A neutrino is an elementary particle that is very similar to an electron, but does not have an electric charge. It has a very small mass, which can even be zero. The neutrino velocity also depends on the mass. The difference in particle arrival time and light is 0.0006% (± 0.0012%). In 2011, the OPERA experiment found that the speed of a neutrino exceeds the speed of light, but independent experience has not confirmed this.
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 they equally participate in weak ones. Particles with such properties are called leptons. In addition to the electron (and its positron antiparticle), the muon (200 electron masses), tau (3500 electron masses) and their antiparticles are attributed to charged leptons. They are called that: electron, muon and tau neutrino. Each of them has an antimaterial component called an antineutrino.
Muon and tau, like an electron, have particles accompanying them. These are muon and tau neutrinos. 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 has a charged lepton and the accompanying neutrino.
Very large and extremely sensitive detectors are needed to detect this particle. 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 registrars 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 is found per day.
Discovery story
Wolfgang Pauli was the first to postulate the existence of a particle in 1930. At that time, a problem arose because it seemed that the energy and angular momentum were not conserved during beta decay. But Pauli noted that if a non-interacting neutral neutrino particle is emitted, then 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, they are not affected by electromagnetic forces and, therefore, they do not cause ionization of the substance. In addition, they only react with matter through weak interactions of insignificant strength. 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 led by Frederick Raines announced the discovery of the electron antineutrino. In her experiments, antineutrinos emitted by a nuclear reactor interacted with protons to form 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 the 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 pi-mesons and sent to the detector in such a way that their reactions with matter could be studied. Despite the fact that they are non-reactive, like other types of these particles, it was found that in those rare cases when they reacted with protons or neutrons, muon neutrinos form muons, but never electrons. In 1998, American physicists Leon Lederman, Melvin Schwartz and Jack Steinberger received the Nobel Prize in Physics for the identification of muon neutrinos.
In the mid-1970s, neutrino physics was replenished with another type of charged leptons - tau. The tau neutrino and tau antineutrino were found to be associated with this third charged lepton. In 2000, physicists at the National Accelerator Laboratory. Enrico Fermi reported the first experimental evidence for the existence of this type of particle.
Weight
All types of neutrinos have a mass that is much less than that of their charged partners. For example, experiments show that the mass of an electron-neutrino should be less than 0.002% of the mass of an electron 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, the first direct evidence was obtained at the Sudbury Neutrino Observatory that electron neutrinos emitted by nuclear reactions in the solar core change their type while they pass through it. Such neutrino “oscillations” are possible if one or several types of particles have a certain small mass. Their studies in 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, in which a large flux of low-energy electrons-antineutrinos is emitted. Supernovae are also predominantly a neutrino phenomenon, since only these particles can penetrate superdense material formed in a collapsing star; only a small fraction 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 fusion reactions. It is likely that most of the dark matter of the Universe consists of neutrinos formed during the Big Bang.
Physics problems
The fields associated with neutrino and astrophysics are diverse and rapidly developing. Current issues, attracting a large number of experimental and theoretical efforts, are as follows:
- What are the masses of different neutrinos?
- How do they affect the cosmology of the Big Bang?
- Do they oscillate?
- Can neutrinos of one type turn into another while they travel through matter and space?
- Are neutrinos fundamentally different from their antiparticles?
- How do stars collapse and form supernovae?
- What is the role of neutrinos in cosmology?
One of the long-standing problems of particular interest is the so-called solar neutrino problem. This name refers to the fact that during several ground-based experiments conducted over the past 30 years, fewer particles were constantly observed than was necessary to produce the energy emitted by the sun. One of its possible solutions is oscillation, i.e., the conversion of electron neutrinos into muon 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 are not observing the correct number of particles on Earth.
Fourth Nobel Prize
The 2015 Nobel Prize in Physics was awarded to Takaaki Kajite and Arthur MacDonald for detecting a neutrino mass. This was the fourth such award related to experimental measurements of these particles. Someone might be interested in the question of why we should be so worried about something that hardly interacts with ordinary matter.
The fact that we can detect these ephemeral particles is evidence of 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 sufficiently large size can detect this.
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 fluid. On average, one neutrino particle interacts with a chlorine atom every day, turning it into argon. Incredibly, Raymond Davis, who was responsible for the detector, came up with a way to detect these several argon atoms, and four decades later in 2002, he was awarded the Nobel Prize for this amazing technical feat.
New astronomy
Because neutrinos interact so weakly, they can travel vast distances. They give us the opportunity to look into places that otherwise we would never have seen. The neutrinos discovered by Davis were formed as a result of nuclear reactions that took place in the very center of the Sun, and were able to leave this incredibly dense and hot place only because they hardly interact with other matter. You can even find neutrinos 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 that the Large Hadron Collider in Geneva, which discovered the Higgs boson, can look into . 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 their expected number. Most physicists believed that the reason for this was a poor knowledge of the astrophysics of the Sun: perhaps models of the bowels of the sun overestimated the amount 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 prevailing theory then they did not possess a mass. But some physicists claimed that in fact the particles had an infinitesimal mass, and this mass was the reason for their shortage.
Three-faced particle
According to the theory of neutrino oscillations, in nature there are three different types of them. If a particle has mass, then as it moves it can go from one type to another. Three types - electron, muon and tau - when interacting with matter, can be converted into the 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. A neutrino, which began its existence as an electronic one, can turn into a muon, and then back. Thus, a particle formed in the core of the Sun, on the way to the Earth, can periodically turn into a muon neutrino and vice versa. Since the Davis detector could only detect electron neutrinos, which could lead to 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 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 Queen's University in Ontario led the team that implemented this at the mine in Sudbury, Ontario. The facility contained tons of heavy water loaned by the Canadian government. 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 stockpiled heavy water, because it is used as a coolant in nuclear reactors. All three types of neutrinos could destroy deuterium to form a proton and neutron, and neutrons were then counted. The detector recorded about three times as many particles as Davis — exactly the amount predicted by the best models of the Sun. This suggests that electron neutrinos can oscillate into its other types.
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 recorded neutrinos coming not from the bowels of the sun, but from the upper atmosphere. When cosmic ray protons collide with the atmosphere, showers of other particles are formed, including muon neutrinos. In a mine, they turned hydrogen nuclei into muons. Kajita's detector 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 their different nature - they were at different points of their oscillation cycles.
The revolution in science
This is all exotic and surprising, but why do the neutrino oscillations and masses attract so much attention? The reason is simple. In the standard model of particle physics, developed over the past fifty years of the twentieth century, which correctly described all other observations in accelerators and other experiments, neutrinos were supposed to be massless. The discovery of the neutrino mass suggests that something is missing. The standard model is not complete. The missing elements have yet to be discovered - with the help of the Large Hadron Collider or another, not yet created machine.