Nobel Prize

On the neutrino trail

Print edition : December 11, 2015

Takaaki Kajita, co-winner of the 2015 Nobel Physics Prize, at the University of Tokyo, Japan, on October 6. Photo: YOSHIKAZU TSUNO/AFP

Arthur McDonald of Canada, co-winner of the 2015 Nobel Prize in Physics, at Queen's University in Kingston, Ontario, on October 6. Photo: Fred Chartrand/AP

THE CMS (Compact Muon Solenoid) Cavern at the European Organisation for Nuclear Research (CERN), in Meyrin, near Geneva, Switzerland. Photo: Richard Juilliart/AFP

This year’s Nobel Prize for Physics underscores the need to study the nature of neutrino, but imagined fears and unfounded allegations are thwarting Indian efforts in that direction.

SINCE the beginning of the year, neutrino, that elusive fundamental particle, has been in the national news for all the wrong reasons. Misinformed activists on one hand and political opportunists on the other have together managed to stall a major Indian initiative to build a world-class underground neutrino research laboratory by invoking baseless fears of harmful impacts of the project, including dangerous radioactive fallout, pollution of water bodies and geological impact on dams.

The laboratory, called the India-based Neutrino Observatory (INO), is proposed to be housed inside a tunnel to be dug in a kilometre-high mountain at the Bodi hills near Pottipuram village in Theni district in Tamil Nadu. The Rs.1,500 crore project is held up following a public interest litigation (PIL) in the Madras High Court and a petition to the Southern bench of the National Green Tribunal (“Neutrino scare”, Frontline, March 6, 2015).

Now neutrino is back in the news, not just national but international. But this time it is for the right reason. The Nobel Prize in Physics for this year has been awarded for the discovery of strange quantum mechanical properties of this mysterious particle. The discovery has helped resolve a four-decade-old problem in solar astronomy and has pointed to the incompleteness of the current theory of the fundamental constituents of matter and the forces of interaction among them, thus emphasising the importance of continued neutrino research for a fuller understanding of the nature of the universe.

The 56-year-old Japanese physicist, Takaaki Kajita, and the 72-year-old Canadian physicist, Arthur B. McDonald, have been jointly awarded the 2015 Nobel Prize in physics “for the discovery of neutrino oscillations, which shows that neutrinos have mass”. To understand this Nobel citation, some description of the physics of neutrinos is necessary.

Physics of neutrinos

What are neutrinos? Like the familiar photon and the electron, the neutrino is one of the known fundamental particles of nature. It is believed that a majority of the neutrinos in the universe were produced soon after the Big Bang about 14 billion years ago. And like the low energy (microwave) relic photons from the Big Bang that fill the universe, low energy (relic) neutrinos (with energies of about ten-thousandths of one electronvolt or eV) also form part of the cosmic background. As a consequence, neutrinos constitute the second most abundant particles in the universe after photons. Compared to the photon density of about 410 photons/cm³, the neutrino density is about 330 neutrinos/cm³.

Besides those of cosmological origin, neutrinos are being continuously produced from terrestrial sources such as nuclear power stations and particle accelerators, from the interactions of cosmic rays with nuclei in the atmosphere resulting in what are called “atmospheric neutrinos”, and from cosmic sources such as the sun, and other cosmic phenomena like births, collisions, and deaths of stars, particularly the explosions of supernovae, and from naturally occurring radioactive decays. Even humans produce about 5,000 neutrinos a second as a result of the radioactive decay of potassium isotopes in our bodies.

Like photons, neutrinos do not carry any electric charge. But unlike all other fundamental particles, they are quite puzzling. They interact very feebly with matter because of which all forms of matter in the universe are nearly transparent to them. Neutrinos can travel unhindered through iron of length equal to the distance travelled by light in a hundred years through empty space. About thousands of billions of neutrinos from the sun and other cosmic sources pass through our bodies every second but we cannot see them or feel them.

Since neutrino interactions are extremely weak and rare, they are very elusive and are not easily detected. A neutrino coming from the sun passing through the entire earth has less than one chance in a thousand billion of being stopped by terrestrial matter. In fact, after hypothesising its existence, Wolfgang Pauli, the Austrian physicist, wrote: “I have done a terrible thing, I have postulated a particle that cannot be detected.” It was in a desperate attempt to save the principle of conservation of energy in a radioactive process called nuclear beta decay that Pauli had suggested in 1930 that some of the energy in beta decay is carried away by an electrically neutral, weakly interacting and very light particle. In 1932, the Italian physicist, Enrico Fermi, named it neutrino and it was finally detected in 1956 by Frederick Reines and Clyde Cowan in a nuclear reactor experiment.

However, because of its elusiveness, not everything about neutrino is well studied or well understood even now. For instance, for a long time neutrinos were believed to be massless. Even though there were suggestions from some tritium beta decay experiments during the 1980s that neutrinos do have a very small mass, these claims could not be verified by other experiments. Cosmological and astronomical data, however, place very stringent limit on the neutrino mass, which is of the order of electronvolt, about a million times smaller than the electron’s mass. The Nobel Prize-winning detection of “neutrino oscillations” has, however, now established that neutrinos do carry mass, however tiny. A neutrino’s mass being not strictly zero means that, unlike a photon, it will not travel at the speed of light but at speeds close to that (highly relativistic) because of its extremely small mass.

Beyond the standard model

According to the Standard Model of fundamental particles and their interactions, there are three types (or species) of neutrinos, or “flavours” in particle physics parlance—the electron-type, the muon-type and the tau-type. The three are respectively associated with their charged particle partners—the familiar electron and its much heavier and short-lived cousins, the muon and the tau. The three neutrinos and their respective partners are collectively called leptons. The six quarks constitute the non-leptonic sector in the Standard Model, and particles made of quarks—such as the proton, the neutron and the pion—are collectively called hadrons. Even though neutrino has zero charge, its antiparticle, the antineutrino, seems to be distinct from it unlike the chargeless photon where the particle and the antiparticle are identical. (The particle produced in nuclear beta decay is actually the electron-antineutrino and so the particle that Reines and Cowan detected in their reactor experiment was also an electron-antineutrino.)

Besides the above family structure of the fundamental particles that make up all of matter (Figure 1), the full Standard Model includes the Salam-Weinberg-Glashow unified theory of electromagnetic and weak interactions and the quantum field theory of strong interactions called quantum chromodynamics (QCD), which enables quarks to bind into hadrons and also forms the basis for neutrons and protons to bind as atomic nuclei.

This Standard Model of particle physics has been an extremely successful description of matter at the fundamental level. Its high point came when the Higgs boson predicted by the model, which is necessary for giving masses to the various fundamental particles and was the key missing piece, was discovered at the Large Hadron Collider (LHC) experiments at the European Organisation for Nuclear Research, known as CERN, in Geneva, in March 2013.

The other predictions of the Standard Model have been verified in many high-precision experiments. Most notably, the Large Electron Positron Collider (LEP) experiments at CERN during the 1980s established that the number of light neutrino flavours is just three, and not more, which is consistent with the number of types that have actually been observed and with the limit on the number set by cosmological considerations. But, despite its enormous success, the Standard Model, as currently formulated, cannot accommodate neutrinos with mass. Thus, neutrinos with mass would indicate that the Standard Model is incomplete and there is physics beyond it that remains to be unravelled and understood.

With direct measurement of neutrino masses being extremely difficult and uncertain, the only firm indication that neutrinos have mass comes from the phenomenon of neutrino oscillations, the first conclusive evidence of which came from the experiments of this year’s Nobel laureates and their colleagues. So, what exactly is this phenomenon?

Neutrino oscillation

Neutrino oscillation is fundamentally a quantum mechanical effect. It refers to a peculiar property by which a neutrino of one type or flavour can morph into another flavour as it travels through space or matter. This metamorphosis is not possible if all the neutrinos are massless. So how is this identity crisis, or “multiple personality disorder”, of neutrinos to be understood?

There are two equivalent descriptions of neutrinos. One is in terms of their masses (m1, m2 and m3) and the other in terms of the charged leptons that they are associated with, that is, their flavour (Figure 2a).

Mathematically speaking, a neutrino with a definite mass is a linear combination of the different neutrino types or flavours. In quantum mechanical language, a neutrino state classified according to its mass (the mass basis) is a “linear superposition” of neutrino states classified in the flavour basis (Figure 2b).

Neutrinos interact only via the weak nuclear interaction. But the weak nuclear interaction senses neutrinos only according to their flavour. So, when a neutrino is produced in a weak interaction process, like radioactive decay, it is of a definite flavour. Or, equivalently, it is a mixture of neutrinos of different masses. However, the speed with which neutrinos propagate depends on their masses and not on their flavours. Also, because the trigger for neutrino detection too is via a weak process, a neutrino detector observes an incoming neutrino by its flavour, and not by its mass. As a neutrino produced at a source travels, the neutrinos in the mixture with different masses will go out of step and by the time they arrive at a neutrino detector at some distance from the source, the detector sees a different mixture of mass states than what the source produced.

The process can be described more accurately in quantum theoretical language so that the use of the words “oscillations” and “superposition” become apparent. According to quantum theory, a neutrino behaves both as a particle as well as a wave. So, the three neutrinos with different masses travel through space as waves with different frequencies. When a flavour type is produced in a weak decay, it is a superposition of three waves corresponding to the three mass states and the three waves start out in phase. But since their frequencies are different, they go out of phase (Figure 3) and at the detector end we have a superposition of the three waves that is different from the original.

The probability of detecting neutrinos of a given flavour produced at some distance from the detector is no longer the same as what is expected from calculations based on the Standard Model, which requires neutrinos to be massless. Since the probabilities would vary from one location to another, neutrinos “oscillate” and assume different identities at different times. That is, of the neutrinos of a given flavour produced at the source end, a fraction has morphed or oscillated away into other types, which the detector, designed to detect a particular neutrino type, can no longer detect. Thus, a smaller number is registered, resulting in a deficit in the number of neutrinos observed. This peculiar behaviour is due to the differences in neutrino masses.

The equations describing this relationship between the mass description and the flavour description involve certain constants called “mixing angles”. So we have three mixing angles corresponding to the three mixings between the mass states 1 & 2, 2 & 3 and 1 & 3 respectively. If these angles can be determined accurately by measuring the oscillations as neutrinos propagate, they will provide important clues for arriving at an improved theory of how fundamental particles of nature behave.

But how did the suggestion that neutrinos might exhibit oscillations arise in the first place? Soon after neutrino was discovered, Russian physicist Bruno Pontecorvo had suggested that like the observed oscillations in the hadron sector between the particle called neutral kaon and its antiparticle, there could be oscillations between the neutrino and the antineutrino. However, no one took that idea seriously at that time. Later, when the muon-type neutrino was discovered in 1962, the idea was revived by Japanese physicists Z. Maki, M. Nakagawa and S. Sakata, who discussed the possibility of mixing between the electron- and the muon-neutrinos.

But only when physics was confronted with a major “deficit problem” in the observation of neutrinos coming from the sun—the so-called Solar Neutrino Problem—was the mixing and oscillations between the electron- and the muon-type neutrinos seriously put forward in 1969 (the tau-type had not been detected yet) by Soviet physicists V.N. Gribov and B.M. Pontecorvo as a particle physics solution to a major solar astronomy problem. But even at that point, the idea did not find many takers.

From the sun

Thermonuclear fusion reactions in the solar core produce energy, and neutrinos. The chain of reactions can be summarised into the following statement: Four hydrogen nuclei (or protons) fuse in the solar interior to form a helium-4 ( 4He) nucleus, two positrons (or antielectrons) and two electron-neutrinos, with release of about 26.73 MeV of energy.

It would seem impossible to test directly the above theory of nuclear burning deep in its interior as the source of energy generation in the sun. Light or heat produced as a result of this would take about 10 million years to leak out from the centre of the sun to the surface. Even when it does finally emerge in the outermost region, and is radiated to the earth, it tells us only about the conditions in these outer regions.

In 1946 itself, Pontecorvo had realised that neutrinos with their very weak interaction with matter of the solar interior would be emitted outwards towards the earth almost instantaneously and can be the perfect probe of the interior of a star, thus enabling verification of the mechanism of energy production. The flux of neutrinos from the sun directed towards the earth could be calculated quite precisely using the Standard Solar Model (SSM) that had been developed by John Bahcall and others during the 1960s. But when the flux of neutrinos from the sun was measured on the earth by Raymond Davis Jr. during 1964-68, in what is known as the Homestake experiment, it was anomalously low compared to theoretical predictions, by a factor of about three. Also, the experiment could not determine the neutrino direction. The experiment used 100,000 gallons of cleaning fluid (perchloroethylene, which is mostly composed of chlorine) in a large tank and the idea was that the flux could be measured by the rate at which the chlorine atoms would be converted to atoms of an isotope of argon ( 37Ar) on interaction with solar electron-neutrinos. The observed deficit of neutrinos from the sun is what is referred to as the Solar Neutrino Problem or, more popularly, the Mystery of the Missing Neutrinos.

This deficit problem brought up many questions and suggestions. Three kinds of explanations were offered to solve the mystery: One, the theoretical calculations could be wrong, which could be for two reasons—either the predicted number of neutrinos on the basis of the SSM was incorrect or the calculated production rate of argon atoms was not right. The suggestion that the neutrino flux prediction could be wrong is, in fact, a reasonable one considering that the flux is proportional to a very high power of T, the core temperature of the sun (T 25, for example, for neutrinos produced from boron-8 reactions). And it is plausible that the temperature is not so well determined. Two, the experiment itself was conceptually wrong. Three, the original Gribov-Pontecorvo idea of oscillations among neutrino types when they travelled long distances.

In 1988, the Kamioka Observatory in Japan, which was actually running the Kamiokande experiment to detect nucleon decay in a large water-Cherenkov detector instrument but could also at the same time detect the solar neutrino flux, reported its first results of measurements. The experiment was located in the Mozumi mine, near Kamioka in Japan, about 1,000 metres underground. By observing the light signals in the Cherenkov counters, which are generated by the superluminal electrons produced by incident neutrinos on interaction with the atoms in the ultra-pure water body, the experiment could also determine the direction and energy of the incident neutrino. The observed neutrino flux was about half the expected value from the SSM. The experiment also clearly showed that the neutrinos came from the direction of the sun.

Subsequently, experiments in Russia called SAGE, experiments in Italy called GALLEX and later GNO, and later again in Japan with a next-generation experiment called Super-Kamiokande, each with different characteristics, all found that the solar neutrino flux detected was somewhat lower than what the SSM predicted. By the end of the millennium, it had become quite clear that the deficit could not be explained away by large uncertainties. Bahcall and associates had refined the SSM further and its predictions could now be independently verified. In 1997, precise measurements of the speed of sound throughout the solar interior, using periodic fluctuations observed in ordinary light from the surface of the sun, showed that the measured values agreed with SSM calculations to within one per cent, demonstrating that there was nothing wrong with the SSM.

The only consistent explanation seemed to require neutrino oscillations: some of the electron-neutrinos produced in the solar core might change flavour during propagation, becoming muon- or tau-neutrinos which are not detected by the radiochemical experiments. Though Super-Kamiokande could, in principle, detect all neutrino flavours, its efficiency was significantly lower for flavours other than electron-type. The problem continued to persist for over 30 years before it was finally resolved thanks to the decisive measurements by Arthur McDonald’s group in Canada with a heavy water (D2O) detector at the Sudbury Neutrino Observatory (SNO), built in a nickel mine in Ontario. The SNO started observations in 1999 and could detect all neutrino flavours simultaneously and efficiently.

The SNO experiment is an ingenious one. Unlike the hydrogen atom in ordinary water, which has only a proton, deuterium in heavy water is made up of a proton and a neutron. So, the experiment could measure the flux in two different ways. One detected the electron-neutrinos from the sun through what are called charged-current (CC) weak interactions (which would convert the neutron in the deuterium to a proton) and hence was sensitive to the neutrino flavour.

The other detected neutrinos through what are called neutral current (NC) weak interactions (which do not convert neutron to a proton) and was therefore insensitive to the neutrino flavour. So, if neutrino oscillations exist, and part of the electron-neutrinos had oscillated away in their 150-million-km journey to other flavours, the CC flux would show a deficit while the NC flux would not irrespective of whether oscillations existed or not.

The SNO instrument used 9,500 light detectors to detect the streaming solar neutrinos in an acrylic spherical tank filled with 1,000 tonnes of heavy water 2 km below the earth’s surface.

The detectors were mounted on a geodesic support structure, and surrounded by ultra-pure water as a shield against radioactive decays in the support structure and the surrounding rock. The rock overburden shielded the instrument from cosmic rays (Figure 4).

During the first two years of operation the SNO captured only three a day. This corresponded only to a third of the expected number of electron-neutrinos. However, the neutrino number of all flavours together (the total flux)—which is what measurement using NC interactions gave—corresponded exactly to what the SSM predicted. The conclusion was that the electron-neutrinos must have changed identities on the way. Thus, the solution to the mystery of the missing solar neutrinos is that neutrinos are, in fact, not missing. The “missing” electron-neutrinos oscillated away into muon- and tau-neutrinos that are more difficult to detect.

The groundbreaking SNO evidence for neutrino oscillations was confirmed a year later by the Japanese-United States experiment that detected electron-antineutrinos emitted from nuclear reactors instead of solar neutrinos. Called KamLAND (Kamioka Liquid Scintillator AntiNeutrino Detector), it began in January 2002. The first results, published in January 2003, showed clear evidence of the disappearance of electron-antineutrinos, consistent with the expectation from the solar neutrino experiments that neutrinos and antineutrinos behave identically.

Kajita’s concern

The Super-K experiment of the other Nobel laureate, Kajita, and his associates was concerned with another, but related, “deficit problem”, which had to do with the ratio of “atmospheric” muon- and electron-neutrinos. When cosmic rays strike the earth`s atmosphere, they create pions, which subsequently decay into muons and muon-neutrinos. Eventually, muons decay into electrons and electron-neutrinos and muon-neutrinos. Atmospheric neutrinos were first discovered in 1964 in the underground experiments in the Kolar Gold Field (KGF) mines.

It is clear that for every electron-neutrino two muon-neutrinos are produced. That is, the ratio of the flux of the two neutrino flavours should be about two. The earliest indications of deviations from the expected value appeared in the mid-1980s. This was called the “atmospheric neutrino anomaly”. Kajita’s group set out to study this problem using the Super-K detector, which was more than 10 times larger than what was used in its predecessor, the Kamiokande experiment. It consists of a 40 m high and 40 m wide tank filled with 50,000 tonnes of ultra-pure water and about 13,000 light detectors located in the tank’s top, sides and bottom. The experiment became operational in 1996.

During its first two years of operation, Super-K obtained about 5,000 neutrino signals. This was a much larger number than in previous experiments. But it was still less than the expected neutrino flux due to cosmic rays (Figure 5).

Cosmic rays come from all directions in space and Super-K detected muon-neutrinos coming straight from the atmosphere above as well as those hitting the detector from below after having traversed the entire earth from the other side.

The number of neutrinos coming from the two directions should be equal since the earth is no hindrance to neutrinos’ passage.

But the muon-neutrinos that came straight down were more than those coming from down below after traversing the earth. Also, the number showed strong zenith angle dependence.

This indicated that muon-neutrinos that travelled longer had time to undergo flavour oscillation as compared to the muon-neutrinos that came straight from above and only had travelled about 50-100 km.

The number of electron-neutrinos did not, however, show any significant zenith angle dependence and was in agreement with expectations indicating that these high energy electron-neutrinos from any direction did not have enough time to switch types and also that the deficit muon-neutrinos had not oscillated into the electron-type.

This led Kajita to conclude that the muon-neutrinos had switched only into tau-neutrinos, which, however, could not be observed in the detector.

Kajita’s oscillation results have later been confirmed by other detectors, such as MACRO and Soudan, by the long-baseline accelerator experiments K2K, MINOS and T2K, and more recently by the large neutrino telescopes ANTARES and IceCube experiment at the South Pole. The appearance of tau-neutrinos in a muon-neutrino beam has also been demonstrated by the OPERA experiment in Gran Sasso, with a neutrino beam from CERN.

Observation of the quantum mechanical phenomenon of neutrino oscillations implies that at least two neutrino species have non-zero mass.

The mechanism which generates neutrino masses is, however, still unknown and this implies a crack in the Standard Model, which must now be extended to include this new physical reality. But several key questions about the nature of neutrino need to be answered before new theories beyond the Standard Model can be developed.

For instance, the results of neutrino oscillation experiments give mass differences between neutrino flavours and the mixing angles. Although mass differences have been determined with precision, individual neutrino masses remain unknown. Why are they so lightweight? Which neutrino is the heaviest and what is the mass ordering among the neutrinos? Also, the neutrino mixing angles are much larger than the quark sector. Why are neutrinos so different from other elementary particles?

Understanding the nature of the neutrino is of prime importance today, not only for elementary particle physics but also for astrophysics and cosmology. The neutrino oscillation experiments have opened a door towards a more comprehensive understanding of the universe we live in. The Nobel Prize-winning experiments are continuing and intense activity is under way around the world to capture neutrinos and examine their properties. It is unfortunate that at this exciting juncture in neutrino research, the Indian INO project has been held up due to imagined fears and unfounded allegations.

It is appropriate to quote Bahcall’s words in 2004 here: “I am astonished when I look back on what has been accomplished in the field of solar neutrino research over the past four decades. Working together, an international community of thousands of physicists, chemists, astronomers, and engineers has shown that counting radioactive atoms in a swimming pool full of cleaning fluid in a deep mine on earth can tell us important things about the centre of the sun and about the properties of exotic fundamental particles called neutrinos. If I had not lived through the solar neutrino saga, I would not have believed it was possible.”

This neutrino trail saga is continuing. It is important, therefore, that Indian scientists’ desire to go on that mystery trail is not thwarted on irrational grounds.

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