Mysteries surround one of the lightest particles in the Universe, but their resolutions can push the frontiers of modern physics.
BY DEBDUTTA PAUL
Physicists are experts at finding objects that do not beg to be noticed. This list includes neutrinos, one of the fundamental particles of nature that zoom around almost at the speed of light. The Sun emits gazillions of them, and hundreds of billions pass through every square centimetre of our bodies each second. Even when we are asleep, they go right across the entire Earth, come out through us, and fly away to space as if we didn’t exist.
If neutrinos sound like ‘neutrons’, that’s no accident. When physicist Wolfgang Pauli hypothesised their existence in 1930, he named them neutrons. In 1932, James Chadwick discovered the neutrons, one of the components of all atomic nuclei (except ordinary hydrogen). Two years later, Enrico Fermi proposed a theory involving neutrinos, the “little neutral ones.”
Experimental physicists found it hard to detect these neutral particles — neutrons and neutrinos — because they do not carry any electric charge. They used the particles’ electric properties to detect them, so these electrically neutral particles remained elusive.
But today, there is remarkable experimental evidence that neutrinos exist. Contrary to neutrons, which are slightly heavier than protons, neutrinos’ masses are tiny. According to the strictest measurements, they are at least a million times less massive than electrons, which are themselves about two thousand times lighter than protons and neutrons.
Hypothesis and detection
How did Wolfgang Pauli predict these extraordinarily light and weakly interacting particles? The answer lies in an age-old principle in physics: energy conservation. According to Einstein’s special theory of relativity, mass is a form of energy, and the total energy of a particle or a group of particles is a sum of their energy due to their masses and kinetic energies. The total remains conserved in any physical process, which is one of nature’s fundamental laws.
When physicists studied how neutrons decayed into protons and electrons, they found that total energy differed before and after the decay. “However, energy conservation is one of the fundamental laws of nature,” said Moon Moon Devi of Tezpur University, who recently visited ICTS-TIFR to attend the two-week program on neutrinos. Its violation would threaten one of the most important principles in physics.
Wolfgang Pauli took this apparent violation of the fundamental principle head-on. He argued that the neutrons’ initial energy was dissipated through other means, hypothesising that another particle carried the remaining energy. Although he predicted that this elusive particle would be hard to detect, Fermi’s calculation of the beta-decay process showed that the energies were coming out just right. In 1956, physicists finally detected neutrinos in special experiments, solving the energy-conservation imbroglio.
The Standard Model of Particle Physics
The mid-twentieth century was a time of hectic activity as far as neutrinos were concerned. While theoretical physicists predicted that neutrinos come in two varieties — electron-neutrinos and muon-neutrinos, experimental physicists successfully tested this hypothesis.
Twentieth-century physics paved the way for discovering a handful of particles. After discovering photons (particles of light), electrons, protons, and neutrons (which make up atoms), physicists discovered muons, pions, neutrinos, and many others. Today, we know that some of these are fundamental particles, for example, photons, electrons, and neutrinos, while others are made up of other fundamental particles, for example, protons and neutrons (consisting of quarks and gluons). Along with each particle is an ‘antiparticle’ — its cousin with similar properties like the mass but distinct properties like the electric charge. If the particle-antiparticle pair comes too close, they annihilate.
The array of fundamental particles and their corresponding antiparticles comprise the Standard Model of Particle Physics. It is a theory about the properties and interactions of these particles. The experimental discovery of the Higgs particle in 2012 showed that the model can explain many mysteries of the Universe. However, it is not a complete theory of nature, and neutrinos have played a role in pointing that out.
Finite neutrino mass
Neutrinos have mass, however small. Because of this tiny mass, neutrinos give the massless photons quite the chase but cannot quite catch up.
Physicists accidentally discovered the non-zero nature of mass while studying neutrinos produced in the Sun. By the mid-twentieth century, physicists had hypothesised that nuclear reactions that power the Sun also create neutrinos. While detecting these solar neutrinos, however, experimental physicists found that the number of neutrinos they measured was one-third of the theoretical predictions.
“The main idea was to test the energy-generation hypothesis in the Sun,” said Srubabati Goswami of Physical Research Laboratory, Ahmedabad, one of the organisers of the ICTS program. The physicists ended up discovering that solar neutrinos come in three varieties: electron-neutrinos, muon-neutrinos, and tau-neutrinos. As their name suggests, these three types are related to electrons, muons, and taus — three fundamental particles in the Standard Model.
Moreover, the physicists inferred from the missing neutrinos that the three types of neutrinos ‘oscillate’ between themselves. That is, they morph into each other while travelling.
“The neutrino oscillations were verified by various experiments: solar experiments, accelerator-based experiments, reactor experiments, [and] atmospheric neutrino experiments,” said Rukmani Mohanta of the University of Hyderabad, another organiser of the ICTS program.
But the catch is that neutrinos with zero mass could not oscillate. So, neutrino oscillation is smoking-gun evidence that not all’s well with the Standard Model of Particle Physics. “It is a very small extension of the Standard Model, but it is an extension [nevertheless],” said Indumathi D of The Institute of Mathematical Sciences, Chennai, who also attended the ICTS program.
Neutrino oscillations
Today, physicists try to understand neutrino oscillations with the help of parameters — a set of numbers that tell them exactly how the oscillations occur. The differences between the squares of the masses of three different types of neutrinos are two such parameters. “The mass-squared differences are like the frequencies [of the oscillations]. But the oscillation experiments cannot infer the absolute mass,” said Srubabati.
Modern neutrino experiments try to measure these parameters and answer other fundamental questions about the nature of neutrinos. For example, we do not know whether neutrinos and antineutrinos are identical or distinct particles.
The answers lie in how neutrinos interact with ordinary matter. However, experimental physicists find it challenging to study neutrinos because they hardly interact with ordinary matter.
Even today, physicists infer about neutrinos by detecting charged particles produced in nuclear reactions. “The detection is always via another charged particle,” said Srubabati.
Mass hierarchy and the INO
One of the fundamental unanswered questions concerns the masses of the three types of neutrinos. While experiments have measured the differences between the squares of the masses, the masses’ ordering is unclear. Physicists have figured out that two types of neutrinos have close-by masses, while the third type has a significantly different mass from the other two. However, an open question is whether this third mass is substantially more or less than the pair’s.
The India-based Neutrino Observatory, a proposed neutrino detector, had the potential to solve this hierarchy problem. Engineers and physicists worked together to run a prototype and demonstrate its capabilities. But the experiment has been financially stopped this year after many years of litigation regarding the site, laments Indumathi, who has been associated with it for two decades.
Uniqueness defined the India-based Neutrino Observatory (INO). It would employ large magnets to detect muons and antimuons and infer the rate of nuclear reactions involving neutrinos and antineutrinos. Moreover, these neutrinos were not ordinary ones. They would travel through the entire Earth for tens of thousands of kilometres and emerge on the other side, encountering the INO and triggering a few nuclear reactions. The physicists would calculate the rates of these reactions by detecting the muons and antimuons produced in them.
It is challenging to build a giant magnet, which the INO would require. Moreover, physicists would have to figure out the rate of neutrinos coming from different directions within the Earth from the dispersion of muons in various directions. The physical conditions of such an experiment differ from all other present-day neutrino detectors, claims Indumathi. “One has to have a very different mindset for this kind of experiment,” she said.
Indumathi thinks that even though the project is already a decade late, its uniqueness still makes it worthwhile to find an alternative site and build it. Other neutrino experiments that have tried to solve the hierarchy problem have not yielded a definitive answer until now. “Also, it’s a completely independent method… You never know what will show up when you do something in a different way,” she said.
The third reason, Indumathi argues, is that there is no substitute for building home-grown experiments to build engineering and scientific expertise in the country. It can also give a lot of freedom. “With your own hands-on experiment, you can always… take it apart, build it, tweak it a little… [if] you want to do something that it was not capable of, you can do that,” she said. “Without the INO, we have lost much more than just one experiment.”
The state-of-the-art
During the ICTS program, scientists discussed many other aspects of neutrinos, including their possible relationship with dark matter. “There is a lot of interest in looking at these two sectors because it’s like saying that there is a desert, and there is an oasis — so everyone is going to come to the oasis,” said Indumathi about the challenges in modern physics.
But these are not the only questions physicists are trying to answer about neutrinos. For example, physicists have yet to figure out the rate at which neutrinos interact with ordinary matter. Since neutrinos participate in only one of two kinds of nuclear reactions, that too hardly, figuring out such basic numbers becomes challenging.
But one thing is clear. In neutrino physics research, experiments and theory go hand-in-hand.
The smallness of neutrinos’ deviation from standard particle physics, quantified by their mass and oscillations, offers a tiny window of exploring physics beyond the current understanding. Many theories try to explain the discrepancies and account for the observable effects. The answers to the questions clouding the neutrinos may offer a concrete path to twenty-first-century particle physics.
The author thanks Moon Moon Devi, Srubabati Goswami, Rukmani Mohanta, and Indumathi D for discussions.