Space chameleon or for what they gave the Nobel Prize in Physics 2015
The 2015 Nobel Prize was awarded for “the discovery of neutrino oscillations, which prove that neutrinos have mass”
In 1998, Takaaki Kajita, a participant at the time of the Super-Kamiokande collaboration, presented data demonstrating the disappearance of atmospheric mu-neutrinos, that is, neutrinos formed during the passage of cosmic rays through the atmosphere, on their way to the detector. In 2001, Arthur B. McDonald, head of the Sudbury Neutrino Observatory (SNO) Collaboration, published evidence of the transformation of solar electron neutrinos into mu and tau neutrinos. These discoveries were of great importance and marked a breakthrough in the physics of elementary particles. Neutrino oscillations and interrelated questions of the nature of neutrinos, neutrino masses and the possibility of symmetry breaking of the charge ratio of leptons are the most important today questions of cosmology and particle physics.
We live in the world of neutrinos. Thousands of billions of neutrinos “flow” through our body every second. They cannot be seen and cannot be felt. Neutrinos rush through space almost at the speed of light and practically do not interact with matter. There are a huge number of sources of neutrinos 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 the explosions of super-new stars, the collapse of stellar supergiants, as well as radioactive reactions to nuclear power plants and natural radioactive decay processes 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 transformation of energy during beta decay (the type of radioactive decay of an atom with electron radiation). In December 1930, he assumed that a part of the energy takes with it 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 the 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, but only neutral (atomic part - neutron), the Italian physicist Enrico Fermi proposed to call the elusive Pauli elementary particle - neutrino.
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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 Prize winner in 1995) conducted an experiment to implement the idea of the Soviet physicist B.M. Pontecorvo on neutrino and antineutrino detection at a nuclear reactor in South Corolina. As a result, he sent a telegram to Wolfgang Pauli (just a year before his death), in which he reported that neutrinos had left traces in their detector. And already in 1957, B.M. Pontecorvo published another pioneering work on neutrinos, in which he first advanced the idea of neutrino oscillations.
Since the 60s, scientists have actively begun to develop a new scientific direction - neutrino astronomy. One of the tasks was to calculate the number of neutrinos born as a result of nuclear reactions to the Sun. But attempts to register the estimated number 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 some neutrinos changed their type. In accordance with the Standard Model currently in force 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 construct in particle physics, describing the electromagnetic, weak, and strong interaction of all elementary particles. The standard model is not a theory of everything, because 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 antiparticles corresponding to them. (http://elementy.ru/LHC/HEP/SM)
Each type of neutrino corresponds to its charged partner - an electron, and two other heavier, possessing a shorter particle lifetime - muon and tau lepton. As a result of nuclear reactions to the Sun, only electron neutrinos are produced and the missing neutrinos could be found if electron neutrinos could turn into mu-neutrinos and tau-neutrinos on their way to Earth.
The search for neutrinos deep underground
The search for neutrinos is carried out continuously, day and night, on installations of colossal size, built deep underground for shielding extraneous noise created by cosmic radiation and spontaneous radioactive reactions in the environment. It is very difficult to distinguish signals from several true solar neutrinos from billions of false ones.
The Neutron Observatory Super-Kamiokande was built in 1996 under the Kamioka mountain, 250 km north-west from Tokyo. Another Sudbury Neutrino Observatory (SNO) observatory was built in 1999 at a nickel mine near Ontario.
Figure 2 - Super-Kamiokande is an atmospheric neutrino detector. When a neutrino interacts with water, an electrically charged particle is formed. This leads to the appearance of Cherenkov-Vavilov radiation, which is detected by light detectors. The shape and intensity of the Cherenkov-Vavilova emission spectrum allows us 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 tank measuring 40 by 40 meters, 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 is halved. In a normal swimming pool, this distance is only a couple of meters away. On the sides of the tank, on its upper and lower parts there are 11,000 light detectors, which allow registering the smallest flash of light in water. A large number of neutrinos pass 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 electronic neutrinos. Around the formed charged particles are formed flashes of blue light. 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 it does not contradict the theory of Einstein, which says 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 a vacuum and, therefore, may be blocked by the speed of movement of a charged particle.
With the passage of cosmic radiation through the layers of the atmosphere, a large number of mu-neutrinos are born, 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 side, passing through the entire thickness of the globe. It was expected that the number of mu-neutrinos detected in two directions would be the same, because the thickness of the earth does not represent any obstacle for neutrinos. However, the number of neutrinos reaching the Super-Kamiokande directly from the atmosphere was much higher. The number of electron neutrinos coming in both directions did not differ. It turns out that that part of the mu-neutrino, which passed a longer way through the thickness of the earth, most likely turned into a tau neutrino in some way. However, it was impossible to register these transformations directly at the Super-Kamiokande observatory.
To get the final answer to the question about the possibility of neutrino transformations or neutrino oscillations, another experiment was carried out in the second neutrino observatory Sudbury Neutrino Observatory (Figure 3). It was built at a depth of 2000 meters under the ground and equipped with 9,500 light detectors. The observatory is designed to detect solar neutrinos, the energy of which is much less than that produced in atmospheric layers. The tank was filled not only with purified water, but with heavy water, in which each hydrogen atom in the water molecule has an additional neutron. Thus, the probability of neutrino interaction with heavy hydrogen atoms is much higher. In addition, the presence of heavy nuclei allows neutrinos to interact with other nuclear reactions, and, therefore, light flashes of a different intensity will be observed. Some types of reactions allow all types of neutrinos to be detected, but unfortunately, they do not accurately distinguish one type from another.
Figure 3 - The Sudbury Neutrino Observatory is a solar neutrino detector. Reactions between heavy hydrogen nuclei and neutrinos make it possible to record as soon as electron neutrinos, and all types of neutrinos simultaneously. (Illustrations 2 and 3 from the website of the Nobel Committee nobelprize.org and the Swedish Academy of Sciences kva.se)
After the start of the experiment, the observatory detected 3 neutrinos per day from 60 billion neutrinos after 1 cm2, arriving to Earth from the Sun. Still, it was 3 times less than the estimated number 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. The generalization of the experimental results of two neutrino observatories, the theory proposed by Pontecorvo on the fundamental possibility of neutrino oscillations, allowed us to prove 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 obtained for the first time and their interpretation was proposed in 2001. In order to finally verify the correctness of the experiments, a year later, in 2002, the KamLAND experiment began (Kamioka Liquid scintillator AntiNeutrino Detector), in which a reactor was used as a source of neutrons. A few years later, after accumulating sufficient statistics, the neutrino conversion results 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 assumes from the point of view of quantum mechanics that neutrinos have a 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, which correspond to neutrinos of various types with different masses. When the waves are sophisticated it is impossible to distinguish one type of neutrino from another. But for a considerable time of movement of the neutrino from the Sun to the Earth, dephasing of the waves can occur and then their subsequent superposition is possible in a different way. Then it becomes possible to distinguish one type of neutrino from another. Such peculiar changes are due to the fact that different types of neutrinos have different masses, but differ by a very small amount. The mass of a neutrino is estimated millions of times smaller than the mass of an electron - this is an insignificant small quantity. However, due to the fact that the 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? Experiments are continuing and there is hope that they will allow us to learn new properties of neutrinos and, thus, bring us closer to an understanding of the history, structure and future of the Universe.
Based on materials from nobelprize.org.
Popular literature and resources on the topic:
1. Hulth, PO (2005) High Energy Neutrinos from the Cosmos, www.nobelprize.org/nobel_prizes/themes/physics/hulth
2. Bahcall, JN (2004) Solving the Mystery of the Missing Neutrinos, www.nobelprize.org/nobel_prizes/themes/physics/bahcall
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
5. nobelprize.org
6. elementy.ru
In 1998, Takaaki Kajita, a participant at the time of the Super-Kamiokande collaboration, presented data demonstrating the disappearance of atmospheric mu-neutrinos, that is, neutrinos formed during the passage of cosmic rays through the atmosphere, on their way to the detector. In 2001, Arthur B. McDonald, head of the Sudbury Neutrino Observatory (SNO) Collaboration, published evidence of the transformation of solar electron neutrinos into mu and tau neutrinos. These discoveries were of great importance and marked a breakthrough in the physics of elementary particles. Neutrino oscillations and interrelated questions of the nature of neutrinos, neutrino masses and the possibility of symmetry breaking of the charge ratio of leptons are the most important today questions of cosmology and particle physics.
We live in the world of neutrinos. Thousands of billions of neutrinos “flow” through our body every second. They cannot be seen and cannot be felt. Neutrinos rush through space almost at the speed of light and practically do not interact with matter. There are a huge number of sources of neutrinos 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 the explosions of super-new stars, the collapse of stellar supergiants, as well as radioactive reactions to nuclear power plants and natural radioactive decay processes 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 transformation of energy during beta decay (the type of radioactive decay of an atom with electron radiation). In December 1930, he assumed that a part of the energy takes with it 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 the 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, but only neutral (atomic part - neutron), the Italian physicist Enrico Fermi proposed to call the elusive Pauli elementary particle - neutrino.
')
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 Prize winner in 1995) conducted an experiment to implement the idea of the Soviet physicist B.M. Pontecorvo on neutrino and antineutrino detection at a nuclear reactor in South Corolina. As a result, he sent a telegram to Wolfgang Pauli (just a year before his death), in which he reported that neutrinos had left traces in their detector. And already in 1957, B.M. Pontecorvo published another pioneering work on neutrinos, in which he first advanced the idea of neutrino oscillations.
Since the 60s, scientists have actively begun to develop a new scientific direction - neutrino astronomy. One of the tasks was to calculate the number of neutrinos born as a result of nuclear reactions to the Sun. But attempts to register the estimated number 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 some neutrinos changed their type. In accordance with the Standard Model currently in force 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 construct in particle physics, describing the electromagnetic, weak, and strong interaction of all elementary particles. The standard model is not a theory of everything, because 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 antiparticles corresponding to them. (http://elementy.ru/LHC/HEP/SM)
Each type of neutrino corresponds to its charged partner - an electron, and two other heavier, possessing a shorter particle lifetime - muon and tau lepton. As a result of nuclear reactions to the Sun, only electron neutrinos are produced and the missing neutrinos could be found if electron neutrinos could turn into mu-neutrinos and tau-neutrinos on their way to Earth.
The search for neutrinos deep underground
The search for neutrinos is carried out continuously, day and night, on installations of colossal size, built deep underground for shielding extraneous noise created by cosmic radiation and spontaneous radioactive reactions in the environment. It is very difficult to distinguish signals from several true solar neutrinos from billions of false ones.
The Neutron Observatory Super-Kamiokande was built in 1996 under the Kamioka mountain, 250 km north-west from Tokyo. Another Sudbury Neutrino Observatory (SNO) observatory was built in 1999 at a nickel mine near Ontario.
Figure 2 - Super-Kamiokande is an atmospheric neutrino detector. When a neutrino interacts with water, an electrically charged particle is formed. This leads to the appearance of Cherenkov-Vavilov radiation, which is detected by light detectors. The shape and intensity of the Cherenkov-Vavilova emission spectrum allows us 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 tank measuring 40 by 40 meters, 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 is halved. In a normal swimming pool, this distance is only a couple of meters away. On the sides of the tank, on its upper and lower parts there are 11,000 light detectors, which allow registering the smallest flash of light in water. A large number of neutrinos pass 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 electronic neutrinos. Around the formed charged particles are formed flashes of blue light. 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 it does not contradict the theory of Einstein, which says 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 a vacuum and, therefore, may be blocked by the speed of movement of a charged particle.
With the passage of cosmic radiation through the layers of the atmosphere, a large number of mu-neutrinos are born, 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 side, passing through the entire thickness of the globe. It was expected that the number of mu-neutrinos detected in two directions would be the same, because the thickness of the earth does not represent any obstacle for neutrinos. However, the number of neutrinos reaching the Super-Kamiokande directly from the atmosphere was much higher. The number of electron neutrinos coming in both directions did not differ. It turns out that that part of the mu-neutrino, which passed a longer way through the thickness of the earth, most likely turned into a tau neutrino in some way. However, it was impossible to register these transformations directly at the Super-Kamiokande observatory.
To get the final answer to the question about the possibility of neutrino transformations or neutrino oscillations, another experiment was carried out in the second neutrino observatory Sudbury Neutrino Observatory (Figure 3). It was built at a depth of 2000 meters under the ground and equipped with 9,500 light detectors. The observatory is designed to detect solar neutrinos, the energy of which is much less than that produced in atmospheric layers. The tank was filled not only with purified water, but with heavy water, in which each hydrogen atom in the water molecule has an additional neutron. Thus, the probability of neutrino interaction with heavy hydrogen atoms is much higher. In addition, the presence of heavy nuclei allows neutrinos to interact with other nuclear reactions, and, therefore, light flashes of a different intensity will be observed. Some types of reactions allow all types of neutrinos to be detected, but unfortunately, they do not accurately distinguish one type from another.
Figure 3 - The Sudbury Neutrino Observatory is a solar neutrino detector. Reactions between heavy hydrogen nuclei and neutrinos make it possible to record as soon as electron neutrinos, and all types of neutrinos simultaneously. (Illustrations 2 and 3 from the website of the Nobel Committee nobelprize.org and the Swedish Academy of Sciences kva.se)
After the start of the experiment, the observatory detected 3 neutrinos per day from 60 billion neutrinos after 1 cm2, arriving to Earth from the Sun. Still, it was 3 times less than the estimated number 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. The generalization of the experimental results of two neutrino observatories, the theory proposed by Pontecorvo on the fundamental possibility of neutrino oscillations, allowed us to prove 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 obtained for the first time and their interpretation was proposed in 2001. In order to finally verify the correctness of the experiments, a year later, in 2002, the KamLAND experiment began (Kamioka Liquid scintillator AntiNeutrino Detector), in which a reactor was used as a source of neutrons. A few years later, after accumulating sufficient statistics, the neutrino conversion results 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 assumes from the point of view of quantum mechanics that neutrinos have a 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, which correspond to neutrinos of various types with different masses. When the waves are sophisticated it is impossible to distinguish one type of neutrino from another. But for a considerable time of movement of the neutrino from the Sun to the Earth, dephasing of the waves can occur and then their subsequent superposition is possible in a different way. Then it becomes possible to distinguish one type of neutrino from another. Such peculiar changes are due to the fact that different types of neutrinos have different masses, but differ by a very small amount. The mass of a neutrino is estimated millions of times smaller than the mass of an electron - this is an insignificant small quantity. However, due to the fact that the 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? Experiments are continuing and there is hope that they will allow us to learn new properties of neutrinos and, thus, bring us closer to an understanding of the history, structure and future of the Universe.
Based on materials from nobelprize.org.
Popular literature and resources on the topic:
1. Hulth, PO (2005) High Energy Neutrinos from the Cosmos, www.nobelprize.org/nobel_prizes/themes/physics/hulth
2. Bahcall, JN (2004) Solving the Mystery of the Missing Neutrinos, www.nobelprize.org/nobel_prizes/themes/physics/bahcall
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
5. nobelprize.org
6. elementy.ru
Source: https://habr.com/ru/post/366623/
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