Congratulations to Japan’s Takaaki Kajita and Canada’s Arthur B McDonald for winning this year’s Nobel Prize in Physics for their discovery that subatomic particles called neutrinos have mass.
I had the pleasure of working with Kajita-san on the Super-Kamiokande experiment located in Gifu Prefecture, Japan. Super-K is an enormous underground neutrino detector containing 50,000 tons of ultrapure water and outfitted with thousands of light detectors.
Neutrinos are famously difficult to detect, but occasionally, they interact with atoms to produce a telltale flash of light. Neutrino detectors like Super-K are constructed to be enormous because it requires a huge target to observe just a handful of neutrinos.
From 2003 to 2008, I was a PhD student working on the Super-K experiment. During this time, Kajita-san led Super-K’s high-energy group in which I worked.
Kajita-san was friendly and easy-going, always happy to take the time to work with students and answer questions. He was also gifted at quickly assessing research presentations to identify weak points.
As a student, you could count on him to critically examine your work, but his feedback was never something to be feared. In this respect, he was a gifted mentor and I was lucky to benefit from his experience.
A matter of mass
Neutrinos come in three “flavours”. Kajita-san and his collaborators showed that neutrinos oscillate from one flavour to another. The discovery was dramatically confirmed by Arthur McDonald and his colleagues working on the SNO experiment.
The results implied that, contrary to the Standard Model of particle physics, neutrinos have mass. We still do not know the mass of individual neutrinos, but we know that the heaviest neutrino is about ten million times less massive than an electron (which itself is about two-thousand times less massive than a neutron).
The fact that neutrinos have mass, and that this mass is very small, is puzzling. Physicists have developed a number of theories to account for the small mass of neutrinos.
But it is not just their small mass that makes neutrinos interesting. They continue to attract the attention of researchers for a host of interesting properties.
For example, it is possible that neutrinos could be what are known as Majorana particles, which would imply that the neutrino is its own antiparticle.
Also, all neutrinos appear to be “left-handed”, preferring to spin clockwise as they travel toward you. Researchers have postulated that the right-handed cousins of left-handed neutrinos may be supermassive particles, yet to be discovered.
Neutrinos have also infiltrated astronomy (neutrino astrophysics was my area of PhD research). Astrophysical neutrinos have been detected from the sun and from supernova 1987A, giving us a new way of understanding the inner workings of stars.
A powerful “engine” is required in order to produce neutrinos at the energy scale observed by IceCube (tens of tera-electronvolts). The young field of high-energy neutrino astronomy therefore probes some of the most extreme environments in the universe.
Kajita-san and I have both transitioned from studying neutrinos to a different area of research. By coincidence, we have both focused our recent research within the burgeoning field of gravitational-wave astronomy.
Kajita-san is now the Principle Investigator for KAGRA, an ambitious Japanese detector, designed to detect faint ripples in the fabric of space-time from cataclysmic events such as the coalescence of two black holes.
I, on the other hand, am based at Monash University, where I work to detect gravitational waves with the Laser Gravitational-wave Observatory (LIGO).
I cannot speak for Kajita-san, but when people ask me why I switched from studying neutrinos to gravitational waves, I joke that neutrinos are “too easy” to detect, and that I wanted a tougher experimental challenge.
Gravitational waves, like neutrinos, are fiendishly difficult to observe. They are yet to be detected, although a number of observatories around the world are fast approaching the required sensitivity.
Like the detection of neutrino oscillations, the discovery of gravitational waves will be a watershed moment in physics, with implications for cosmology, astrophysics and even fundamental physics.
As I ponder Kajita-san’s remarkable research to date, I cannot help but wonder if neutrino oscillations are just the first chapter in his exciting career in physics. Meanwhile, neutrino physics, the sub-field that he and I left behind, continues to grow and evolve in exciting ways.