What a class, I have an essay! – Nobel Prize in Physics for the scientists who revealed the secrets of neutrinos

Cracking the neutrino riddles may help scientists understand fundamental questions about the history of the universe, such as why there is much more matter in the universe than conventional models predicted, and help us understand the processes that took place in the young universe immediately after the big bang

In 1930, the Austrian physicist Wolfgang Pauli studied the process known as beta decay - a radioactive process in which an electron (or positron, a similar particle with a positive electric charge) is emitted from the nucleus of an atom. Pauli carefully examined the data, and concluded that some of the particles' energy simply disappeared. When he found no other solution, he was forced to assume that this energy disappears with another tiny particle emitted in decay. With that, Pauli himself did not really believe it, and even said (probably) to his colleagues: "I did a terrible thing - I predicted the existence of a particle that cannot be discovered". Pauli's work was continued by another famous physicist, Enrico Fermi, and developed a theory that includes such a particle, which carries energy and lacks an electric charge. The Italian Fermi gave the particle a name that has a sound in his language - Neutrino - meaning a small neutron. However, actually discovering the neutrino was an almost impossible task. Such particles do not react with almost any physical force, therefore they are not affected by phenomena such as gravity or magnetic and electric fields, and they pass easily through solid matter. In fact, as you read these lines, trillions of neutrinos are passing through your body, without leaving any impression. They also easily pass through a much more solid material, such as the Earth. In fact, to capture neutrinos effectively, a wall of lead several light-years thick is required. The existence of the neutrino was scientifically proven only in 1956, when two American physicists, Frederick Reines and Clyde Cowan, developed a sophisticated detector that was able to detect a reaction created by the impact of the tiny particle.

the missing particles

In the following decades, scientists built more sophisticated neutrino detectors. Most of them are located deep in the ground (for example in abandoned mines), to reduce the chance that other particles will be absorbed in them. These detectors were able to detect neutrinos reasonably well, but then a new question arose. The great majority of neutrino particles reach us from outer space - many of them from solar radiation. However, when the scientists began to count the few neutrino particles that they were able to detect from the direction of the sun, and try to use them to calculate how many neutrinos actually reach us, they reached about a third of the amount that should come from the sun. Where do two thirds of the neutrinos go? One of the hypotheses was that the neutrinos do not disappear, but change identity. According to the currently accepted model of the structure of matter (the standard model, as physicists call it), there are three different types of neutrinos - neutrino-electron, neutrino-tau (tau) and neutrino-muon (muon). The sun only produces an electron neutrino, and it also lasts longer than the other two, which decay at a very high rate. If it turns out that the neutrino from the sun transforms on the way to the other types of the particle, this may be the solution to the mystery of the missing particles.

deep in the ground

If the discovery of neutrinos is a complex task, then distinguishing between the different types of neutrinos is an even greater task. In 1996, the Super-Kamiokande detector started operating in Japan - in an old zinc mine southwest of Tokyo. A kilometer below the surface of the earth is placed a huge tank - its length and width is 40 m, and it contains 50,000 tons of distilled water at an almost perfect level of purification. Most of the neutrino particles pass through the tank without leaving a trace, but some of them collide with particles, and in these collisions a very tiny light is produced. 11,000 very sensitive detectors around the tank are able to pick up these tiny flashes of light, and distinguish whether it is an electron neutrino or a muon neutrino (the detector cannot distinguish the third type, tau) and from which direction it is coming. The neutrino particles do not come from the sun alone, but are created in the atmosphere due to cosmic radiation coming from every direction in space. Therefore, the researchers expected that the neutrino particles would also reach the detector from all directions, both from above and from below, after passing through the entire earth. The researchers were surprised to discover with this that the number of muon neutrinos coming from above is much higher than those coming from below. They hypothesized that in the short time difference necessary for neutrinos to pass through the Earth, many of the particles became the third type - tau neutrinos - which, as mentioned, are not detected in this facility.

Meanwhile in Canada

Three years after the start of the work of the Japanese detector, in 1999 another sophisticated detector began to operate in Sudbury, Canada. This detector, at a depth of about two kilometers, consists of a spherical tank containing 1,000 tons of heavy water. This is water whose hydrogen atom contains a neutron (in normal hydrogen there are no neutrons in the nucleus), the abundance of neutrons greatly increases the chance of the neutrinos colliding with them. Two types of collisions are created in the tank - in one only electron neutrinos are detected, and in the other - all three types of neutrinos. This allowed the researchers to focus on neutrinos coming from the direction of the Sun, and compare the number of collisions of the two types. Although the detector detected only a few of the 60 billion neutrinos that pass through every centimeter of it every second, the data clearly showed that one-third of the neutrinos coming from the Sun are electron neutrinos, and the other two-thirds are of the muon and tau type. The conclusion was that the neutrino-electron, emitted from the sun, change shape on the way and become the other neutrinos.

Not just energy

Although the neutrino's existence was predicted in 1930, and proved 26 years later, scientists have not been able to solve one major problem - does the neutrino have mass, or is it a particle with only energy. The popular belief was that the neutrino is just energy, similar to light particles (photons), but the findings of the Canadian detector and the Japanese detector blew the cards. The change of shape can only be explained (in terms of quantum physics) if the neutrino particle has its own tiny mass - a discovery that changed everything scientists had thought before.
The scientists who will distribute the prize are the directors of the detectors where the observations were made. Arthur (Art) Bruce McDonald (McDonald), was born in 1943 in Nova Scotia in eastern Canada, where he did his bachelor's and master's degrees, before going to the California Institute of Technology for doctoral studies. After spending a few years at a nuclear research center in Canada, he got a professorship at the prestigious Princeton University, but in 1989 he returned home to Canada, and got a position at Queen's University in Kingston, and by virtue of his position he managed the research group at the neutrino detector in Sudbury, not far from Kingston.
Takaaki Kajita was born in 1959 in Higashimtsuime in central Japan. At the age of 27, he had already completed a doctorate in physics at the University of Tokyo, and was appointed a faculty member at the University's Cosmic Radiation Research Institute. He led the discovery of the "missing" neutrinos, and later, as the head of the research team at the Kamiokanda detector, he was responsible for the discovery of the shape change of neutrinos originating from cosmic radiation.

The discovery of the neutrino's mass had serious consequences for the static model of matter, which is based on the assumption that neutrinos have no mass but only energy. The findings oblige the physicists to adapt the standard model to the new data, and to answer many questions that arise regarding the structure of matter. Among the questions that are still open: What is actually the mass of the neutrino? Are there other types of neutrinos? And what are their characteristics? Furthermore, the scientists believe that the discovery that neutrinos have mass means that there are more subatomic particles that we do not know about, and more forces acting on them. In terms of the bigger picture, cracking the neutrino puzzles may help scientists understand fundamental questions about the history of the universe, such as why there is a much larger amount of matter in the universe than conventional models predicted, and help us understand the processes that took place in the young universe immediately after the big bang.

Particularly sophisticated neutrino detectors, such as the one operating deep in the Antarctic ice sheet, may be used not only in the study of the particles themselves, but as a kind of sophisticated telescopes, capable of answering questions that cannot be solved with the help of optical telescopes.

More of the topic in Hayadan:

• Two researchers who separately discovered that neutrinos have mass won the Nobel Prize in Physics
• Science on ice
• Ghost lighthouses of new physics
• The dark matter puzzle