Electron Neutrinos and Antineutrinos

The electron neutrino (a lepton) was first postulated in 1930 by Fermi to explain why the electrons in beta decay were not emitted with the full reaction energy of the nuclear transition. The apparent violation of conservation of energy and momentum was most easily avoided by postulating another particle. Fermi called the particle a neutrino, but it was not experimentally observed until 1953. This elusive particle has no charge and almost no mass, so could penetrate vast thicknesses of material without interaction. The mean free path of a neutrino in water would be on the order of 10x the distance from the Earth to the Sun. In the standard Big Bang model, the neutrinos left over from the creation of the universe are the most abundant particles in the universe. This remnant neutrino density is put at 100 per cubic centimeter at an effective temperature of 2K (Simpson). The background temperature for neutrinos is lower than that for the microwave background (2.7K) because the neutrino transparency point came earlier. The sun emits vast numbers of neutrinos which can pass through the earth with little or no interaction. This leads to the statement "Solar neutrinos shine down on us during the day, and shine up on us during the night!" . Bahcall's modeling of the solar neutrino flux led to the prediction of about 5 x 10⁶ neutrinos/cm²s.

A remarkable opportunity for observing neutrinos came with Supernova 1987A when the Japanese observing team detected neutrinos almost coincident with the discovery of the light from the supernova.

Neutrinos interact only by the weak interaction. Their interactions are usually represented in terms of Feynman diagrams.

Detection of Neutrinos

The first experimental observation of the neutrino interacting with matter was made by Frederick Reines, Clyde Cowan, Jr, and collaborators in 1956 at the Savannah River Plant in South Carolina. Their neutrino source was a nuclear reactor (it actually produced antineutrinos from beta decay).

Modern neutrino detectors at IMB in Ohio and Kamiokande in Japan detected neutrinos from Supernova 1987A. A new neutrino detector at Sudbury, Ontario began collecting data in October of 1999. Another Japanese neutrino detector called Super Kamiokande became operational in April 1996.

An early set of experiments with a facility called the solar neutrino telescope, measured the rate of neutrino emission from the sun at only one third of the expected flux. Often referred to as the Solar Neutrino Problem, this deficiency of neutrinos has been difficult to explain. Recent results from the Sudbury Neutrino Observatory suggest that a fraction of the electron neutrinos produced by the sun are transformed into muon neutrinos on the way to the earth. The observations at Sudbury are consistent with the solar models of neutrino flux assuming that this "neutrino oscillation" is responsible for observation of neutrinos other than electron neutrinos.

Sudbury Neutrino Observatory

The new Sudbury Neutrino Observatory (SNO) consists of a 1000 metric ton bottle of heavy water suspended in a larger tank of light water. The apparatus is located in Sudbury, Ontario, Canada at a depth of about 2 km down in a nickel mine. A 18 m diameter geodesic array of 9,500 photomultiplier tubes surrounds the heavy water to detect Cerenkov radiation from the neutrino interaction which dissociates deuterium:

Show other detection reactions for SNO

The distinctive characteristic of the heavy water observatory is that it can measure both the electron neutrino flux and the total neutrino flux (electron, muon and tau neutrinos). It should allow them to determine whether neutrinos change flavors. If so, it could explain the solar neutrino problem and would show that the neutrinos have mass.

SNO began operating in production mode in October, 1999, and as of Summer 2000 had collected a sizable number of neutrino events both from the sun (the main focus of the experiment) and from atmospheric events with pions and muons. The Cerenkov cones of the solar neutrinos center about the direction opposite the sun, showing about the same flux at night as during the day. This was an expected result, since the mean free path of a neutrino in matter is about 22 lightyears in lead and having the earth in the path makes little difference. A sizable number of the atmospheric neutrino events come from below, having traveled all the way through the earth and forming the Cerenkov cone in the photomultiplier tubes at the top of the spherical heavy-water ball. These Cerenkov cones are scattered all around the sphere, while the solar ones of course show a precise anti-solar direction.

The depth of the detector protects it from the intense bombardment of cosmic ray muons which reaches the earth's surface. The detector measures only about 70 muon events per day, and they are easily distinguished from neutrino events since the muon interacts by the electromagnetic interaction and produces a much larger signal in the detector array.

In order to detect the ring of light which is the signature of Cerenkov radiation, the responses of all the photomultiplier tubes (PMTs) are monitored with a very short time scale. In order to be counted as an "event" in the detector, at least 20 PMTs must be triggered within an interval of 100 nanoseconds.

The Solar Neutrino Telescope

Raymond Davis of Brookhaven National Laboratory constructed a neutrino detector 1.6 km underground in the Homestake Gold Mine in Lead, South Dakota. The detector consists of a 378,000 liter tank of perchloroethylene, which is further isolated by being submerged in water. Theoretical expections were about one neutrino-chlorine interaction per day, but the measured solar neutrino events were about a third of that, raising serious questions about the abundance of solar neutrinos (the Solar Neutrino Problem).

The detection of neutrinos by this instrument was based on the interaction of neutrinos with chlorine nuclei to produce argon. The argon can be removed from the tank and measured so that the number of neutrinos captured in a given time interval can be determined.

The argon decays back to the chlorine isotope from which it was created by the process of electron capture. The detection of this transition is aided by the definite energy of the x-ray emitted during the electron capture process. This mine experiment was able to detect about 15 argon atoms a month, according to Simpson.

Perchloroethylene is ordinary dry-cleaning fluid, but 400,000 gallons is a lot of cleaning fluid. Davis denies the story that he was besieged by wire coat-hanger salesmen after the large purchase.

Detection of Supernova Neutrinos

Since the neutrino can pass through the entire Earth without interaction, it takes specialized techniques to detect one. After being postulated by Fermi in 1930 to explain anomalies in beta decay, they were not actually detected until 1953 by Reines and Cowan.

Detection of neutrinos is now well developed and a classic opportunity for neutrino detection occurred with Supernova 1987A. A burst of ten neutrinos was detected within a time interval of about 15 seconds at a neutrino detector deep in a mine in Japan. They had to penetrate the Earth to get to the detector.

More detail Energies in eV

Neutrino Mass?

No definite mass has been measured for the neutrino, and the standard comment about most experiments is "the results are consistent with zero mass for the neutrino". But this raises certain theoretical problems and there have been many attempts to set a range for the mass of the neutrino. Since its mass is evidently very small, if non-zero, the mass is usually stated in terms of its energy equivalent in electron volts. Most experiments conclude that the mass equivalent of the neutrino is less than 50 eV.

One of the recent pieces of information about neutrino mass came from the neutrinos observed from Supernova 1987A. Ten neutrinos arrived within 15 seconds of each other after traveling 180,000 light years, and they differed by a up to factor of three in energy. This limits the neutrino rest mass energy to less than about 30 eV (Rohlf).

New experimental evidence from the Super-Kamiokande neutrino detector in Japan represents the strongest evidence to date that the mass of the neutrino is non-zero. Models of atmospheric cosmic ray interactions suggest twice as many muon neutrinos as electron neutrinos, but the measured ratio was only 1.3:1. The interpretation of the data suggested a mass difference between electron and muon neutrinos of 0.03 to 0.1 eV. Presuming that the muon neutrino would be much more massive than the electron neutrino, then this implies a muon neutrino mass upper bound of about 0.1 eV.

The recent neutrino measurements at the Sudbury Neutrino Observatory are consistent with the modeled total neutrino flux and add evidence for neutrino oscillation, a process which can only occur if the neutrinos have mass.