Solar neutrino problem solved

Posted: June 20, 2001

Illustration of Sudbury Neutrino Observatory. Photo: SNO
Data from an unusual underground observatory have helped scientists solve a key mystery about the Sun, but have in turn raised new questions about fundamental particle physics, scientists announced Monday.

Physicists from Canada, the United States, and the United Kingdom said Monday that the first scientific results from the Sudbury Neutrino Observatory (SNO) show that the Sun generates as many neutrinos as predicted by existing models, but arrive at Earth in different forms. The results were released at the annual conference of the Canadian Association of Physics in Victoria, British Columbia.

The results may put to rest one of more vexing mysteries of modern astronomy: why past experiments detected as few as one-third the number of neutrinos predicted by models of solar physics, an issue that become known in the scientific community as the "solar neutrino problem."

"We now have high confidence that the discrepancy is not caused by problems with the models of the Sun but by changes in the neutrinos themselves as they travel from the core of the Sun to the earth," said Art McDonald, SNO project director.

The Sun generates its tremendous energy through nuclear fusion, combining hydrogen atoms to form helium. This process releases energy and also generates an additional, nearly massless particle called a neutrino. First predicted to exist by theorists in 1930, it took scientists a quarter-century to discover them, in large part because neutrinos can pass through matter and rarely interact with it.

The first efforts to measure the neutrino flux from the Sun started in the late 1960s. Physicist Ray Davis filled a tank with 600 tons of dry cleaning fluid, composed primarily of chlorine, and set up equipment to measure the amount of argon created in those rare cases when a neutrino collided with a chlorine atom. The tank was placed deep in an old mine in South Dakota to shield it from cosmic rays.

The results were surprising: the experiment detected only about one-third the neutrinos expected based on models of nuclear fusion in the Sun. Later experiments conducted elsewhere also found far fewer neutrinos than predicted. This meant that either the models for nuclear fusion in the Sun were wrong, or that something was happening to the neutrinos between their creations deep in the Sun and their arrival at the Earth.

To study this, a consortium of Canadian, American, and British universities constructed the Sudbury Neutrino Observatory. Located two kilometers below the surface in a nickel mine in Sudbury, Ontario, the observatory uses heavy water -- where the two atoms of hydrogen in each molecule are replaced with deuterium, a heavy isotope of hydrogen -- as a detector fluid. When neutrinos interact with heavy water, an electron is ejected from the molecule at a speed greater than the speed of light in the water itself, generating a flash of light known as Cerenkov radiation. By measuring those flashes scientists can measure the number of neutrinos and compare those figures with models.

Unlike past experiments, the SNO detector is sensitive to not only the neutrinos generated by the nuclear fusion process, known as electron neutrinos, but two other types, called mu and tau neutrinos. The SNO data showed that the total number of neutrinos detected was equal to the number of electron neutrinos predicted to come from the Sun. Thus, some of the neutrinos changed, or oscillated, to the other neutrino types during transit from the Sun to the Earth.

"Earlier measurements had been unable to provide definitive results showing that this transformation from solar electron neutrinos to other types occurs," said McDonald. "The new results from SNO, combined with previous work, now reveal this transformation clearly, and show that the total number of electron neutrinos produced in the Sun are just as predicted by detailed solar models."

While the results are a vindication for solar physicists, the results raise new problems for particle physicists, who canšt yet explain why neutrino oscillation takes place. "This transformation of neutrino types is not allowed in the Standard Model of elementary particles," said David Wark of the Rutherford/Appleton Laboratory and the University of Sussex. "Theoreticians will be seeking the best way to incorporate this new information about neutrinos into more comprehensive theories."

The results also provide some insights into cosmology. The evidence of neutrino oscillation, along with past studies, allowed physicists to put upper limits on the estimated mass of neutrinos. Combined with the number of neutrinos expected to exist in the universe, physicists estimate that the combined mass of those neutrinos roughly equals the total mass of all the visible stars in the universe.

While an impressive figure, the result is far less than the estimated mass of "dark matter" in the universe. "Even though there is an enormous number of neutrinos in the universe, the mass limits show that neutrinos make up only a small fraction of the total mass and energy content of the universe," said Hamish Robertson of the University of Washington.

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