Neutrinos are electrically neutral elementary particles part of the standard model of physics with half-integer spin. Meaning "little neutral one" in Italian they are denoted by the Greek letter ν (nu).
Current evidence suggests that neutrinos have mass but that their masses are tiny, even by the standards of subatomic particles.
Neutrinos are leptons, along with the charged electrons, muons, and taus, and come in three flavours:
- electron neutrinos (ν e)
- muon neutrinos (ν μ)
- tau neutrinos (ν τ)
Each flavour is also associated with an antiparticle, called an "antineutrino", which also has no electric charge and half-integer spin. Neutrinos are produced in a way that conserves lepton number; i.e., for every electron neutrino produced, a positron (anti-electron) is produced, and for every electron antineutrino produced, an electron is produced as well.
They don’t interact much with matter
Neutrinos do not carry any electric charge, which means that they are not affected by the electromagnetic force that acts on charged particles, and are leptons, so they are not affected by the strong force that acts on particles inside atomic nuclei. Neutrinos are therefore affected only by the weak subatomic force and by gravity. The weak force is a very short-range interaction, and gravity is extremely weak on the subatomic scale. Thus, neutrinos typically passes through normal matter unimpeded and undetected.
How they are created
Neutrinos can be created in several ways, including in certain types of radioactive decay, in nuclear reactions such as those that take place in the Sun, in nuclear reactors, and when cosmic rays hit atoms.
The majority of neutrinos in the vicinity of the earth are from nuclear reactions in the Sun. In fact, about 65 billion (6.5×1010) solar neutrinos per second pass through every square centimeter perpendicular to the direction of the Sun in the region of the Earth.
Neutrinos change as they go along
Neutrinos are now understood to oscillate between different flavors in flight. That is, an electron neutrino produced in a beta decay reaction may arrive in a detector as a muon or tau neutrino.
This oscillation requires that the different neutrino flavors have different masses, although these masses have been shown to be tiny. From cosmological measurements, we know that the the sum of the three neutrino masses must be less than one millionth that of the electron.
A rare moment when a neutrino, that is essentially invisible, collides with a proton. image: rgonne National Laboratory - Image courtesy of Argonne National Laboratory
Source adapted from: Neutrino. (2015, June 14). In Wikipedia, The Free Encyclopedia. Retrieved 07:31, June 17, 2015, from https://en.wikipedia.org/w/index.php?title=Neutrino&oldid=666948610