Changing identities
M S S Murthy, Oct 27, 2015,
The Nobel Prize in physics 2015 has been awarded to Takaaki Kajita of Japan and Arthur B McDonald of Canada for the discovery of neutrino oscillations, which shows that neutrinos have mass. The Royal Swedish Academy of Sciences further says, “The discovery has changed our understanding of the inner most working of matter and can prove crucial to our understanding of the universe.”
What are neutrinos and why are they important? Neutrinos are the second most abundant subatomic particles in the universe, second only to photons. They are
generated through radioactive decay processes — as a result of nuclear reactions that take place in the sun and other stars and by the interaction of cosmic radiation with gas molecules. These particles stream out of their sources in all directions in Earth’s atmosphere.
Three types (or flavours) of neutrinos are known to occur: electron-neutrino, muon-neutrino and tau-neutrino. Hypothesised to be chargeless and massless, they rarely interact with matter and pass straight through solid objects such as planets. Physicists had to build giant detectors, buried deep underground, to detect the products of these rare interactions to establish the existence of elusive neutrinos.
Oscillating neutrinos
In the many attempts to determine the flux of solar neutrinos, which are primarily of electron-neutrino type, the number of neutrinos detected was only a third of the expected value. What happened to the rest of them? There were many suggestions to explain this intriguing observation. One of them was that neutrinos may be changing identities. That is, some of the solar neutrinos, the majority of which are electron-neutrinos, might be morphing into other types of neutrinos — muon-neutrinos and tau-neutrinos. This process, which escaped detection, came to be known as ‘neutrino oscillation’. However, the confirmation of this interesting phenomenon had to wait for bigger and more sophisticated neutrino detectors.
The first break came when the Japanese gigantic Super-Kamiokande detector
became operational in 1996. It caught muon-neutrinos coming straight from the atmosphere above, as well as those hitting the detector from below, after having traversed the entire globe. Since Earth does not constitute any considerable obstacle to neutrinos, there ought to be equal numbers of neutrinos coming from the two directions.
But, Takaaki Kajita found that the muon-neutrinos that came straight down to Super-Kamiokande were more in number than those first passing through the globe. The only possible explanation was that muon-neutrinos that travelled longer had time to undergo an identity change, which was not the case for the muon-neutrinos that came straight from above and only had travelled a few dozen kilometres. The former must have switched into the third type — tau-neutrinos. However, their passage could not be observed in the detector.
A more direct evidence for neutrino oscillation came in 1999 from Sudbury Neutrino Observatory (SNO) at Ontario, Canada. The system was capable of detecting not only the electron-neutrinos separately, but also the amount of all three types of neutrinos together, without distinguishing them from each other. At SNO, Arthur B McDonald was studying solar neutrinos, which primarily consisted electron-neutrinos, expected that both ways of measuring the number of neutrinos should yield the same result. But again, there was a surprise. The number of electron-neutrinos was only a third of the expected number. Two-thirds had disappeared. However, the sum, counting all three types together, matched the expected number of neutrinos. The electron-neutrinos had changed identities during their 150-million-km-long journey from the sun to Earth.
Together, these two experiments confirmed the suspicion that neutrinos can change from one identity to another. The implications of this conclusion are even more far reaching: If neutrinos oscillate between types they must have mass. This is contradicts one of the most successful theories, the “Standard Model”, which deals with all the fundamental particles and forces in the universe. Standard Model requires that neutrinos have no mass. Now, the new observation that neutrinos have mass is clearly a dent on the
theory and calls for rethinking on the part of theoretical physicists to accommodate this new finding.
The Indian tryst
India has a long history of neutrino observation. As early as in the 1960s scientists from the Tata Institute of Fundamental Research (TIFR), Mumbai had set up a neutrino observatory at a depth of 2,000 metres in the gold mines of Kolar to observe the atmospheric neutrinos. A collaboration of particle physicists from TIFR, Osaka University of Japan and Durham University, UK recorded the first cosmic ray neutrino interaction in this underground facility in Kolar. However, with the closure of the mines the project had to be abandoned in the 1990s. In 1998, physicists from the Institute of Mathematical Sciences, Chennai studied the mathematical parameters in connection with neutrino oscillation, which were later confirmed by the Daya Bay Neutrino Experiment in China.
Now scientists from TIFR and 26 participating institutions in the country have proposed a new facility called India-based Neutrino Observatory (INO) with an investment of about Rs 1,350 crore, jointly funded by the Department of Atomic Energy and the Department of Science and Technology. The observatory will be set up in an underground cavern with a rock cover of more than one km from all sides in the Bodi Hills on the Western Ghats in Tamil Nadu’s Theni district, about 100 km from Madurai.
The observatory will also study the neutrinos produced by cosmic rays in Earth’s atmosphere with an aim to make precision measurements of the parameters related to neutrino oscillations and also the mass of neutrinos. The environmental clearance and cabinet approval have been obtained for this recently. Scientists now hope that excavation work may start now and they will be able to collect the first data on neutrinos by 2020.
The work of this year’s Nobel laureates has only peeped into the hidden world of the neutrinos. Experiments in more than 30 neutrino observatories around the world are continuing to capture the neutrinos and further examine their properties. The results of these investigations are expected to change our understanding of the history, structure and the future of our universe.
What are neutrinos and why are they important? Neutrinos are the second most abundant subatomic particles in the universe, second only to photons. They are
generated through radioactive decay processes — as a result of nuclear reactions that take place in the sun and other stars and by the interaction of cosmic radiation with gas molecules. These particles stream out of their sources in all directions in Earth’s atmosphere.
Three types (or flavours) of neutrinos are known to occur: electron-neutrino, muon-neutrino and tau-neutrino. Hypothesised to be chargeless and massless, they rarely interact with matter and pass straight through solid objects such as planets. Physicists had to build giant detectors, buried deep underground, to detect the products of these rare interactions to establish the existence of elusive neutrinos.
Oscillating neutrinos
In the many attempts to determine the flux of solar neutrinos, which are primarily of electron-neutrino type, the number of neutrinos detected was only a third of the expected value. What happened to the rest of them? There were many suggestions to explain this intriguing observation. One of them was that neutrinos may be changing identities. That is, some of the solar neutrinos, the majority of which are electron-neutrinos, might be morphing into other types of neutrinos — muon-neutrinos and tau-neutrinos. This process, which escaped detection, came to be known as ‘neutrino oscillation’. However, the confirmation of this interesting phenomenon had to wait for bigger and more sophisticated neutrino detectors.
The first break came when the Japanese gigantic Super-Kamiokande detector
became operational in 1996. It caught muon-neutrinos coming straight from the atmosphere above, as well as those hitting the detector from below, after having traversed the entire globe. Since Earth does not constitute any considerable obstacle to neutrinos, there ought to be equal numbers of neutrinos coming from the two directions.
But, Takaaki Kajita found that the muon-neutrinos that came straight down to Super-Kamiokande were more in number than those first passing through the globe. The only possible explanation was that muon-neutrinos that travelled longer had time to undergo an identity change, which was not the case for the muon-neutrinos that came straight from above and only had travelled a few dozen kilometres. The former must have switched into the third type — tau-neutrinos. However, their passage could not be observed in the detector.
A more direct evidence for neutrino oscillation came in 1999 from Sudbury Neutrino Observatory (SNO) at Ontario, Canada. The system was capable of detecting not only the electron-neutrinos separately, but also the amount of all three types of neutrinos together, without distinguishing them from each other. At SNO, Arthur B McDonald was studying solar neutrinos, which primarily consisted electron-neutrinos, expected that both ways of measuring the number of neutrinos should yield the same result. But again, there was a surprise. The number of electron-neutrinos was only a third of the expected number. Two-thirds had disappeared. However, the sum, counting all three types together, matched the expected number of neutrinos. The electron-neutrinos had changed identities during their 150-million-km-long journey from the sun to Earth.
Together, these two experiments confirmed the suspicion that neutrinos can change from one identity to another. The implications of this conclusion are even more far reaching: If neutrinos oscillate between types they must have mass. This is contradicts one of the most successful theories, the “Standard Model”, which deals with all the fundamental particles and forces in the universe. Standard Model requires that neutrinos have no mass. Now, the new observation that neutrinos have mass is clearly a dent on the
theory and calls for rethinking on the part of theoretical physicists to accommodate this new finding.
The Indian tryst
India has a long history of neutrino observation. As early as in the 1960s scientists from the Tata Institute of Fundamental Research (TIFR), Mumbai had set up a neutrino observatory at a depth of 2,000 metres in the gold mines of Kolar to observe the atmospheric neutrinos. A collaboration of particle physicists from TIFR, Osaka University of Japan and Durham University, UK recorded the first cosmic ray neutrino interaction in this underground facility in Kolar. However, with the closure of the mines the project had to be abandoned in the 1990s. In 1998, physicists from the Institute of Mathematical Sciences, Chennai studied the mathematical parameters in connection with neutrino oscillation, which were later confirmed by the Daya Bay Neutrino Experiment in China.
Now scientists from TIFR and 26 participating institutions in the country have proposed a new facility called India-based Neutrino Observatory (INO) with an investment of about Rs 1,350 crore, jointly funded by the Department of Atomic Energy and the Department of Science and Technology. The observatory will be set up in an underground cavern with a rock cover of more than one km from all sides in the Bodi Hills on the Western Ghats in Tamil Nadu’s Theni district, about 100 km from Madurai.
The observatory will also study the neutrinos produced by cosmic rays in Earth’s atmosphere with an aim to make precision measurements of the parameters related to neutrino oscillations and also the mass of neutrinos. The environmental clearance and cabinet approval have been obtained for this recently. Scientists now hope that excavation work may start now and they will be able to collect the first data on neutrinos by 2020.
The work of this year’s Nobel laureates has only peeped into the hidden world of the neutrinos. Experiments in more than 30 neutrino observatories around the world are continuing to capture the neutrinos and further examine their properties. The results of these investigations are expected to change our understanding of the history, structure and the future of our universe.
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