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 look like being 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 2002]. What’s more, these ghostly particles have ‘chameleon’ type
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 have chameleon type properties and change
type en-route to Earth.

Says Pro”2 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 – 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 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 minimise
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 only happens if the
different types have different masses.

For further information

Additional information about the conference presentations, the SNO
laboratory, the neutrino measurements being made and the participating
institutions can be found at: www.sno.phy.queensu.ca A set of high
resolution lab photos can be downloaded from the SNO web site as well.

Video footage/photos of SNO can be obtained by contacting media relations
officer:
Paul De la Riva, Laurentian University at (705) 675-1151 Ext. 3406.

Interviews with SNO scientists can be arranged by contacting:
Paul de la Riva (Laurentian University) at (705) 675-1151 Ext. 3406,
or Nancy Marrello (Queen’s University) at (613) 533-6000, Ext. 74040.

Contacts:

Peter Barratt, PPARC Press Office

Tel: 01793 442025

Email: peter.barratt@pparc.ac.uk

U.K. Co-spokesmen are:

Professor David Wark, RAL/University of Sussex

01235 445094

Dr. Nick Jelley, Oxford University

011 441 865 273360

Dr. Art McDonald, SNO Project Director, Queen’s University.

(in Albuquerque via cell phone (613) 541-1405).

Dr. David Sinclair, Associate Director, SNO Project, Carleton University.

(in Albuquerque via cell phone (613) 371-3747.

Dr. Doug Hallman, SNO Director of Communications, Laurentian University.

(in Sudbury via cell phone (705) 691-5495)

Dr. Aksel Hallin, Queen’s University.

(in Kingston at (613) 533-6766)

Dr. George Ewan , Professor Emeritus, Queen’s University, original SNO
Co-spokesman.

(in Kingston at (613) 533-2698)

The principal scientific investigators at the eleven SNO institutions are
identified on the SNO website, together with contact information.

U.S. Co-spokesmen are:

Dr. R.G. Hamish Robertson, University of Washington

(206) 616-2745. Cell (425) 345-2471

(Doubletree Hotel, Albuquerque NM during the conference)

Dr. Eugene Beier, University of Pennsylvania

(215) 898 5979.

(Hyatt Hotel, Albuquerque NM during the conference)

Background Information

The Sudbury Neutrino Observatory is a unique neutrino telescope, the size of
a ten-storey building 2 kilometers underground in INCO’s Creighton Mine near
Sudbury Ontario planned, constructed and operated by a 100 member team of
scientists from Canada, the United States and the United Kingdom. Through
its use of heavy water, the SNO detector provides unique ways to detect
neutrinos from the sun and other astrophysical objects and measure their
properties. For many years, the number of solar neutrinos measured by other
underground detectors had been found to be smaller than expected from
theories of energy generation in the sun. This had led scientists to infer
that either the understanding of the Sun was incomplete, or that the
neutrinos were changing from one type to another in transit from the core of
the Sun. In results presented in June 2001, SNO scientists compared the
number of electron-type neutrinos reaching the SNO detector to the number of
neutrinos seen by a second reaction which includes contributions from the
other two types of neutrinos, making use of additional data from the
Super-Kamiokande detector in Japan. The observed difference in these two
numbers showed conclusively that neutrinos change their type enroute to
Earth, and arrive as a mixture of electron neutrinos and the other two
types. The results to be announced this month are based on the SNO
detector’s ability, through a third type of neutrino reaction, to measure
independently the total rate of all of the three known types of neutrinos.
The new data provides independent and more accurate information on the
neutrino changes and on the accuracy of models of the sun.

The SNO detector consists of 1000 tonnes of ultrapure heavy water enclosed
in a 12 meter diameter acrylic plastic vessel, which in turn is surrounded
by ultrapure ordinary water in a giant 22 meter diameter by 34 meter high
cavity. Outside the acrylic vessel is a 17 meter diameter geodesic sphere
containing 9600 light sensors or photomultiplier tubes, which detect tiny
flashes of light emitted as neutrinos are stopped or scattered in the heavy
water. The flashes are recorded and analyzed to extract information about
the neutrinos causing them. At a detection rate of about one neutrino per
hour, many days of operation are required to provide sufficient data for a
complete analysis. The laboratory includes electronics and computer
facilities, a control room, and water purification systems for both heavy
and regular water.

The construction of the SNO Laboratory began in 1990 and was completed in
1998 at a cost of $80M CDN with support from the Natural Sciences and
Engineering Research Council of Canada, the National Research Council of
Canada, the Northern Ontario Heritage Foundation, Industry, Science and
Technology Canada, INCO Limited, the United States Department of Energy, and
the Particle Physics and Astronomy Research Council of the UK. The heavy
water is on loan from Canada’s federal agency AECL with the cooperation of
Ontario Power Generation, and the unique underground location is provided
through the cooperation and support of INCO Limited. Measurements at the
SNO Laboratory began in 1999, and the detector has been in almost continuous
operation since November 1999 when, after a period of calibration and
testing, its operating parameters were set in their final configurations.

In June 2001, the second phase of measurements with the SNO detector was
begun, in which ultra pure sodium chloride (salt) was added to the heavy
water core of the detector, to enhance signals for some of SNO’s neutrino
reactions and add further to the accuracy of SNO’s neutrino determinations.
Plans call for the observatory to continue measurements until at least the
end of 2005, with a third phase of measurements set to begin in 2003.

Further background information can be found on the SNO website:
www.sno.phy.queensu.ca