Frequently Asked Questions

What are Neutrinos?

Neutrino flavours
Neutrino flavours

Neutrinos are subatomic particles that are very peculiar due to being so elusive despite being the most abundant particles in the universe. Neutrinos come in three “flavors”: electron neutrinos, muon neutrinos, and tau neutrinos which respectively associated with the electron, muon, and tau (more about flavours later!). Each neutrino also has a corresponding antiparticle, called an antineutrino, which also has no electric charge and half-integer spin (Read more about spin and other properties of subatomic particles here). Neutrinos have no electric charge so they’re unaffected by the electromagnetic force, unlike other well known particles such as the proton and neutron. They are also are unaffected by the strong force (one of four fundamental forces that overcomes the nature of like-charged particles to repel each other and holds atomic nuclei together). They do experience the weak force (which is the 3rd fundamental force) however this does not aid in detection as this is very short range force and therefore particles have to be extremely close together in order to feel it. Finally, they are affected gravity, but just barely, as neutrinos are almost massless (they were first discovered they were assumed to be massless, as the standard model predicts, and it was only recently that they were proven to have mass through neutrino oscillations, for which the 2015 Nobel prize was awarded to Takaaki Kajita and Arthur B. McDonald ). Essentially neutrinos are barely affected by anything else in the universe, making them extremely difficult to detect. Take a look at this video to understand more about why neutrinos are so elusive and what they are.

The origin of the neutrino dates back to 1914 when J. Chadwick first demonstrated that the beta−decay(a type of radioactive decay which results in the nucleus receiving an extra proton and an electron being released) energy spectrum of a radioactive element was continuous, which seemed to imply energy and momentum were being lost in the decay – or in other words energy was not being conserved! However this conundrum was resolved when in 1930 W.Pauli postulated an electrically neutral particle which would account for the energy and momentum that was being lost. He dubbed this particle the neutron, however 2 years later when Chadwick discovered a larger, neutral and strongly interacting particle that was similar to the proton, it made more sense to call this the neutron. E.Fermi aided in the renaming of Pauli’s ghostly particle, proposing to add the suffix -ino (which mean “small” in Italian) to part of the original name, making it a neutrino, as it was possible the particle was massless.

Due to its extremely small or non-existent mass and its elusive nature Pauli was not expecting the neutrino to ever be observed. However, approximately 20 years after its postulation F. Reines and C.L. Cowan Jr., set up an experiment at the Savannah River nuclear reactor in South Carolina to demonstrate that neutrinos produced in the reactor from beta decay occasionally interacted with protons in the detector medium. Each reaction of these neutrinos would result in a neutron and a positron(an anti-electron) which could be detected and would be unambiguous proof of the existence of the neutrino. In June 1956, only two years before Pauli’s death, Reines and Cowan sent an unexpected telegram to Pauli, informing him of the neutrinos discovery.

What Do “Neutrino Oscillations” Entail?

Neutrinos come in three “flavours”; electron, muon and tau. Neutrino oscillation is the phenomenon in which neutrinos can change from one flavour to another. The observation of this was critical to neutrino science as if all three neutrinos flavours had a mass of zero (as was initially thought), or even the same arbitrary mass, this would not be allowed. Therefore, it was shown that neutrinos had mass due to the observation of neutrino oscillations, with muon neutrino oscillations being discovered in 1998 at the Super-Kamiokande in Japan and electron and tau neutrino oscillations being found in 2001/2002 at the Sudbury Neutrino Observatory.

In order to observe neutrino oscillations the neutrinos must be allowed to travel for a large distance through earth’s surface as the mantle acts as an amplifying medium for neutrino oscillations. This is because as they travel they are affected by electrons in the Earth’s atoms, through subatomic forces, which increases the likelyhood of these mutating particles to transform in to a different flavour.

How do you create neutrinos?

In the DUNE project, physicists will use one of Fermilab’s existing particle accelerators, known as the Main Injector, to generate neutrinos and it has been in operation since 2004. It accelerates protons using very strong magnets and electric fields and then hurls them into a piece of graphite or similar material where they collide with atoms in the material and produce daughter particles. The particles that emerge from these collisions generate neutrinos which will then travel 800 miles to the Stanford detector. This short video is an excellent summary of the neutrino-generating process.