Published 6 October 2015 by Stephanie Hanel

2015 Nobel Prize in Physics: Changeable ‘ghost particles’

The awarding of the Physics Nobel Prize started somewhat mystically: The topic was the change of identities “of some of the most abundant inhabitants of the universe”. A short time later we got to know who changes its appearance: neutrinos!

Neutrinos are those ‘ghost particles’ which – according to Prof. Anne L’Huillier from the Nobel committee – pass through our bodies in gigantic numbers every second without us being able to see or feel them. And as paradox as this may sound: Especially because they are so incredibly ‘small’, we need enormously large measuring equipment in order to track them down.

In the case of the new Nobel Prize in Physics, the issue was not the detection of the particles, for which Raymond Davis Jr. and Masatoshi Koshiba were awarded the Nobel Prize in 2002, but their number and their particular properties.



Different types of neutrinos exist: The so-called electron neutrinos, the muon and tau neutrinos. And they are created in different ways: Some go back to the times of the Big Bang, others are formed by the explosion of a supernova, or are the consequence of nuclear fusion processes in the Sun.

What had been the problem of the assignment of the neutrinos to date? It was a double challenge which the two physics Nobel laureates Takaaki Kajita from Japan and the Canadian researcher Arthur B. McDonald took on. The Japanese research group was able to prove that muon neutrinos convert into tau neutrinos on the way into the underground installation. And the Canadian research group found out that the conversion of the neutrinos was also the explanation why around two thirds of all electron neutrinos which leave the Sun could not be found. The neutrinos do not ‘disappear’, but convert. Consequently the Nobel Prize Committee called them: “The chameleons of space”.



For physical research this means that the theoretical prediction of Bruno Pontecorvo (1957), which assumed that a condition for a possible neutrino conversion (the specialists call it neutrino oscillation) was that neutrinos must have mass, was confirmed. The standard model of particle physics listed neutrinos as particles without mass, however. The new Nobel laureates in physics therefore detected more than just particles which convert. Their discoveries prove as well that neutrinos have a mass, albeit a very small one. It had not yet been possible to quantify it accurately, responded Prof. Olga Botner on the question of a journalist present at the press conference, this remained a challenge for the future, but it was possible to give a limit for the mass as “more than a million times lighter than an electron”.



But let us return again to the detectors: The Japanese Super-Kamiokande is located more than one kilometre below the earth’s surface in order to shield it from all atmospheric radiation – apart from the neutrinos. A tank which contains more than 50,000 tonnes of ultrapure water represents the heart of the installation. The approx. 10,000 photomultipliers detect the so-called Cherenkov radiation. This radiation results from free electrons and muons which are formed by the interaction of the neutrinos with the water molecules. In 1988, Prof. Kajita’s team discovered the “atmospheric neutrino anomaly”, which it was then able to attribute to the neutrino oscillation in 1998, using data of Super-Kamiokande. The major research facility is the successor of the detector which was used already to detect the first neutrinos. Takaaki Kajita was born in Japan in 1959 and has been working at the Institute for Cosmic Ray Research of the University of Tokyo since 1988, whose director he became in 1999.



Prof. Arthur B. McDonald undertook his research at the Sudbury Neutrino Observatory in Ontario, Canada. He answered questions of the journalists present live on the telephone. Prof. McDonald recounted that he met his co-laureate only three weeks ago at a conference and that they had frequently communicated over the years. The major research facility where McDonald made his discovery is situated in a former nickel mine. In it, a spherical, acrylic-glass tank with 1,000 tonnes of heavy water forms the ‘collecting tank’ for the neutrinos. Electron neutrinos can be distinguished from the other neutrinos in this installation. It was therefore possible to detect the missing neutrinos as muon and tau neutrinos and thus prove experimentally that a neutrino oscillation took place. McDonald and his team succeeded with this crucial breakthrough in 2001. Arthur Bruce McDonald was born in 1943 in Sydney and is emeritus professor of Queen’s University Kingston in Canada.



Professor McDonald called the award of the Nobel Prize to him a “very daunting experience”, his colleague Kajita called it “kind of unbelievable“ – we hope to welcome them both at the 66th Lindau Nobel Laureate Meeting in 2016 (26 June-1 July)!

Pictures used in slider graphic: Ill. N. Elmehed. © Nobel Media AB 2015


Stephanie Hanel

Stephanie Hanel is a journalist and author. Her enthusiasm for the people behind science grew out of her work as an online editor for AcademiaNet, an international portal that publishes profiles of excellent female scientists. She is an interested observer of new communication channels and narrative forms as well as a dedicated social media user and science slam fan.