New SNO Data Resolves Solar Neutrino Problem

The solar neutrino problem has been settled and the ability of neutrinos to change from one type, or "flavor," to another established directly for the first time by the efforts of the Sudbury Neutrino Observatory (SNO) collaboration.

This finding gives physicists new confidence that they understand how energy is produced in the sun's core and that neutrinos are just as quirky as we thought.

The benevolent sunlight we receive on Earth has its origin in the sun's central fusion furnace, whence the light must fight its way outwards in a series of scatterings that takes, on average, hundreds of thousands of years.

Solar neutrinos, setting out from the same place, flee unhindered, thus providing the most unadulterated indication of activity at the core. Measurements dating back to the 1960's of this neutrino flux were puzzling: only a fraction of the expected number arrived at detectors on Earth.

Suspicion naturally fell on the experiments and on the standard solar model (SSM) used to calculate the flux. Soon, however, the neutrinos themselves were implicated.

If on their journey to Earth some of the neutrinos had changed into muon-or tau-neutrinos, then terrestrial detectors designed only to spot electron neutrinos (e-nu's) would be cheated of their rightful numbers.

SNO is a unique neutrino telescope, the size of a ten-story building, two kilometers underground in INCO's Creighton Mine near Sudbury, Ontario, operated by a 100-member team of scientists from Canada, the U.S. and the U.K.

It is designed to scrutinize a particular reaction in the sun: the decay of boron-8 into beryllium-8 plus a positron and an e-nu. The experimental goals are threefold: to prove neutrinos change their flavor; to measure the number of neutrinos coming from the Sun; and to determine the relative masses of neutrinos.

SNO's gigantic apparatus consists of 1000 tons of heavy water held in an acrylic vessel surrounded by a galaxy of phototubes, the whole residing 2 km beneath the Earth's surface in an Ontario mine, the better to filter out distracting background.

Last year SNO reported first results based on reactions in which a solar neutrino enters the detector and either (1) glances off an electron in one of the water molecules, or (2) combines with a deuteron to create an electron and two protons, a reaction referred to as a "charged current" (CC) interaction since it is propagated by the charged W boson.

The SNO data, when supplemented with ES data from the Super Kamiokande experiment in Japan, provided preliminary evidence a year ago for the neutrino-oscillation solution for the solar neutrino problem.

Now the definitive result has been tendered by SNO scientists at the Albuquerque meeting. The new findings update last year's CC and ES data and introduce, for the first time, evidence deriving from a reaction in which the incoming neutrino retains its identity but the deuteron (D) is sundered into a proton and neutron; this is why SNO went to such trouble and expense of using the D2O for the weakly-bound neutron inside each D. This interaction, called a neutral-current (NC) reaction because the weak interaction is carried by a neutral Z boson, is fully egalitarian when it comes to neutrino scattering; unlike last year's ES data, the NC reaction allows e-nu's, mu-nu's, and tau-nu's to scatter on an equal footing.

The upshot: all the nu's from the sun are directly accounted for. The missing nu-e flux shows up as an observable mu-nu and tau-nu flux. This conclusion is established with a statistical surety of 5.3 standard deviations, compared to the less robust 3.3 of a year ago. The measured e-nu flux (in units of one million per sq. cm per second) is 1.7 while that for the mu-nu and tau-nu combined is 3.4. (When one includes neutrinos of all energies, the flux from the sun is billions/sq. cm/sec.)

"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," said Hamish Robertson of the University of Washington, one of the collaboration scientists. "There's absolutely no question the neutrino type changes, and now we know quite precisely the mass differences between these particles."

The issue of how the neutrino changes from one flavor to another can even be addressed by viewing the day-night asymmetry of neutrino flux. When the whole of the earth is between the sun and the detector (night viewing), the oscillation process, which depends on a density of matter through which the nu proceeds, should be speeded up. This type of measurement also contributes to the study of neutrino masses and mixings.

An experiment like SNO can measure not mass but the square of the mass difference between nu species. Even if the nu mass is quite small (much lighter than the previously lightest known particle, the electron) it might still have played a large role in cosmology, where it might have been instrumental in shepherding galaxies; in supernovas, neutrinos might carry away as much as 99% of an exploding star's energy.

Editor's Note: The SNO team has submitted its results to Physical Review Letters; preprints are available at the online preprint server: nucl-ex/0204008 and 0204009; see also http://www.sno.phy.queensu.ca.