Photomultiplier tubes lining the walls of the Daya Bay neutrino detector, October 5, 2012. Photo: Roy Kaltschmidt, Lawrence Berkeley National Laboratory
- On June 2, scientists from a big physics project in China presented the most precise value yet of an important parameter in neutrino research at a conference in Seoul.
- The measurement allows physicists to move a step closer to solving a few puzzles – but it also recalls the triumph of a Chinese project and the demise of an Indian one.
- In 2012, researchers from the Daya Bay project had reported the first conclusive measurement of the parameter, called θ13, and its value was much higher than expected.
- It encouraged an Indian group to push to have the India-based Neutrino Observatory (INO) approved, and got its members excited about what they might find.
- Physicists in China were able to build on their success to design and start building a next-generation neutrino detector, while the INO’s prospects have fizzled out.
On June 2, scientists from a big physics project in China presented the most precise value yet of an important parameter in neutrino research at a conference happening in Seoul, South Korea. The measurement allows physicists to move one step closer to solving a few longstanding puzzles – but nearer home, it recalls the triumph of a Chinese project and the demise of an Indian one that sought to get there first.
Both the Chinese and the planned Indian projects were designed to study neutrinos – the most-common subatomic particle in the universe that has mass. Neutrinos are extremely light, have no electric charge and rarely interact with matter, so they are very hard to detect and study. Scientists have nevertheless built large detectors that sit quietly for months or even years together, and record every neutrino that happens to bump into the detector material, and its properties.
Studying neutrinos is worthwhile because, like every other subatomic particle, they have also affected the universe in some way. But neutrinos are special because they behave in a unique way that we don’t understand, which means we don’t know which puzzles they can help us solve. Many physicists believe that neutrinos can tell us about how dark matter decays (if it does) and provide insights into the quantum nature of gravity.
Theta-one-three
Key to getting all these answers is a number known as θ13 (pronounced “theta-one-three”). Nine years ago, I had a chat with MVN Murthy, a theoretical physicist and now a retired professor at the Institute of Mathematical Sciences, Chennai, where he told me about it. My memories of news about the INO have since been indelibly linked to θ13.
Neutrinos come in three types, or three flavors: electron neutrino, muon neutrino and tau neutrino. When a neutrino of one flavor flies through space, it can transform into a neutrino of a different flavour. When this happens, the neutrino is said to have oscillated. We don’t know how exactly neutrino oscillation happens. We also don’t know which neutrino flavor is the heaviest and which is the lightest. But we would like to.
Scientists use a grid of nine parameters to answer physics questions about neutrinos. Six of them are expected to tell us about the neutrino mass hierarchy. Three of them, called the mixing angles, have to do with how the three neutrino flavors mix with each other. If we need answers to those questions, we need the values of each of these parameters. In 2007, when Chinese physicists designed the Daya Bay Neutrino Detector Experiment, the value of only one of the three mixing angles remained unknown: θ13. Its value described the frequency with which the electron neutrino oscillated into one of the other flavours.
In 2015, the Indian government approved a proposal by a group of Indian physicists to build the India-based Neutrino Observatory (INO) in Theni, Tamil Nadu, and allocated Rs 1,500 crore. Murthy and other members of the INO collaboration told me that the observatory would help measure θ13 with higher precision as well as resolve some important, related issues.1
Neutrinos oscillate differently when they are passing through air versus through matter, like solid rock. So the INO detector – which would be placed inside a hollowed-out mountain, creating a natural barrier to filter out other radiation – would track muon neutrinos coming from above, through the atmosphere, and those from below, through the earth. Based on the fraction of ‘missing’ electron neutrinos (because they would have oscillated) in each source, the project would provide scientists with a way to estimate the probability of the electron neutrino flavor oscillating to another, and the hierarchy of masses as well.
Today, however, the INO is staring at a bleak future – if there is a future at all.
The Jiangmen observatory
At the physics conference in Seoul, researchers from the Daya Bay project presented the most precise value of θ13 so far. In 2012, researchers from the same project had reported the first conclusive measurement of θ13 – a value much higher than expected. It heightened interest in neutrino physics and encouraged the INO collaboration to push for the project’s approval and also got them excited about what they might find.
Some believe the answer to why the universe has more matter than antimatter lies hidden in the differences between neutrinos and antineutrinos. “Having measured all three mixing angles, physicists can now pursue the next set of ambitious experiments to study what is called CP violation, or charge-conjugation and parity violation,” a 2012 press release from Caltech, which was collaborating on the Daya Bay project , quoted his physicist Robert McKeown as saying. “If CP violation is true, then particle reactions can occur at rates that differ from those of reactions involving the particles’ antimatter counterparts.”
The announcement did two more things: it told Chinese researchers that they were on the right track and it told the world that these researchers knew how to build and operate a neutrino detector. Both of them helped China stitch together an international collaboration that contributed funds and knowledge for the Jiangmen Underground Neutrino Observatory, or JUNO. JUNO is expected to succeed Daya Bay and provide the next big insights into neutrino oscillation – together with a few of its state-of-the-art peers around the world. These are DUNE in the US and Super-Kamiokande in Japan (which is being upgraded).
The Indian physicists had hoped INO would be one of them, but that seems unlikely. The Indian government approved the INO on January 5, 2015, while the groundbreaking ceremony for JUNO, in Southern China, happened five days later. But since then the trajectories of the two projects have taken opposite directions. According to an update from the Chinese Academy of Sciences on April 28, 2022, the construction for JUNO is well underway and will be completed next year.
On the other hand, in February this year, the Tamil Nadu government filed an affidavit in the Supreme Court asking that the Union government call the INO off. The application followed an appeal by Tamil Nadu Chief Minister MK Stalin to Prime Minister Narendra Modi last year to shut the project. It was the latest hurdle in a long line of hurdles, motivated at various times by distrust of science, mistrust of government, procedural irregularities, pseudoscience, ecological concerns and – inevitably – a loss of purpose.
BigScience
The Daya Bay announcement in 2012 had spurred work on both INO and JUNO, in two different countries. Initially, members of the collaboration told me that once the INO began operations, it would recapture the pole position in neutrino astronomy that India once possessed. They also said that if both instruments were to begin operations together, they could work together to complement and validate each others’ findings.
But in time, and even as the construction delays stretched on, physicists also began to pitch ideas to adapt or repurpose the project site to include experiments related to dark matter and other areas. It was the sensible thing to do: the proposal would need only a few tweaks, the logistics had already been worked out, the collaboration had almost all the approvals necessary and, most of all, repurposing would keep years of work towards India’s first Big Science project from being divided.
Members of the INO collaboration were hopeful, at least until last year, that their project would one day come to life. But it also seems obvious that the interest in the next big neutrino project has shifted away from India. (The country’s first Big Science project is now expected to be the LIGO detector coming up in Maharashtra.) There is only so much appetite internationally to fund and support a neutrino experiment. This is why the ‘I’ in ‘INO’ stands for India-based, not Indian. Yet JUNO will certainly produce results before INO.
As of October 2021, based on findings at different neutrino projects around the world, the value of θ13 had been estimated to be 8.20º to 8.93º. At the Seoul conference, the Daya Bay collaboration reported that it had been able to improve the accuracy of the measured value to 2.8%. 2 The Daya Bay project has also officially closed, passing its baton on to JUNO. And JUNO can apparently take the precision below 1% and thus provide sharper insights into the universe’s mysteries.
Meanwhile, let us ponder the mysteries of the INO’s future.