As neutrino astronomy comes of age, the Nobel Foundation has decided to award Takaaki Kajita and Arthur B. McDonald with the physics prize for 2015 for their discovery of neutrino oscillations – a property which indicates that the fundamental particle has mass.
Takaaki Kajita is affiliated with the Super-Kamiokande neutrino detector in Japan. He and Yoji Totsuka used the detector to report in 1998 that neutrinos produced when cosmic rays struck Earth’s atmosphere were ‘disappearing’ as they travelled to the detector. Then, in 2002, McDonald of the Sudbury Neutrino Observatory in Canada reported that incoming electron neutrinos from the Sun were metamorphosing into muon- or tau-neutrinos. Electron-neutrino, muon-neutrino and tau-neutrino are three kinds of neutrinos (named for particles they are associated with: electrons, muons and taus).
What McDonald and Kajita had together found was that neutrinos were changing from one kind to another as they travelled – a property called neutrino oscillations – which is definite proof that the particles have mass. Sadly, Totsuka died in 2008, and may not have been considered for the Nobel Prize for that reason.
This was an important discovery for astroparticle physics. For one, the Standard Model group of equations that defines the behaviour of fundamental particles hadn’t anticipated it. For another, the discovery also made neutrinos a viable candidate for dark matter, which we’re yet to discover, and for what their having mass implies about the explosive deaths of stars – a process that spews copious amounts of neutrinos.
Neutrino oscillations were first predicted by the Italian nuclear physicist Bruno Pontecorvo in 1957. In fact, Pontecorvo has laid the foundation of a lot of concepts in neutrino physics whose development has won other physicists the Nobel Prize (in 1988, 1995 and 2002), though he’s never won the prize himself.
Although it was a tremendous discovery that neutrinos have mass, a discovery that forced an entrenched theory of physics to change itself, the questions that Pontecorvo, Kajita, McDonald and others asked have yet to be fully answered: one of the biggest unsolved problems in physics today is what the neutrino-mass hierarchy is. In other words, physicists haven’t yet been able to find out – via theory or experiment – which of the three kinds neutrinos is the heaviest and which the lightest. The implications of the mass-ordering are important for physicists to understand certain fundamental predictions of the Standard Model. As it turns out, the model has many unanswered questions, and some physicists hope that a part of the answer may lie in the unexpected properties of neutrinos.
Exacerbating the scientific frustration is the fact that neutrinos are notoriously hard to detect because they rarely interact with matter. For example, the IceCUBE neutrino observatory operated by the University of Wisconsin-Madison near the South Pole in Antarctica employs thousands of sensors buried under the ice. When a neutrino strikes a water molecule in the ice, the reaction produces a charged lepton – electron, muon or tau, depending on the neutrino. That lepton moves faster through the surrounding ice than the speed of light in ice, releasing energy called Cherenkov radiation that’s then detected by the sensors. Building on similarly advanced principles of detection, India and China are also constructing neutrino detectors.
At least, India is supposed to be. China on the other hand has been labouring away for about a year now in building the Jiangmen Underground Neutrino Observatory (JUNO). India’s efforts with the India-based Neutrino Observatory (INO) in Theni, Tamil Nadu have, on the other hand, ground to a halt. The working principles behind both INO and JUNO are targeted at answering the mass-ordering questions. And if answered, it would almost definitely warrant a Nobel Prize in the future.
INO’s construction has been delayed because of a combination of festering reasons with no end in sight. The observatory’s detector is a 50,000-ton instrument called the iron calorimeter that is to be buried underneath a kilometre of rock so as to filter all particles but neutrinos out. To acquire such a natural shield, the principal institutions involved in its construction – the Department of Atomic Energy (DAE) and the Institute of Mathematical Sciences, Chennai (Matscience) – have planned to hollow out a hill and situate the INO in the resulting ‘cave’. But despite clearances acquired from various pollution control boards as well as from the people living in the area, the collaboration has faced repeated resistance from environmental activists as well as politicians who, members of the collaboration allege, are only involved for securing political mileage.
The DAE, which obtained approval for the project from the Cabinet and the funds to build the observatory, has also been taking a hands-off approach and has until now not participated in resolving the face-off between the scientists and the activists.
At the moment, the construction has been halted by a stay issued by the Madurai Bench of the Madras High Court following a petition filed with the support of Vaiko, founder of the Marugmalarchi Dravida Munnetra Kazhagam. But irrespective of which way the court’s decision goes, members of the collaboration at Matscience say that arguments with certain activists have degenerated of late, eroding their collective spirit to persevere with the observatory – even as environmentalists continue to remain suspicious of the DAE. This is quite an unfortunate situation for a country whose association with neutrinos dates back to the 1960s.
At that time, a neutrino observatory located at a mine in the Kolar Gold Fields was among the first in the world to detect muon neutrinos in Earth’s atmosphere – the same particles whose disappearance Takaaki Kajita was able to record to secure his Nobel Prize for. Incidentally, a Japanese physicist named Masatoshi Koshiba was spurred by the KGF discovery to build a larger neutrino detector in his country, called Kamioka-NDE, later colloquialised to Kamiokande (Koshiba won the Nobel Prize in 2002 for discovering the opportunities of neutrino astronomy). Kamiokande was later succeeded by Super-Kamiokande, which in the late-1990s became the site of Kajita’s discovery. The KGF observatory, on the other hand, was shut in the 1992 as the mines were closed.
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For the broader physics community, brakes applied on the INO’s progress count for little because there are other neutrino detectors around the world – like JUNO – as well as research labs that can continue to look for answers to the mass-ordering question. In fact, the Nobel Prize awarded to Kajita and McDonald stands testimony to the growing realisation that, like the particles of light, neutrinos can also be used to reveal the secrets of the cosmos. However, for the Indian community, which has its share of talented theoretical physicists, the slowdown signifies a slipping opportunity to once again claim leadership of experimental neutrino physics.