UW Today

January 31, 2002

UW scientists learn how many neutrinos sun generates

News and Information

Neutrinos are among the tiniest particles in the universe. They’re also among the more perplexing problems physicists face.

Scientists working at huge underground laboratories in Japan and Canada have made major strides in understanding neutrinos during the last three years. Now a team working with a particle accelerator at the UW has added another significant finding, determining with the greatest precision yet just how many energetic neutrinos are generated in the sun’s nuclear furnace.

They found that the fusion rate producing those subatomic particles in the sun is 17 percent greater than previously thought, said Kurt Snover, a UW research professor in physics who heads the team. And, he added, the new number is accurate to within 3 or 4 percentage points, compared to 15 points for the old standard. The findings mean the sun must be producing 17 percent more energetic neutrinos (the highest-energy solar neutrinos) than scientists previously thought.

The research, published in the Jan. 28 edition of Physical Review Letters, was done by Snover, Arnd Junghans, Erik Mohrmann, Tom Steiger, Eric Adelberger, Jean-Marc Casandjian and Erik Swanson, all of the UW Center for Experimental Nuclear Physics and Astrophysics. Also taking part were Lothar Buchman, Sehwan Park and Alex Zyuzin from Canada’sCanada’s National Laboratory for Particle and Nuclear Physics in Vancouver, British Columbia.

Neutrinos come from several natural sources. The highest-energy neutrinos are produced by cosmic rays outside the solar system, while solar neutrinos have lower energies. Either way, the particles come in three types — what physicists call “flavors” — electron, muon and tau. A fourth type, called a sterile neutrino, also could be a factor.

Experiments at the Super-Kamiokande detector in Japan and the Sudbury Neutrino Observatory in Canada in the last three years have shown that these particles can change from one flavor to another, proof that neutrinos have mass. That finding is important for researchers trying to find the so-called “missing mass” of the universe, mass that is theorized to have resulted from the Big Bang but which so far has been unaccounted for.

For more than two decades there has been a physics problem associated with solar neutrinos. An experiment begun in 1965 at Homestake Gold Mine in South Dakota proved that the particles were bombarding Earth, but at far lower levels than expected.

“People didn’t know if the calculations of how many neutrinos coming from the sun was wrong, whether there was a problem with the experiment or a third possibility, that neutrinos traveling from the sun changed their character,” Snover said.

Together, the Sudbury and Super-Kamiokande experiments demonstrated the third possibility was the correct one. All solar neutrinos start in the electron “flavor,” but the two underground experiments showed that some of those change to the tau or muon varieties, and that the total of all three varieties coming from the sun totals roughly what physicists would expect to see.

The solar neutrino production rate, determined from calculations performed at other institutions, is figured from the sun’s temperature; the amounts of beryllium, hydrogen, helium and other materials used for fuel; and the rates at which the materials combine with each other, including the fusion rate of beryllium-7 with hydrogen.

“We determined this fusion rate much more precisely with our experiment,” Snover said.

The UW team used a particle accelerator to fire protons (nuclei of hydrogen atoms) in a particle beam at a tiny piece of beryllium-7, a radioactive metal isotope. In the experiment, the beryllium-7 is held on a rotating arm that moves the metal in front of the particle beam, where it is bombarded by high-speed protons and transformed into boron-8. The arm then quickly swings the metal fragment in front of a detector that verifies that boron-8 has been produced. From there the scientists deduce neutrino production, since the new isotope has a half-life of less than a second before it gives off a neutrino. By studying the boron-8 production in the experiment, Snover’s group is able to determine a key factor in calculating the solar neutrino production rate.

The U.S. Department of Energy and the Natural Sciences and Engineering Research Council of Canada finance the work, which is a step in improving the understanding of particle physics.

“It helps us understand better the differences in mass, as well as other properties, among these character-changing neutrinos,” Snover said.

“It’s factors like these that go into the soup pot when you’re trying to figure out what are the properties of a neutrino.”