This year’s Nobel prize for physics was awarded today to two experimentalists Takaaki Kajita of the University of Tokyo and Arthur B. McDonald of Queen’s University in Canada for their independent work that showed that neutrinos have mass. You can read the official announcement form the Royal Swedish Academy of Sciences here.
Of all the known particles, neutrinos are far and away the most elusive and hard to detect. This is because the detection of a particle involves using some property that it has that a detector can grab hold of. But neutrinos have no charge or magnetic moment, the easiest properties to use in detectors, and their masses were so small as to be directly undetectable and for a long time it was assumed that they were exactly zero.
Furthermore, neutrinos only interact with other particles via what is known as the weak interaction, which means that it hardly ever reacts with anything. In fact, at any given time, billions of these little fellas, emitted from the Sun, are passing freely through the Earth (and our bodies) and we have no idea that they are doing so because they don’t affect us in any way. So detecting neutrinos involved building huge detectors deep underground and looking for extremely rare collisions with the liquids in the detectors. These experiments are devilishly difficult because you need huge amounts of detector material, have to eliminate all manner of background noise, and have very long time-scales because the collisions are so rare.
So how do you find out that they have mass? Well, for a long time it was noticed that the number of the most common flavor of neutrino produced by the Sun and detected on the Earth was much lower than the expected values predicted using known nuclear fusion reactions. How could they disappear on their way to the Earth?
According to our theories of particles, neutrinos come in three kinds (called flavors) and this anomaly could be explained if neutrinos had non-zero masses because then one flavor could change to another via the mass matrix, a process known as neutrino oscillations. The two groups headed by these researchers showed that neutrinos did indeed switch flavors in the expected amounts, thus resolving the problem of missing neutrinos. In the process, this implies that neutrinos must have some mass, however small.