'Missing' neutrinos found

On 18 June 2001, the first scientific results of Sudbury Neutrino Observatory (SNO) were published, bringing the first clear evidence that neutrinos oscillate (i.e. that they can transmute into one another), as they travel in the sun. This oscillation in turn implies that neutrinos have non-zero masses. The total flux of all neutrino flavours measured by SNO agrees well with the theoretical prediction. Further measurements carried out by SNO have since confirmed and improved the precision of the original result.

The first measurements of the number of solar neutrinos reaching the earth were taken in the 1960s, and all experiments prior to SNO observed a third to a half fewer neutrinos than were predicted by the Standard Solar Model. As several experiments confirmed this deficit the effect became known as the solar neutrino problem. Over several decades many ideas were put forward to try to explain the effect, one of which was the hypothesis of neutrino oscillations. All of the solar neutrino detectors prior to SNO had been sensitive primarily or exclusively to electron neutrinos and yielded little to no information on muon neutrinos and tau neutrinos.

Way back in 1930 theoretical physicist Wolfgang Pauli predicted that there should be such a thing as a neutrino. When physicists make such predictions, they are working with data and equations, and they sometimes find that something is missing in their data—the math doesn't come out right unless some other thing, still unknown, exists. Pauli was in that situation—he thought that there must be a particle as large as an electron, but with no charge. He named this not-yet-discovered particle a neutron.
A few years later, another physicist discovered a much larger neutral particle that he named neutron. So the as-yet-undiscovered particle got renamed neutrino.
In 1956, physicists were able to detect neutrinos (well, it turned out that they were closely-related particles called anti-neutrinos) created in nuclear reactors. And in the 1960s, scientists were able to detect and even count neutrinos coming from the Sun, using huge tanks of dry-cleaning liquid deep, deep underground. (Those few incoming neutrinos that hit a chlorine atom in the liquid changed the chlorine to argon. The amount of argon in the liquid was then measured.)
Once they were able to detect and measure neutrinos, scientists noticed that Earth was receiving only one third to one half of the expected solar neutrinos. Again, our equations were scrutinized—this time, the equations that modeled how nuclear fusion operates deep in the Sun's core. Could there be something wrong with our model? Could something be missing?
The Sudbury Neutrino Observatory reported on this day in 2001 that the missing neutrinos were there, all along, but had changed “flavors” in their long journey through the outer layers of the Sun, through space, and through our own atmosphere. The earlier neutrino detectors had only detected electron neutrinos, but the SNO used heavy-water detectors that also detected muon and tau neutrinos. With data on all three flavors of neutrinos, the expected number was found, and our model of how the Sun operates was confirmed

Neutrinos are elementary particles of matter with no electric charge and very little mass. There are three types: the electron-neutrino, the muon-neutrino and the tau-neutrino. Electron-neutrinos, which are associated with the familiar electron, are emitted in vast numbers by the nuclear reactions that fuel the Sun. 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 something wrong with either the theories of the Sun, or the understanding of neutrinos.
The determination that the electron neutrinos from the Sun transform into neutrinos of another type is very important for a full understanding of the Universe at the most microscopic level. This transformation of neutrino types is not allowed in the Standard Model of elementary particles. Theoreticians will be seeking the best way to incorporate this new information about neutrinos into more comprehensive theories.

Sudbury Neutrino Observatory (SNO)
The Sudbury Neutrino Observatory (SNO) is a neutrino observatory located 6,800 feet (about 2 km) underground in Vale Inco's Creighton Mine in Sudbury, Ontario, Canada. It is a unique neutrino telescope, the size of a ten-storey building, constructed and operated by a 100-member team of scientists from Canada, the United States and the United Kingdom. The detector was designed to detect solar neutrinos through their interactions with a large tank of heavy water. The SNO detector, which is located 2000 meters below ground in INCO's Creighton nickel mine near Sudbury, Ontario, uses 1000 tonnes of heavy water to intercept about 10 neutrinos per day. The detector was turned on in May 1999, and was turned off on 28 November 2006. While new data is no longer being taken, the SNO collaboration will continue to analyze the data taken during that period for the next several years. The underground laboratory has been enlarged and continues to operate other experiments at SNOLAB. The SNO equipment itself is currently being refurbished for use in the SNO+ experiment.