Space chameleon or for what they gave the Nobel Prize in Physics 2015

2015 Nobel Prize awarded for “discovery of neutrino oscillations that prove that neutrinos have mass”

In 1998, Takaaki Kajita, a member of the Super-Kamiokande collaboration at the time, presented data demonstrating the disappearance of atmospheric mu-neutrinos, i.e. neutrinos, generated by cosmic rays passing through the atmosphere, on their way to the detector. In 2001, Arthur B. McDonald, director of the Sudbury Neutrino Observatory (SNO) Collaboration, published evidence of the conversion of solar electron neutrinos into mu and tau neutrinos. These discoveries were of great importance and marked a breakthrough in elementary particle physics. Neutrino oscillations and interrelated issues of the nature of neutrinos, neutrino masses, and the possibility of breaking the symmetry of the lepton charge ratio are the most important issues of cosmology and elementary particle physics today.

We live in a neutrino world. Thousands of billions of neutrinos “flow” through our body every second. They cannot be seen and cannot be felt. Neutrinos travel through space almost at the speed of light and practically do not interact with matter. There are a huge number of neutrino sources both in space and on Earth. Part of the neutrino was born as a result of the Big Bang. And now the sources of neutrinos are explosions of super new stars, and the decay of stellar supergiants, as well as radioactive reactions to a nuclear power plant and the processes of natural radioactive decay in nature. Thus, neutrinos are the second largest elementary particles after photons, particles of light. But despite this, for a long time their existence has not been determined.

The possibility of the existence of neutrinos was proposed by the Austrian physicist Wolfgang Pauli as an attempt to explain the conversion of energy during beta decay (a type of radioactive decay of an atom with the emission of electrons). In December 1930, he suggested that part of the energy is taken away with an electrically neutral, weakly interacting particle with a very small mass (possibly massless). Pauli himself believed in the existence of such a particle, but at the same time, he understood how difficult it is to detect a particle with such parameters by methods of experimental physics. He wrote about this: “I did a terrible thing, I postulated the existence of a particle that cannot be detected.” Soon, after the discovery in 1932 of a massive, strongly interacting particle similar to a proton,

The ability to detect neutrinos appeared only in the late 50s, when a large number of nuclear power plants were built and the neutrino flux increased significantly. In 1956, F. Raines (also later a Nobel laureate in 1995) conducted an experiment to implement the idea of ​​the Soviet physicist B.M. Pontecorvo for the detection of neutrinos and antineutrinos in a nuclear reactor in South Corolin. As a result, he sent a telegram to Wolfgang Pauli (just a year before his death), in which he reported that neutrinos left traces in their detector. And already in 1957 B.M. Pontecorvo published another pioneering work on neutrinos, in which he was the first to propose the idea of ​​neutrino oscillations.
Since the 60s, scientists have actively begun to develop a new scientific field - neutrino astronomy. One of the tasks was to calculate the number of neutrinos born as a result of nuclear reactions on the Sun. But attempts to register the estimated amount of neutrinos on Earth showed that about two-thirds of the neutrinos are missing! Of course, there could be errors in the calculations made. But one of the possible solutions was that part of the neutrinos changed their type. In accordance with the Standard Model in force today in elementary particle physics (Figure 1), there are three types of neutrinos - electron neutrinos, mu neutrinos and tau neutrinos.

Figure 1 - The standard model is a theoretical construction in elementary particle physics that describes the electromagnetic, weak and strong interactions of all elementary particles. The standard model is not the theory of everything, since it does not describe dark matter, dark energy and does not include gravity. It contains 6 leptons (electron, muon, tau lepton, electron neutrino, muon neutrino and tau neutrino), 6 quarks (u, d, s, c, b, t) and 12 corresponding antiparticles. (

Each type of neutrino corresponds to its charged partner - an electron, and two other heavier particles with a shorter lifetime - muon and tau lepton. As a result of nuclear reactions on the Sun, only electronic neutrinos are born and the missing neutrinos could be found if, on the way to Earth, electronic neutrinos could turn into mu-neutrinos and tau-neutrinos.

Search for neutrinos deep underground The

search for neutrinos is carried out continuously, day and night, on colossal installations built deep underground to shield extraneous noise created by cosmic radiation and spontaneous radioactive reactions in the environment. It is very difficult to distinguish the signals of several real solar neutrinos from billions of false ones.

Super-Kamiokande Neutron Observatory was built in 1996 under Mount Kamioka, 250 km northwest of Tokyo. Another Sudbury Neutrino Observatory (SNO) was built in 1999 in a nickel mine near Ontario.

Figure 2 - Super-Kamiokande is an atmospheric neutrino detector. When a neutrino interacts with water, an electrically charged particle forms. This leads to the appearance of Cherenkov-Vavilov radiation, which is detected by light detectors. The shape and intensity of the Cherenkov-Vavilov radiation spectrum makes it possible to determine the type of particle and where it came from.

Super-Kamiokande is a giant detector built at a depth of 1000 meters. It consists of a 40 by 40 meter tank filled with 50,000 tons of water. The water in the tank is so clean that the light can travel a distance of 70 meters before its intensity decreases by half. In a regular swimming pool, this distance is only a couple of meters. On the sides of the tank, on its upper and lower parts, 11,000 light detectors are located, allowing to register the smallest flash of light in water. A large number of neutrinos passes through a tank of water, but only some of them interact with atoms and / or electrons to form electrically charged particles. Muons are formed from mu neutrinos and electrons from electron neutrinos. Around the formed charged particles, flashes of blue light form. This is the so-called Cherenkov-Vavilov radiation, which occurs when charged particles move at a speed exceeding the speed of light in a given medium. And this does not contradict Einstein's theory, which states that nothing can move at a speed higher than the speed of light in a vacuum. In water, the speed of light is only 70% of the speed of light in vacuum and, therefore, can be blocked by the speed of movement of a charged particle.

When cosmic radiation passes through the atmosphere, a large number of mu-neutrinos are generated, which need to travel only a few tens of kilometers to the detector. Super-Kamiokande can detect mu-neutrinos coming directly from the atmosphere, as well as those neutrinos that fall on the detector from the back, passing through the entire thickness of the globe. It was expected that the number of mu-neutrinos detected in two directions will be the same, because the earth’s thickness does not represent any obstacle for neutrinos. However, the number of neutrinos falling on Super-Kamiokande directly from the atmosphere was much larger. The number of electron neutrinos arriving in both directions did not differ. It turns out that the part of mu-neutrino, which passed a greater path through the thickness of the earth, most likely turned into a tau neutrino somehow. However, it was not possible to register the conversion data directly at the Super-Kamiokande observatory.

In order to get the final answer to the question of the possibility of neutrino transformations or neutrino oscillations, another experiment was carried out at the second neutrino observatory Sudbury Neutrino Observatory (Figure 3). It was built at a depth of 2000 meters underground and is equipped with 9,500 light detectors. The observatory is designed to detect exactly solar neutrinos, whose energy is much less than that generated in the atmosphere. The tank was filled not just with purified water, but with heavy water, in which every hydrogen atom in the water molecule has an additional neutron. Thus, the probability of interaction of neutrinos with heavy hydrogen atoms is much higher. In addition, the presence of heavy nuclei allows neutrinos to interact with other nuclear reactions, and therefore flashes of light of a different intensity will be observed. Some types of reactions make it possible to detect all types of neutrinos, but unfortunately, they do not accurately distinguish one type from another.

Figure 3 - Sudbury Neutrino Observatory is a solar neutrino detector. Reactions between the heavy nuclei of hydrogen and neutrinos make it possible to register both electronic neutrinos and all types of neutrinos simultaneously. (Figures 2 and 3 from the Nobel Committee website and the Swedish Academy of Sciences

After the start of the experiment, the observatory detected 3 neutrinos per day from 60 billion neutrinos through 1 cm2 arriving on Earth from the Sun. And still, it was 3 times less than the estimated amount of electron solar neutrinos. The total number of all types of neutrinos detected at the observatory corresponded with high accuracy to the expected number of neutrinos emitted by the Sun. A generalization of the experimental results of two neutrino observatories, the theory proposed by Pontecorvo on the fundamental possibility of neutrino oscillations, has proved the existence of neutrino transformations on the way from the Sun to Earth. In these two observatories Super-Kamiokande and Sudbury Neutrino Observatory, the described results were first obtained and in 2001 their interpretation was proposed. To finally verify the accuracy of the experiments, a year later, in 2002, the KamLAND experiment (Kamioka Liquid scintillator AntiNeutrino Detector) began, in which a reactor was used as a neutron source. A few years later, after accumulating sufficient statistics, the results on the conversion of neutrinos were confirmed with high accuracy.

To explain the mechanism of neutrino transformations or neutrino oscillations, scientists turned to the classical theory of quantum mechanics. The effect of the conversion of electron neutrinos into mu and tau neutrinos suggests from the point of view of quantum mechanics the presence of neutrinos in the mass, otherwise this process is impossible even theoretically. In quantum mechanics, a particle of a certain mass corresponds to a wave of a certain frequency. Neutrinos are a superposition of waves that correspond to neutrinos of various types with different masses. When the waves are in phase it is impossible to distinguish one type of neutrino from another. But for a considerable time of neutrino motion from the Sun to the Earth, waves can be dephased and then their subsequent superposition in another way is possible. Then it becomes possible to distinguish one type of neutrino from another. Such peculiar changes occur due to the fact that different types of neutrinos have different masses, but differ by a very small amount. The neutrino mass is estimated to be millions of times less than the mass of an electron - this is an insignificant small quantity. However, due to the fact that neutrino is a very common particle, the sum of the masses of all neutrinos is approximately equal to the mass of all visible stars.

Despite such successes of physicists, many questions still remain unresolved. Why are neutrinos so light? Are there other types of neutrinos? Why are neutrinos so different from other elementary particles? The experiments are ongoing and there is hope that they will allow us to learn new properties of neutrinos and, thus, bring us closer to understanding the history, structure and future of the Universe.

Based on materials from the site

Related Literature and Resources:

1. Hulth, PO (2005) High Energy Neutrinos from the Cosmos,
2. Bahcall, JN (2004) Solving the Mystery of the Missing Neutrinos ,
3. McDonald, AB, Klein, JR och Wark, DL (2003) Solving the Solar Neutrino Problem, Scientific American, Vol. 288, Nr 4, April
4. Kearns, E., Kajita, T. och Totsuka, Y. (1999) Detecting Massive Neutrinos, Scientific American, Vol. 281, Nr 2, August

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