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When Neutrinos Change Type, Some Must Have Mass

The Sun emits electron-neutrinos, elementary particles of matter that have no electric charge and very little mass, created in vast numbers by the thermonuclear reactions that fuel our parent star.

Since the early 1970s, several experiments have detected neutrinos arriving on Earth, but they have found only a fraction of the number expected from detailed theories of energy production in the Sun.

This meant there was either something wrong with our theories of the Sun, or our understanding of neutrinos.

It turns out that our theories of how the Sun is powered seem to be correct, according to a team of scientists from the UK, the US and Canada whose latest results from research into solar neutrinos were announced on Saturday (20 April).

What's more, these ghostly particles have "chameleon" capabilities, changing from one type of neutrino into another on their journey from the Sun to Earth.

The scientists used data taken entirely from the Sudbury Neutrino Observatory (SNO) in Canada, which shows without doubt that the number of observed solar neutrinos is only a fraction of the total emitted from the Sun -- clear evidence that they change type en route to Earth.

Says Project Director Art McDonald of Queen's University, Canada, "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 in excellent agreement with calculations of the nuclear reactions powering the Sun. The SNO team is really excited because these measurements enable neutrino properties to be defined with much greater certainty in fundamental theories of elementary particles."

Neutrinos 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."

The results announced on Saturday at the Joint American Physical Society/American Astronomical Society meetings in Albuquerque, New Mexico, show that the number of electron-neutrinos detected is about 1/3 of the number expected according to calculations based on the latest sophisticated models of the solar core.

The SNO detector uses the unique properties of heavy water -- in which 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 total number of all three types of neutrino agrees well with the calculations. 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 said, "In order to make these measurements we had to restrict the radioactivity in the detector to minute levels and determine both neutrino signals and the detector background 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 last year, results from the detection of electron neutrinos at 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 -- or so it was thought. However, these conclusions were based on comparisons of the SNO results with those from a different experiment, the Super-Kamiokande detector, located in Japan.

Prof. Dave Wark of the University of Sussex and the Rutherford Appleton Laboratory, Oxford, commented, "Whenever a scientific conclusion relies on two experiments, and on the theory connecting them, it is twice as hard to be certain that you understand what is going on. We are therefore much more certain now that we have really shown that solar neutrinos change type."

The latest results, entirely from the SNO detector, (and which have been submitted to Physical Review Letters), are 99.999% accurate, and are of great importance because of the way in which physicists think that the neutrinos -- long thought to be massless particles -- change types, since that only happens if the different types have different masses.

Related website:

Sudbury Neutrino Observatory


[Contact: Peter Barratt ]

23-Apr-2002

 

 

 

 

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