April 20, 2002
Sensitive Measurement by SNO Observes Solar Neutrinos in a New Way
(This press release is being simultaneously issued by several collaborating institutions.* )
A team of scientists from Canada, the US and the UK today announced the results of a unique new measurement of the total number of all known neutrino types reaching the Earth from the Sun. Using data entirely from the Sudbury Neutrino Observatory (SNO) in Canada they are also able to determine that the observed number of electron neutrinos (the type produced by the Sun) is only a fraction of the total number. This shows with great certainty that neutrinos from the Sun change from one type to another before reaching the Earth.
Says Project Director Art McDonald of Queen’s University, ”These new results show in a clear, simple and accurate way that solar neutrinos change their type. The total number of neutrinos we observe is also in excellent agreement with calculations of the nuclear reactions powering the Sun. The SNO team is really excited because these measurements enable neutrino properties such as mass to be specified with much greater certainty for fundamental theories of elementary particles.”
Neutrinos are particles with no electric charge and very little mass. They are known to exist in three types related to three different charged particles - the electron and its lesser known relatives the muon and the tau. The Sun emits electron-neutrinos, which are created in the thermonuclear reactions in the solar core. Previous experiments have found fewer electron-neutrinos than suggested by calculations based on how the Sun burns – the famous “solar neutrino problem.”
SNO uses the unique properties of heavy water – where the hydrogen has an extra neutron in its nucleus – to detect not only electron-neutrinos through one type of reaction, but also all three known neutrino types through a different reaction.
The results presented today at the Joint American Physical Society/American Astronomical Society meetings in Albuquerque, New Mexico show that the number of electron-neutrinos observed is only about 1/3 of the total numbe r reaching the Earth. This shows unambiguously that electron-neutrinos emitted by the Sun have changed to muon- or tau-neutrinos before they reach Earth.
Dr. Andre Hamer of Los Alamos National Laboratory told the meeting, “In order to make these measurements we had to restrict the radioactivity in the detector to minute levels and determine the background effects very accurately to show clearly that we are observing neutrinos from the Sun. The care taken throughout this experiment to minimize radioactivity and the careful calibration and analysis of our data has enabled us to make these neutrino measurements with great accuracy.”
In June 2001, results from the detection of electron-neutrinos in SNO first indicated, with a certainty of 99.9%, that neutrinos change type on their way from the Sun, thus solving the long-standing problem. However, these conclusions were based on comparisons of results from SNO with those from a different experiment, the Super-Kamiokande detector in Japan. The new results, obtained entirely from the SNO, are so accurate that it is 99.999% probable that solar neutrinos change type before reaching Earth. The results, which have been submitted to Physical Review Letters, are of great importance because the way in which the neutrinos – long thought to be massless particles - change types is thought to be linked to neutrino mass and mass differences between various neutrino types.
Says Professor Hamish Robertson of the University of Washington, "It was a dramatic and exciting moment for us when we first saw the neutrons being produced by this type of neutrino interaction and realized there were three times as many as you would get if only electron neutrinos were coming from the Sun. There's absolutely no question the neutrino type changes and now we know quite precisely the mass differences between these particles.”
Dr. Richard (Dick) Hahn, leader of the Brookhaven National Laboratory group that is working in SNO, agreed. “These results are exciting because they demonstrate the full potential of the SNO neutrino detector,” said Hahn. “All of the collaboration’s hard work over many years is really paying off now.”
Brookhaven’s history of neutrino research dates to the early 1970s, when scientist Ray Davis’s pioneering work in a South Dakota gold mine sent the neutrino world into an uproar by first documenting the missing electron neutrinos.
* Collaborating institutions -- From Canada: Queen's University, Carleton University, Laurentian University, University of Guelph, University of British Columbia, Chalk River Laboratories (to 1996). From the U.S.: Lawrence Berkeley National Laboratory, Los Alamos National Laboratory, University of Pennsylvania, University of Washington, Brookhaven National Laboratory, Princeton University (to 1992), University of California at Irvine (to 1989). From the U.K.: Oxford University