Neutrinos and Their Detection

Neutrinos are a type of subatomic particle known both for their ubiquity and their disinclination to interact with other forms of matter. They have zero electric charge and very little mass even compared to other fundamental particles (though not none, more on this later) so they are not affected by electromagnetic forces and only slightly by gravity.

Since neutrinos are so elusive, it is not surprising that their existence was first surmised indirectly. In 1930, while studying a type of radioactive decay known as beta decay, physicist Wolfgang Pauli noticed a discrepancy. Through beta decay (shown above), a neutron is converted into a proton. This is a common process by which unstable atomic nuclei transmute into more stable ones. It was known that an electron was also released in this process. However, Pauli found that this left some momentum unaccounted for. As a result, he postulated the existence of a small, neutral particle (this properties eventually led to the name "neutrino"). The type emitted in this sort of decay is now known as an electron antineutrino (all the types will be enumerated below).

However, they were speculative for some decades before a direct detection occurred in 1956 in the Cowan-Reines Neutrino Experiment, named after physicists Clyde Cowan and Frederick Reines.

The experiment relied upon the fact that nuclear reactors were expected to release a large flux of electron antineutrinos during their operation, providing a concentrated source with which to experiment. The main apparatus of the experiment was a volume of water that electron antineutrinos emerging from the reactor would pass through. Occasionally, one would interact with a proton in the tank, producing a neutron and a positron (or anti-electron, denoted e⁺) through the reaction shown on the bottom left. This positron would quickly encounter an ordinary electron and the two would mutually annihilate to form gamma rays (γ). These gamma rays would then be picked up by scintillators around the water tanks. To increase the certainty that these gamma ray signatures in fact came from neutrinos, the experimenters added a second layer of detection by dissolving the chemical cadmium chloride (CdCl) in the water. The addition of a neutron (the other product of the initial reaction) to the common isotope Cd-108 creates an unstable state of Cd-109 which releases a gamma ray after a period of a handful of microseconds. Thus, the detection of two gamma rays simultaneously and then a third after a small delay would definitively indicate a neutrino interaction. The experiment was very successful and the rate of interactions, about three per hour, matched the theoretical prediction well. The neutrino had been discovered.

The Standard Model of particle physics predicted the existence of three "generations" of neutrinos corresponding to three types of particles called leptons.

The above diagram shows the three types of leptons and their corresponding neutrinos. In addition to this, every particle type has a corresponding antiparticle which in a way has the "opposite" properties (though some properties, such as mass, remain the same). The electron antineutrino discussed above is simply the antiparticle corresponding to the electron neutrino, for example. The discoveries of the others occurred at particle accelerators, where concentrated beams could be produced: the muon neutrino in 1962, and the tau neutrino in 2000. These results completed the expected roster of neutrino types under the Standard Model. In its original form, though, the Standard Model predicted that all neutrinos would have exactly zero mass. Note that this hypothesis (though later proved incorrect) is not disproven by the fact that neutrinos account for the "missing momentum" Pauli originally identified; massless particles, such as photons (particles of light), can still carry momentum and energy.

All of the neutrino physics described so far concerns artificially produced particles. However, these discoveries were only the beginning. Countless neutrinos also originate in the cosmos, motivating the area of neutrino astronomy. For more on this field and its value to both astronomy and particle physics, see the next post (coming January 22).

Sources: http://www.astro.wisc.edu/~larson/Webpage/neutrinos.html, http://hyperphysics.phy-astr.gsu.edu/hbase/particles/cowan.html, https://perimeterinstitute.ca/files/page/attachments/Elementary_Particles_Periodic_Table_large.jpghttp://www.scienceinschool.org/sites/default/files/articleContentImages/19/neutrinos/issue19neutrinos10_xl.jpg, http://www.fnal.gov/pub/presspass/press_releases/donut.html