Particle of a neutrino: definition, properties, description. Oscillations of neutrinos are ...

A neutrino is an elementary particle, which is very similar to an electron, but has no electric charge. It has a very small mass, which can even be zero. The neutrino velocity also depends on the mass. The difference in the arrival time of the particle and light is 0.0006% (± 0.0012%). In 2011, during the OPERA experiment, it was found that the speed of neutrinos is faster than light, but independent experience has not confirmed this.

The Elusive Particle

This is one of the most common particles in the universe. Since it interacts very little with matter, it is incredibly difficult to detect. Electrons and neutrinos do not participate in strong nuclear interactions, but equally participate in the weak. Particles possessing such properties are called leptons. In addition to the electron (and its antiparticle, the positron), the charged leptons include a muon (200 electron masses), tau (3500 electron masses) and their antiparticles. They are called so: electron, muon, and tau neutrinos. Each of them has an antimaterial component, called antineutrinos.

The muon and tau, like an electron, have particles accompanying them. It is a muon and a tau neutrino. Three types of particles differ from each other. For example, when muon neutrinos interact with a target, they always produce muons, and never tau or electrons. In the interaction of particles, although electrons and electron-neutrinos can be created and destroyed, their sum remains unchanged. This fact leads to the separation of leptons into three types, each of which possesses a charged lepton and the accompanying neutrino.

To detect this particle, very large and extremely sensitive detectors are needed. Typically, low-energy neutrinos will travel for many light years before interacting with matter. Consequently, all ground-based experiments with them rely on measuring their small fraction interacting with recorders of reasonable size. For example, in the Sudbury neutrino observatory, containing 1000 tons of heavy water, about 1012 solar neutrinos per second pass through the detector. And only 30 per day are found.

History of the discovery

Wolfgang Pauli was the first to postulate the existence of a particle in 1930. At that time, a problem arose, because it seemed that energy and angular momentum did not persist in beta decay. But Pauli noted that if a non-interacting neutral neutrino particle is emitted, the law of conservation of energy will be observed. The Italian physicist Enrico Fermi in 1934 developed the theory of beta decay and gave the particle its name.

Despite all the predictions, for 20 years neutrinos could not be detected experimentally due to its weak interaction with matter. Since the particles are not electrically charged, electromagnetic forces do not act on them, and, consequently, they do not cause ionization of the substance. In addition, they react with matter only through weak interactions of negligible force. Therefore, they are the most penetrating subatomic particles capable of passing through a huge number of atoms without causing any reaction. Only 1 in 10 billion of these particles, traveling through matter at a distance equal to the diameter of the Earth, reacts with a proton or neutron.

Finally, in 1956 a group of American physicists, headed by Frederick Raines, reported the discovery of electron-antineutrinos. In her experiments, antineutrinos emitted by a nuclear reactor interacted with protons, forming neutrons and positrons. The unique (and rare) energy signatures of these latter by-products have become evidence of the existence of the particle.

The discovery of charged muon leptons became the starting point for the subsequent identification of a second type of neutrino-muon. Their identification was carried out in 1962 on the basis of the results of an experiment in a particle accelerator. High-energy muon neutrinos were formed by the decay of pions and were directed to the detector in such a way that their reactions with matter could be studied. Although they are not reactive, like other types of these particles, it was found that in those rare cases when they reacted with protons or neutrons, the muon neutrinos form muons, but never electrons. In 1998, American physicists Leon Lederman, Melvin Schwarz and Jack Steinberger received the Nobel Prize in Physics for the identification of muon neutrinos.

In the mid-1970s, neutrino physics was supplemented by another type of charged leptons - tau. Tau neutrinos and tau antineutrinos were found to be associated with this third charged lepton. In 2000, physicists in the National Accelerator Laboratory. Enrico Fermi reported the first experimental evidence for the existence of this type of particles.

Weight

All types of neutrinos have a mass that is much smaller than their charged partners. For example, experiments show that the electron-neutrino mass should be less than 0.002% of the electron mass and that the sum of the masses of the three species should be less than 0.48 eV. For many years it seemed that the mass of the particle was zero, although there was no convincing theoretical evidence why this should be so. Then, in 2002, at the Neutrino Observatory in Sudbury, the first direct evidence was obtained that the electron-neutrino emitted by nuclear reactions in the Sun's core as they pass through it change their type. Such "oscillations" of neutrinos are possible if one or more species of particles have a certain small mass. Their studies on the interaction of cosmic rays in the Earth's atmosphere also indicate the presence of mass, but further experiments are required to more accurately determine it.

Sources

Natural sources of neutrinos are the radioactive decay of elements in the bowels of the Earth, under which a large flow of low-energy electrons-antineutrinos is emitted. Supernovae are also predominantly a neutrino phenomenon, since only these particles can penetrate the superdense material formed in the collapsing star; Only a small part of the energy is converted into light. Calculations show that about 2% of the energy of the Sun is the energy of neutrinos formed in reactions of thermonuclear fusion. It is likely that most of the dark matter of the universe consists of neutrinos formed during the Big Bang.

Problems of physics

The areas associated with neutrinos and astrophysics are diverse and rapidly evolving. Current issues, involving a large number of experimental and theoretical efforts, are as follows:

• What are the masses of different neutrinos?
• How do they influence the cosmology of the Big Bang?
• Do they oscillate?
• Can a neutrino of one type turn into another as long as they travel through matter and space?
• Are neutrinos fundamentally different from their antiparticles?
• How do stars break down and form supernovae?
• What is the role of neutrinos in cosmology?

One of the longstanding problems of particular interest is the so-called problem of solar neutrinos. This name refers to the fact that during several ground experiments conducted during the last 30 years, less particles were constantly observed than necessary for the production of energy radiated by the sun. One of its possible solutions is oscillation, i.e., the conversion of electronic neutrinos to muonic or tau while traveling to Earth. Since it is much more difficult to measure low-energy muon- or tau-neutrinos, this kind of transformation could explain why we do not observe the correct number of particles on the Earth.

The Fourth Nobel Prize

The Nobel Prize in Physics for the year 2015 was awarded to Takaaki Kadzite and Arthur MacDonald for discovering the neutrino mass. This was the fourth such award, related to the experimental measurements of these particles. Someone may be interested in the question of why we should worry so much about something that hardly interacts with ordinary matter.

The mere fact that we can detect these ephemeral particles is a testament to human ingenuity. Since the rules of quantum mechanics are probabilistic, we know that, despite the fact that almost all neutrinos pass through the Earth, some of them will interact with it. A detector of a large enough size can register it.

The first such device was built in the sixties deep in a mine in South Dakota. The mine was filled with 400 thousand liters of cleaning liquid. On the average, one particle of neutrinos interacts with the chlorine atom every day, converting it into argon. Incredibly, Raymond Davis, in charge of the detector, came up with a way to detect these several argon atoms, and four decades later in 2002, for this amazing technical feat, he was awarded the Nobel Prize.

New astronomy

Because neutrinos interact so weakly, they can travel a great distance. They give us the opportunity to look into places that otherwise we would never have seen. Neutrinos, discovered by Davis, were formed as a result of nuclear reactions that took place in the very center of the Sun, and could leave this incredibly dense and hot place only because they hardly interact with other matter. One can even detect a neutrino flying from the center of an exploding star at a distance of more than one hundred thousand light-years from Earth.

In addition, these particles make it possible to observe the Universe at its very small scales, much smaller than those in which the Large Hadron Collider in Geneva can see, which discovered the Higgs boson. It is for this reason that the Nobel Committee decided to award the Nobel Prize for the discovery of another type of neutrino.

Mysterious shortage

When Ray Davis observed solar neutrinos, he found only a third of the expected number of neutrinos. Most physicists believed that the reason for this is a poor knowledge of the astrophysics of the sun: perhaps, the models of the bowels of the sun reassessed the number of neutrinos produced in it. Nevertheless, for many years, even after the solar models improved, the deficit persisted. Physicists drew attention to another possibility: the problem could be related to our ideas about these particles. In accordance with the then prevailing theory, they did not have a mass. But some physicists claimed that in fact the particles had an infinitesimal mass, and this mass was the reason for their lack.

Three-faced particle

According to the theory of neutrino oscillations, there are three different types of neutrinos in nature. If a particle has a mass, then as it moves, it can go from one type to another. Three types - electron, muon and tau - can interact with matter into a corresponding charged particle (electron, muon, or tau lepton). "Oscillation" is due to quantum mechanics. The type of neutrino is not constant. It changes over time. The neutrino, which began its existence as an electronic one, can turn into a muon, and then back. Thus, a particle formed in the Sun's core, on the way to the Earth, can periodically turn into a muon neutrino and vice versa. Since the Davis detector could detect only an electron-neutrino capable of leading to a nuclear transmutation of chlorine into argon, it seemed possible that the missing neutrinos turned into other types. (As it turned out, neutrinos oscillate inside the Sun, and not on the way to the Earth).

Canadian experiment

The only way to verify this was to create a detector that worked for all three types of neutrinos. Since the 90s, Arthur MacDonald of the Royal University of Ontario headed the team that carried it out in the mine in Sudbury, Ontario. The installation contained tons of heavy water provided by the Government of Canada. Heavy water is a rare but naturally occurring form of water in which hydrogen containing one proton is replaced by its heavier isotope deuterium, which contains a proton and a neutron. The Canadian government has stored heavy water, since it is used as a coolant in nuclear reactors. All three types of neutrinos could destroy deuterium with the formation of a proton and neutron, and neutrons were then counted. The detector recorded about three times the number of particles compared to Davis - exactly the amount predicted by the best models of the Sun. This allowed us to assume that the electron-neutrino can oscillate to other types.

The Japanese experiment

Around the same time, Takaaki Kajita from the University of Tokyo conducted another remarkable experiment. A detector installed in a mine in Japan registered neutrinos that do not come from the depths of the sun, but from the upper layers of the atmosphere. In the collision of protons of cosmic rays with the atmosphere, showers of other particles, including muon neutrinos, form. In the mine they turned the nucleus of hydrogen into muons. The detector of Kajita could observe particles coming in two directions. Some fell from above, coming from the atmosphere, while others moved from below. The number of particles was different, which indicated a different nature - they were at different points of their oscillation cycles.

Coup in Science

It's all exotic and amazing, but why do oscillations and neutrino masses attract so much attention to themselves? The reason is simple. In the standard model of elementary particle physics, developed during the last fifty years of the twentieth century, which correctly described all the other observations in accelerators and other experiments, neutrinos had to be massless. The discovery of neutrino mass indicates that something is missing. The standard model is not complete. Missing elements have yet to be discovered - with the help of the Large Hadron Collider or another, not yet created machine.