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Tuesday, July 17, 2018

The Ghost Particle

Neutrinos are everywhere. But that’s just what makes them difficult to detect.

Written by Amitabh Sinha | Published: February 8, 2015 12:34:20 am

Neutrinos are everywhere. But that’s just what makes them difficult to detect. Five years from now, a lab in TN will attempt just that — thus embarking on the study of one of the most exciting areas of particle physics

At the Bodi West hills in Theni district, the site for the  neutrino observatory. At the Bodi West hills in Theni district, the site for the
neutrino observatory.

It’s not God’s Particle, but it’s no less elusive. And the hunt for it will begin a few years from now at a lab located underground at a depth of over 2,000 metres from the highest point of a mountain in the Bodi West Hills in Theni district of Tamil Nadu. The India-based Neutrino Observatory, or INO as it is called, will embark on one of the most exciting and alive areas of research in particle physics —  the study of neutrinos.

The project, expected to be ready by 2020, is one of the most ambitious scientific inquiries Indian scientists have worked on in recent times. Several groups in different countries are carrying out parallel research on neutrinos — an omnipresent and yet hard-to-detect sub-atomic particle which holds important clues to some of the basic questions on how the universe is made up.

Neutrinos — not to be confused with neutrons, which along with electrons and protons constitute an atom — are high-energy sub-atomic particles of neutral charge produced during natural radioactive decays and all sorts of nuclear reactions in nuclear power reactors, particle accelerators or nuclear bombs. But the most common sources of neutrinos are celestial phenomena —  the birth and death of stars, collisions and explosions happening in space.
The core of the sun is an important source of neutrinos. Every fusion reaction in the sun, in which two hydrogen atoms get fused into one helium atom, releases two neutrinos. And there are billions of these fusions happening every minute. Most of the neutrinos present in the universe are supposed to have been produced at the time of the Big Bang.

Dr Naba K Mondal, project director of INO, says every cubic centimetre of space has about 330 neutrinos that were produced at the time of the Big Bang.

Abundant though they are —  the second most abundant particle in the universe after photons, or light particles —  neutrinos have an extremely low propensity to interact with any object, thereby making their detection very difficult. They are so small that they remained undiscovered till 1956. That is why their properties are not very well known and also why scientists have to go underground to set up special detectors to cut out noise and disturbance.

Like many other groups around the world, the scientists at INO will be keen to determine the mass of the neutrinos. Three types of neutrinos — electron, muon and tau — are currently known and it is possible that these have different masses. The mass-ordering of these three types of neutrinos is the other main subject of inquiry at INO.

Because most of these neutrinos belong to the Big Bang age and have remained unchanged ever since due to their weak interactive nature, scientists believe they can reveal some information about the way the universe started and the manner it changed over time.
Though the INO is the latest attempt at neutrino research, India is not new to this field. An underground laboratory for the study of neutrinos was created in 1951 in the gold mines of Kolar in Karnataka, but it shut down when the mines closed in 1992. In fact, this laboratory was the first one to establish that the interaction of cosmic rays with the atmosphere also generated neutrinos. Such neutrinos were detected in this laboratory in 1965 and it was the first concrete proof of what was until then only a theory.

“Before going for fresh construction in Tamil Nadu, we tried to see whether the laboratory at Kolar can be used again. But those mines are over 100 years old and there is over 1,000 km of tunneling inside. The laboratory could not have been operated in isolation and maintaining the whole place would have been financially unviable,” says Mondal.

So the search for a site to locate the new observatory began. The issue was to find a geological area which would sustain a huge cavity inside it and where the seismic area was very low. Scientists at the Geological Survey of India suggested that the project be located in the southern part of India, below 13 degrees north latitude, where the rock quality was “strong”. A site was selected near Mudumalai national park north of Ooty where tunneling was already going on for a hydel power project. But this area was later declared a tiger reserve and environmental approval was denied for the INO project.

It was after this that the present site was chosen. But even as construction began, the scientists at INO faced a very different kind of hurdle. There have been protests against the project, with some claiming that it posed huge health risks and others alleging that the project was a disguise for dumping nuclear waste. The INO website, therefore, devotes considerable amount of space to explaining the project and debunking the myths.

At the Bodi West hills in Theni district, the site for the  neutrino observatory. At the Bodi West hills in Theni district, the site for the
neutrino observatory.

There have also been comparisons with the Large Hadron Collider (LHC) at CERN (European Organisation for Nuclear Research), an underground 100-km circular particle accelerator facility on the borders of France and Switzerland. The LHC, often described as the world’s biggest laboratory, recently established the existence of Higgs Boson.

Mondal, who has worked at CERN for several years, says that while the nature of scientific inquiry is similar —  study of high-energy particle physics —  INO is considerably smaller in scale and does not use an accelerator but only a magnet as a detector.

The INO will have an initial run of at least 10 years so that neutrino detections are large enough to be statistically significant. But that is not going to be the end of the underground facility.

“Already several groups of scientists have expressed an interest in using the facility for their own experiments. This underground facility will develop into a full-fledged laboratory for many kinds of studies in physics, biology, geology and other disciplines. It would become a huge scientific asset for basic science research,” says Mondal.

Besides, there are spin-off benefits too. A big one would be in the development of indigenous technology itself, says Mondal. “For a project like this, the equipment are not available off the shelf. They have to be designed and built in a customised manner. Detector technologies have implications for development of other equipment such as medical imaging. So there will be spin-offs. Everyone knows that the Internet developed as a spin-off technology at CERN,” he says.

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