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 fluxes of muon netrinos arriving at the detector directly from the atmosphere with a flux of neutrinos from opposite side Earth, passing to the detector through the entire thickness of the Earth. 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.

Takaaki Kajita and Arthur B. McDonald

The 2015 Nobel Prize in Physics was awarded for the discovery of neutrino oscillations. The prize winners were scientists Arthur B. McDonald from Canada and Takaaki Kajita from Japan.

For them important contribution in experiments that demonstrated that neutrinos can change state. This metamorphosis requires that neutrinos have mass. Scientists' discovery has changed our understanding of matter and may decisively change our understanding of the Universe, the Nobel Committee said.

McDonald is a professor at the California Institute of Technology (USA) and professor emeritus at Queen's University (Canada). Takaaki Kaita heads the Cosmic Ray Research Institute and is a professor at the University of Tokyo.

Last year Nobel Prize in physics were awarded to Isamu Akasaki (Japan), Hiroshi Amano (Japan) and Shuji Nakamura (USA) for the invention of a new energy-efficient and environmentally friendly light source - a blue light-emitting diode (LED). The invention of scientists is an effective alternative to conventional light bulbs.

The largest number of prizes in physics were awarded for research into elementary particles (34), in nuclear physics(28), condensed matter physics (28) and quantum mechanics(11), reports the BBC. The most famous Nobel laureate of all times, disciplines and peoples became Albert Einstein. In 1921, he received the Nobel Prize in Physics - as it was said, for his services to the field of theoretical physics, and in particular for the discovery of the photoelectric effect.

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 at the center of the Sun during thermonuclear fusion, they accompany those reactions due to which the Sun shines. 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 (carrier - 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 weak interactions(carrier - 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, in just such a search, it will be possible to find a certain theory that will replace the Standard Model, tie together several observations and allow a natural way to explain both neutrino masses and oscillations, and 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.

MOSCOW, October 6 - RIA Novosti. Canadian physicist Arthur MacDonald, who received the 2015 Nobel Prize together with Japanese Takaaki Kajita for the discovery of neutrino oscillations, dreams of measuring the exact mass of neutrinos, which would allow scientists to uncover the secret of the birth of the Universe, which he announced at a press conference in Stockholm.

“Yes, we really still have a lot of questions about what neutrinos are and how their transformations fit into the Standard Model of physics. We don’t yet know what the mass of neutrinos is, and now experiments are being carried out in our laboratories in which we We’re trying to calculate it and understand whether other types of these particles exist,” the scientist said.

McDonald and Khajita won the 2015 Nobel Prize in Physics for their discovery in 1998 of the phenomenon of neutrino oscillations - the ability of these elusive particles to "switch" between three types: electron, muon and tau neutrinos.

Neutrinos are electrically neutral elementary particles that arise as a result nuclear reactions of various types, in particular nuclear reactors, or are born on the Sun and fall to Earth with cosmic rays. They are distinguished by extremely high penetrating ability. A neutrino can fly through hundreds of meters of concrete and “not notice” the obstacle.

The ability of different types of neutrinos to transform into each other can only exist if this particle has a non-zero mass. Estimates of the mass of the Universe, and therefore ideas about its future fate, depend on the presence of mass in neutrinos. In addition, the non-zero neutrino mass can explain the fact that the Universe consists of matter, and there is practically no antimatter in it, although at the moment big bang equal quantities of both must have arisen.

Macdonald and Khajita's discovery was only finally confirmed in the summer of 2015, when CERN physicists detected a fifth tau neutrino in a stream of muon neutrinos moving from Switzerland to Italy, where the famous OPERA detector is located, which gave rise to the "superluminal neutrino" sensation in 2011, which was soon refuted.

Now it is impossible to predict how the results of neutrino studies will be used, experts say. However, these studies already have some practical results or can be expected in the near future.

As Russian scientists told RIA Novosti as part of Science Monday, using neutrinoscopies of the Earth, it is possible to map rocks in the Earth's interior, study the history of volcanic eruptions and melting ice in the Antarctic, as well as monitor the operation of nuclear power plants and monitor nuclear weapons tests.