarcevic said that in addition to having no charge, these particles rarely interact with any kind of matter. For example, millions of these particles pass through our bodies every second. Also, because these particles are highly stable and neutral, they can travel in a direct line from the astrophysical source, undeflected by intervening magnetic fields, thus holding great promise for probing the deepest reaches of stars and galaxies.

Sarcevic has been one of the leaders in studies of high-energy neutrino interactions and neutrino oscillations with extragalactic sources.

She said that because understanding neutrino properties is crucial to both fundamental particle structure and to our knowledge of the history and eventual fate of the universe, it has long been recognized that detection of neutrinos is crucial for the advance of both particle physics and cosmology.

Sarcevic has proposed the way to search for extraterrestrial neutrinos, neutrinos produced in jets beamed from the black holes of Active Galactic Nuclei, neutrinos from gamma-ray bursters and neutrinos that originate in the topological defects formed in the early universe. She has proposed how Earth might be used as a particle sieve, namely as a filter for atmospheric muons and other particles that could be mistaken as byproducts of neutrino-nucleon interactions.

She has shown that a high-energy neutrino can beat the odds and collide with nucleons, due to the large density of quarks inside the nucleon, thus providing a possibility for detection of the extragalactic neutrinos and their oscillations.

Another UA scientist, Professor Adam Burrows of the departments of physics and astronomy, also does neutrino research. He works on the role of neutrinos in supernova explosions, where all the neutrino species important in the solar context are also of interest.

Burrows says that the recent findings are central to supernova research as the detailed interaction of neutrinos with matter and neutrinos with other neutrinos is determined.

"The results from the SNO collaboration have been long anticipated and are good news for the entire neutrino physics community. Something new under the sun," he said.


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