The 2015 Nobel Prize was awarded for “the discovery of neutrino oscillations, which prove that neutrinos have mass.”
In 1998, Takaaki Kajita, then a member of the Super-Kamiokande collaboration, presented data demonstrating the disappearance of atmospheric mu-neutrinos, that is, neutrinos produced 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 for the conversion of solar electron neutrinos into mu and tau neutrinos. These discoveries were of great significance and marked a breakthrough in particle physics. Neutrino oscillations and the interrelated questions of the nature of neutrinos, neutrino mass and the possibility of breaking the symmetry of the charge ratio of leptons are the most important issues of cosmology and elementary particle physics today.
We live in a 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 at almost 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. Some neutrinos were born as a result of the Big Bang. And now the sources of neutrinos are explosions of super novae, and the decay of supergiant stars, as well as radioactive reactions at nuclear power plants and the processes of natural radioactive decay in nature. Thus, neutrinos are the second most numerous elementary particles after photons, particles of light. But despite this, their existence was not determined for a long time.
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 (a type of radioactive decay of an atom with the emission of electrons). In December 1930, he proposed that some of the energy was taken away by an electrically neutral, weakly interacting particle with a very low mass (possibly massless). Pauli himself believed in the existence of such a particle, but at the same time, he understood how difficult it was to detect a particle with such parameters using experimental physics methods. He wrote about this: “I did a terrible thing, I postulated the existence of a particle that could not be detected.” Soon, after the discovery in 1932 of a massive, strongly interacting particle similar to a proton, but only neutral (part of an atom is a neutron), the Italian physicist Enrico Fermi proposed that Pauli call the elusive elementary particle a neutrino.
The opportunity 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. Rhines (also later a 1995 Nobel Prize laureate) conducted an experiment to implement the idea of the Soviet physicist B.M. Pontecorvo on the detection of neutrinos and antineutrinos at a nuclear reactor in South Carolina. As a result, he sent a telegram to Wolfgang Pauli (just a year before his death) informing him that neutrinos had left traces in their detector. And already in 1957 B.M. Pontecorvo published another pioneering work on neutrinos, in which he pioneered 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 count the number of neutrinos produced as a result of nuclear reactions in the Sun. But attempts to register the estimated number of neutrinos on Earth showed that approximately two-thirds of neutrinos were missing! Of course, there could be errors in the calculations made. But one possible solution was that some of the neutrinos changed their type. In accordance with the Standard Model currently in force in 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 that describes the electromagnetic, weak and strong interactions 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. 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. (http://elementy.ru/LHC/HEP/SM)
Each type of neutrino corresponds to its charged partner - the electron, and two other heavier particles with a shorter lifetime - the muon and the tau lepton. As a result of nuclear reactions on the Sun, only electron neutrinos are born, and the missing neutrinos could be found if, on their way to Earth, electron neutrinos could turn into mu-neutrinos and tau-neutrinos.
The search for neutrinos deep underground
The search for neutrinos is carried out continuously, day and night, in colossal installations built deep underground to screen out extraneous noise created by cosmic radiation and spontaneous radioactive reactions in the environment. It is very difficult to distinguish the signals of a few real solar neutrinos from billions of false ones.
The Super-Kamiokande Neutron Observatory was built in 1996 under Mount Kamioka, 250 km northwest of Tokyo. Another observatory, the 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 is created. This leads to the appearance of Cherenkov-Vavilov radiation, which is recorded 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 tank measuring 40 by 40 meters, filled with 50,000 tons of water. The water in the tank is so pure that the light can travel 70 meters before its intensity is halved. In a regular swimming pool, this distance is only a couple of meters. On the sides of the tank, on its top and bottom, there are 11,000 light detectors that allow you to register the slightest flash of light in the water. A large number of neutrinos pass through a tank of water, but only a few of them interact with atoms and/or electrons to form electrically charged particles. Muons are formed from mu-neutrinos and electrons from electron neutrinos. Flashes of blue light are formed around the charged particles formed. 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 faster 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, can be blocked by the speed of a charged particle.
When cosmic radiation passes 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 enter the detector from the opposite side, passing through the entire thickness of the globe. It was expected that the number of mu-neutrinos detected in both directions would be the same, because the thickness of the earth does not present any barrier to neutrinos. However, the number of neutrinos hitting Super-Kamiokande directly from the atmosphere was much greater. The number of electron neutrinos arriving in both directions did not differ. It turns out that that part of the mu-neutrino that traveled a longer path through the thickness of the earth most likely somehow turned into a tau-neutrino. However, it was impossible to register these transformations directly at the Super-Kamiokande observatory.
To get a final answer to the question about 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 2,000 meters underground and equipped with 9,500 light detectors. The observatory is designed to detect solar neutrinos, whose energy is significantly lower than those generated in the layers of the atmosphere. The tank was filled not just with purified water, but with heavy water, in which each hydrogen atom in a 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 make it possible to detect all types of neutrinos, but unfortunately, they do not allow one type to be accurately distinguished from another.
Figure 3 - Sudbury Neutrino Observatory is a solar neutrino detector. Reactions between heavy hydrogen nuclei and neutrinos make it possible to detect both only 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 experiment began, the observatory detected 3 neutrinos per day out of 60 billion neutrinos arriving to Earth from the Sun every 1 cm2. And still it was 3 times less than the calculated 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. A generalization of the experimental results of two neutrino observatories, the theory proposed by Pontecorvo about the fundamental possibility of neutrino oscillations made it possible to prove the existence of neutrino transformations on the way from the Sun to the Earth. In these two observatories, Super-Kamiokande and Sudbury Neutrino Observatory, the described results were first obtained and their interpretation was proposed in 2001. To finally verify the correctness 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. Several years later, after sufficient statistics had been accumulated, the results on neutrino transformation 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 transformation of electron neutrinos into mu- and tau-neutrinos assumes, from the point of view of quantum mechanics, that neutrinos have 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 different types with different masses. When the waves are in phase, it is impossible to distinguish one type of neutrino from another. But during a significant time of movement of neutrinos from the Sun to the Earth, dephasing of the waves can occur and then their subsequent superposition in a different 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 they differ by a very small amount. The mass of a neutrino is estimated to be millions of times less than the mass of an electron - this is an insignificant amount. However, due to the fact that neutrinos are very common particles, 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 ongoing and there is hope that they will reveal new properties of neutrinos and, thus, bring us closer to understanding the history, structure and future of the Universe.
Prepared from materials from the website nobelprize.org.
Popular literature and resources
It should be added that all of this initial evidence in favor of neutrino oscillations was obtained in “vanishing experiments.” These are the type of experiments where we measure the flux, see that it is weaker than expected, and guess that the neutrinos we are looking for have turned into a different variety. To be more convincing, you need to see the same process directly, through the “experiment on the emergence” of neutrinos. Such experiments are also now being conducted, and their results are consistent with extinction experiments. For example, at CERN there is a special accelerator line that “shoots” a powerful beam of muon neutrinos in the direction of the Italian Gran Sasso laboratory, located 732 km away. The OPERA detector installed in Italy looks for tau neutrinos in this stream. Over the five years of operation, OPERA has already caught five tau neutrinos, so this definitively proves the reality of the previously discovered oscillations.
Act Two: Solar Anomaly
The second mystery of neutrino physics that required resolution concerned solar neutrinos. Neutrinos are born in the center of the Sun during thermonuclear fusion; they accompany the reactions that make the Sun shine. Thanks to modern astrophysics, we know well what should happen in the center of the Sun, which means we can calculate the rate of neutrino production there and their flow reaching the Earth. By measuring this flow experimentally (Fig. 6), we will be able to look directly into the center of the Sun for the first time and check how well we understand its structure and operation.
Experiments to detect solar neutrinos have been carried out since the 1960s; part of the Nobel Prize in Physics for 2002 went just for these observations. Since the energy of solar neutrinos is small, on the order of MeV or less, a neutrino detector cannot determine their direction, but only records the number of nuclear transformation events caused by neutrinos. And here, too, a problem immediately arose and gradually grew stronger. For example, the Homestake experiment, which operated for about 25 years, showed that, despite fluctuations, the flux it recorded was on average three times less than that predicted by astrophysicists. These data were confirmed in the 90s by other experiments, in particular Gallex and SAGE.
The confidence that the detector was working correctly was so great that many physicists were inclined to believe that astrophysical theoretical predictions were failing somewhere - the processes were too complex at the center of the Sun. However, astrophysicists refined the model and insisted on the reliability of the predictions. Thus, the problem persisted and required an explanation.
Of course, here too, theorists have long been thinking about neutrino oscillations. It was assumed that on the way from the solar interior, some electron neutrinos turn into muon or tau. And since experiments like Homestake and GALLEX, by virtue of their design, exclusively catch electron neutrinos, they are undercounted. Moreover, in the 70-80s, theorists predicted that neutrinos propagating inside the Sun should oscillate slightly differently than in vacuum (this phenomenon was called the Mikheev-Smirnov-Wolfenstein effect), which could also help explain the solar anomaly .
To solve the problem of solar neutrinos, it was necessary to do a seemingly simple thing: build a detector that could capture the full flux of all types of neutrinos, as well as, separately, the flux of electron neutrinos. It will then be possible to make sure that neutrinos produced inside the Sun do not disappear, but simply change their type. But due to the low energy of neutrinos, this was problematic: after all, they cannot turn into a muon or tau lepton. This means that we need to look for them in some other way.
The Super-Kamiokande detector tried to cope with this problem by using the elastic scattering of neutrinos on the electrons of an atom and recording the recoil that the electron receives. Such a process, in principle, is sensitive to neutrinos of all types, but due to the peculiarities of the weak interaction, the overwhelming contribution to it comes from electron neutrinos. Therefore, the sensitivity to the total neutrino flux turned out to be weak.
And here another neutrino detector, SNO, said the decisive word. In it, unlike Super-Kamiokande, it used not ordinary, but heavy water containing deuterium. The deuterium nucleus, the deuteron, is a weakly bound system of a proton and a neutron. From the impact of a neutrino with an energy of several MeV, a deuteron can break up into a proton and a neutron: \(\nu + d \to \nu + p + n\). This process, caused by the neutral component of the weak interaction (the carrier is the Z-boson), has the same sensitivity to neutrinos of all three types, and it is easily detected by the capture of a neutron by deuterium nuclei and the emission of a gamma quantum. In addition, SNO can separately detect purely electron neutrinos by the splitting of a deuteron into two protons, \(\nu_e + d \to e + p + p\), which occurs due to the charged component of weak interactions (the carrier is the W boson).
The SNO collaboration began collecting statistics in 1998, and when enough data had accumulated, it presented the results of measuring the total neutrino flux and its electron component in two publications, 2001 and 2002 (see: Measurement of the Rate of ν e +d→p+p+e B And ). And somehow everything suddenly fell into place. The total neutrino flux actually matched what the solar model predicted. The electronic part was indeed only a third of this flow, in agreement with numerous earlier experiments of the previous generation. Thus, solar neutrinos were not lost anywhere - simply, having been born in the center of the Sun in the form of electron neutrinos, they actually turned into neutrinos of a different type on their way to Earth.
Act three, continuing
Then, at the turn of the century, other neutrino experiments were carried out. And although physicists have long suspected that neutrinos oscillate, it was Super-Kamiokande and SNO who presented irrefutable arguments - this is their scientific merit. After their results, a phase transition suddenly occurred in neutrino physics: the problems that tormented everyone disappeared, and oscillations became a fact, the subject of experimental research, and not just theoretical reasoning. Neutrino physics has undergone explosive growth and is now one of the most active areas of particle physics. New discoveries are regularly made there, new experimental installations are launched all over the world - detectors of atmospheric, space, reactor, accelerator neutrinos - and thousands of theorists are trying to find hints of New Physics in the measured neutrino parameters.
It is possible that sooner or later it will be possible in such a search to find a certain theory that will replace the Standard Model, link together several observations and allow us to naturally explain neutrino masses and oscillations, dark matter, and the origin of the asymmetry between matter and antimatter in our world, and other mysteries. That the neutrino sector has become a key player in this search is largely due to Super-Kamiokande and SNO.
Sources:
1) Super-Kamiokande Collaboration. Evidence for Oscillation of Atmospheric Neutrinos // Phys. Rev. Lett. V. 81. Published 24 August 1998.
2) SNO Collaboration. Measurement of the Rate of ν e +d→p+p+e− Interactions Produced by 8 B Solar Neutrinos at the Sudbury Neutrino Observatory // Phys. Rev. Lett. V. 87. Published 25 July 2001.
3) SNO Collaboration. Direct Evidence for Neutrino Flavor Transformation from Neutral-Current Interactions in the Sudbury Neutrino Observatory // Phys. Rev. Lett. V. 89. Published 13 June 2002.
STOCKHOLM, October 6. /Corr. TASS Irina Dergacheva/. The 2015 Nobel Prize in Physics was awarded on Tuesday to Takaaki Kajita (Japan) and Arthur MacDonald (Canada) for the discovery that neutrinos oscillate, indicating they have mass.
This was announced by the Nobel Committee at the Royal Swedish Academy of Sciences.
The bonus amount is one million Swedish kronor, which is approximately 8 million rubles at the current exchange rate. The award ceremony will take place on the day of Alfred Nobel's death, December 10, in Stockholm.
The laureates managed to solve a problem that physicists had been struggling with for a very long time. They proved that neutrino particles have mass, albeit very small. This discovery is called epoch-making for particle physics.
"This discovery has changed our understanding of the internal structure of matter and may prove decisive for our understanding of the Universe," the committee explained.
Neutrino is an elementary particle that is “responsible” for one of the four fundamental interactions, namely the weak interaction. It underlies radioactive decay.
There are three types of neutrinos: electron, muon and tau neutrinos. In 1957, Italian and Soviet physicist Bruno Pontecorvo, who worked in Dubna, predicted that neutrinos of different types can transform into each other - this process is called oscillations of elementary particles. However, in the case of neutrinos, the existence of oscillations is only possible if these particles have mass, and since their discovery, physicists have believed that neutrinos are massless particles.
The scientists' guess was experimentally confirmed simultaneously by Japanese and Canadian groups of researchers led, respectively, by Takaaki Kajita and Arthur MacDonald.
Kajita was born in 1959 and currently works at the University of Tokyo. MacDonald was born in 1943 and works at Queen's University in Kingston, Canada.
Physicist Vadim Bednyakov on neutrino oscillation
Almost simultaneously, a group of physicists led by second laureate Arthur MacDonald analyzed data from the Canadian SNO experiment collected at the Sudbury Observatory. The observatory observed streams of neutrinos flying from the Sun. The star emits powerful streams of electron neutrinos, but in all experiments scientists observed the loss of about half of the particles.
During the SNO experiment, it was proven that simultaneously with the disappearance of electron neutrinos, approximately the same number of tau neutrinos appear in the beam stream. That is, McDonald and colleagues proved that oscillations of electron solar neutrinos occur in tau.
Proving that neutrinos have mass required a rewrite of the Standard Model, the basic theory that explains the properties of all known elementary particles and their interactions.
In 2014, the most prestigious scientific award in physics went to Japanese scientists Isamu Akasaki, Hiroshi Amano and Suji Nakamura for the invention of blue light-emitting diodes (LEDs).
About the award
According to Alfred Nobel's will, the physics prize should be awarded to "whoever makes the most important discovery or invention" in this field. The prize is awarded by the Royal Swedish Academy of Sciences, located in Stockholm. Its working body is the Nobel Committee on Physics, whose members are elected by the academy for three years.
The first to receive the prize in 1901 was William Roentgen (Germany) for the discovery of radiation named after him. Among the most famous laureates are Joseph Thomson (Great Britain), recognized in 1906 for his research on the passage of electricity through gas; Albert Einstein (Germany), who received the prize in 1921 for his discovery of the law of the photoelectric effect; Niels Bohr (Denmark), awarded in 1922 for his atomic research; John Bardeen (USA), two-time winner of the prize (1956 - for research into semiconductors and the discovery of the transistor effect, 1972 - for creating the theory of superconductivity).
Scientists from different countries have the right to nominate candidates for the prize, including members of the Royal Swedish Academy of Sciences and Nobel Prize laureates in physics who have received special invitations from the committee. Candidates can be proposed from September until January 31 of the following year. Then the Nobel Committee, with the help of scientific experts, selects the most worthy candidates, and in early October the Academy selects the laureate by a majority vote.
Russian scientists have won the Nobel Prize in Physics ten times. Thus, in 2000, Zhores Alferov was awarded it for his development of the concept of semiconductor heterostructures for high-speed optoelectronics. In 2003, Alexey Abrikosov and Vitaly Ginzburg, together with Briton Anthony Leggett, received this award for their innovative contributions to the theory of superconductors. In 2010, Konstantin Novoselov and Andre Geim, now working in the UK, were awarded an award for creating the world's thinnest material - graphene.
Physicists, laureates Nobel Prize 2015, discovered the phenomenon, incompatible with generally accepted Standard Model of Elementary Particles. Independently of each other, they experimentally confirmed that neutrinos have mass. The Higgs mechanism of formation of masses of elementary particles cannot explain this phenomenon. According to the Standard Model, neutrinos should have no mass.
Many questions arise, and a wide field for new research opens up.
Back in 60s last century Bruno Pontecorvo, famous Italian and Soviet(immigrated to USSR in 1950) physicist, who worked in Joint Institute for Nuclear Research V Dubna, suggested that neutrinos have mass, and proposed the idea of an experiment to test this hypothesis. Proof of the presence of mass in neutrinos can be observed by observing their oscillations. Oscillations are repeating processes in the state of a system.
For neutrinos this is repeating transformation of three types of neutrinos(electron, muon and tau neutrinos) into each other. It followed from the theory that the duration of the oscillation periods is determined by the difference in the squares of the neutrino masses passing from one type to another. It was believed that the electron neutrino had the smallest mass, the muon neutrino had a little more, and the tau neutrino had even more. By observing oscillations, it is possible to estimate the difference in the squares of the masses and thereby prove that neutrino masses exist, but in this experiment it is impossible to estimate the value of the masses of each type of neutrino separately.
Nobel Prize Laureate Arthur MacDonald studied the flux of solar neutrinos at the Sudbury Neutrino Observatory in Canada. Neutrino fluxes from the Sun have been studied many times at various underground observatories around the world, and it has always turned out that the observed neutrino flux is three times less than expected. The expected flux was estimated in accordance with the neutrino yield from thermonuclear reactions occurring in the solar core. As a result of these reactions, a stream of electron neutrinos flows out of the Sun. It was this type of neutrino that the detectors were able to detect. It has long been assumed that on their way from the Sun, neutrinos can transform from electron to other types. Arthur MacDonald was able to observe the fluxes of all three types of neutrinos and show that in total they corresponded to what was expected. It was shown that the period of oscillations is shorter than the time it takes for the neutrino flow to travel from the Sun to the Earth, and during this time a large number of electron neutrinos manage to turn into muon and tau. Thus, the process of oscillations was experimentally discovered and, consequently, it was confirmed that the neutrino has mass.
Nobel Prize Laureate Takaaki Khajiit conducted observations of high-energy neutrinos at the Super-Kamiokande neutrino telescope. High-energy neutrinos arise in the Earth's atmosphere as a result of the action of cosmic rays. The experiment consisted of comparing the fluxes of muonic netrinos arriving at the detector directly from the atmosphere with the flux of neutrinos from the opposite side of the Earth, passing through the entire thickness of the Earth to the detector. It turned out that in the second stream some of the muon neutrinos turned into electrons. Thus, it was independently proven that oscillations occur in neutrino fluxes, and, therefore, neutrinos have mass.
In reality, both the processes themselves and their observations are many orders of magnitude more complex than those described in this text.
