Nobel Neutrinos

The 2015 Nobel prize in physics has been announced - congratulations Takaaki Kajita and Arthur McDonald.

This award recognises their work on neutrinos, specifically "for the discovery of neutrino oscillations, which shows that neutrinos have mass".

So, what are neutrinos?

Neutrinos are electrically neutral elementary particles, which come in three 'flavours' or types - electron neutrinos (νe), muon neutrinos (νμ) and tau neutrinos (ντ). Each flavour has its own antiparticle, called antineutrinos. Neutrinos can be created in different ways - radioactive decay, in nuclear reactions and in nuclear reactors. The majority of the neutrinos near Earth are produced from nuclear reactions in the Sun. Theoretical calculations say that about 65 billion neutrinos pass through every square centimeter region of the Earth's surface every second, yet they are incredibly difficult to detect.

Because neutrinos are neutral they are not affected by the electromagnetic force that acts upon charged particles, and also as they are leptons they are not affected by the strong force that acts upon particles within atomic nuclei. They do interact with the weak force but this is a short range interaction, and also interact with gravity but this is weak on the subatomic scale. Therefore typically neutrinos will pass straight through normal matter unaffected and undetected.

Neutrino detectors are huge structures. There is the Super-kamiokande detector in Japan, where Professor Kajita works, and the Sudbury Neutrino Observatory in Canada run by Professor McDonald. The Super-K detector is located 1km underground. It consists of a huge steel tank holding 50,000 tons of ultra-pure water. On the sides are mounted 11,000 photomultiplier devices to detect photons. When a neutron interacts with electrons in the water, a charged particle can be produced. This charged particle travels faster than the speed of light in the water, giving off Cherenkov radiation, a burst of light similar to a sonic boom. This light is registered, indicating that a neutrino has interacted and hence been detected.

Why is the study of neutrinos important in physics?

Their work solved a puzzle - that only about 1/3 of all the neutrinos theoretically expected to be bombarding Earth were actually detected. The solution to the mystery was that these missing neutrinos had infact changed indentities. For example, an electron neutrino produced in a Β-decay reaction may arrive at a detector as a muon neutrino. For this to be true it implies that neutrinos, for a long time thought to be massless, must have mass. This mass is incredibly small, less than one millionth the mass of an electron, but the fact it is non-zero is of great significance.

The use of neutrinos for communication has been demonstrated, potentially allowing for information to be coded in coherent beams of neutrinos which could travel immense distances throgh dense materials. In astrophysics neutrinos are useful for probing astronomical sources because they are not significantly affected by their travel through interstellar space, whereas photons can be obscured by things such as dust or background radiation. The observation of supernovae, the explosions produced by dying stars, is possible by detecting neutrinos which are the only particles known to escape. Neutrinos are currently identified as a candidate for what makes up dark matter, the mysterious entity making up most of the Universe.

Neutrino event in bubble chamber - Argonne National Laboratory

Inside Super-K detector - Kamioka Observatory, ICCR, University of Tokyo