Neutrino for the scruff

The Nobel Physics Prize in 2015 was awarded to the Japanese Takaaki Kajita and Canadian Arthur B. McDonald " for the opening of neutrino osocillations showing that they have a lot." The doct is told about the dramatic history of research of the “elusive particle” . physical .- Mat. sciences, head. Department of High Energy Physics Institute of Nuclear Research of the Russian Academy of Sciences Yuri Kudenko and Dokt. physical .- Mat. Sciences, Vedas. scientific. sore. Iya RAS and Astrosmic Center Fian Boris Stern .

The neutrino is perhaps the most famous and popular particle from the entire “zoo” of the so -called standard model. And the story of its study is the most detective of the stories in the physics of elementary particles. Neutrino’s existence guessed by the lack of energy in the beta-decay of nuclei (Wolfgang Pauli, 1930); Not yet being discovered, this particle was firmly inscribed in the theory of weak interactions (Enriko Fermi, 1934). Finally, more than a quarter of a century passed after the hypothesis nomination, and neutrino was found by its interaction in the detector, which was installed near the nuclear reactor (Rainnes and Cowen, 1956). The elusiveness of the neutrino (i.e., a very small section of interaction with the substance) added popularity: “ If you fill the whole space between the Sun and Earth with iron , then the neutrino easily overcome this thickness ,” they wrote in popular articles of the 1960s. And Vladimir Vysotsky sang:

Let you not catch the neutrino by the beard
And you can't put it in the test tube , -
It would be great to Pontecorvo
He took him stronger by the scruff !

It was Bruno Pontecorvo who was mentioned in this song. Back in 1957 (already being a Soviet physicist, a JINR employee), he put forward the idea that neutrinos could be oscillating. Here we are obliged to stop and explain what particle oscillations are.

This is a purely quantumomechanical effect, which is quite difficult to describe on the fingers - like the entire quantum mechanics. However, you should not complex about this: as Richard Feynman said, no one understands quantum mechanics, just some with its help can get the right results. The effect occurs when there are two (or more) types of particles A and B. Moreover, their masses differ slightly, and there are no laws that prevent the transition A B and vice versa. Quantum mechanics allow such a thing that neither A nor B, but only their combination (mass state) can safely live in a free state. And they can be born or participate in interaction only in its pure form - a or V. So, for example, a particle of A. was born, but it cannot calmly exist in this form. And what does it mean "restless"? It will turn into in, then again in a and so on. - along the sinusoid (we lower the details for simplicity). This is the oscillations of particles. For the first time, oscillations were found in the beams of neutral K-Mesons. And now the idea was put forward that the same effect could be subject to neutrino. It was already known that there are at least two types of neutrinos, but it was believed that their masses are most likely exactly equal to zero and that there may be a strict law of preservation of the so -called lepton number, prohibiting transitions between different types of neutrinos. The assumption of Pontecorvo was ahead of its time.

* * *

The next chapter in a neutrine detective was opened by an experiment on registration of solar neutrino, led by Raymond Davis (USA). The idea of ​​the experiment was put forward by the same Bruno Pontecorvo: to use the reaction of the transformation of chlorine into a radioactive isotope of the Argon under the influence of neutrino. In general, the methodology of radio chemical experiments on the registration of the neutrino looks fantastic: to pull out a few atoms (!) From the huge volume of the detector (!) Formed from the interaction of the neutrino, and to count them according to the decay.

By the mid-1960s, a huge installation with a tank containing 378 cubic meters of perchlor-evelene (detergent), located in the mine at a depth of one and a half kilometers, was completed. In the late 1960s, the first results of the Davis experiment appeared: the stream of neutrino from the sun was registered, but it turned out to be about three times less theoretical, designed by John Bakal.

The chlorine-argon method has a rather high threshold in neutrino energy. He “sees” the neutrino not from the main proton cycle, but from the side, boron, which gives a small contribution to the solar luminosity. Because of this, suspicions fell primarily on the model of the sun used by the bacal to calculate the neutrino flow-maybe the flow for the boron cycle is overestimated? However, it was not possible to find an error in the calculations, the Sun model turned out to be stiff enough to allow such a deviation.

The next radical step was two Galli-German experiment. Ideologically, their scheme is the same as that of the Davis experiment, only as a working substance - Gallium. The difference is that the gallium -gallium reaction threshold is much lower in energy, so that the experiment sees the neutrino from the main proton cycle (the method is proposed by Vadim Kuzmin, Iyai RAS). Here the flow of neutrino is directly determined by the luminosity of the sun. In 1988, the Galli-German experiment earned in the Elbrus at the Baksan neutrinum observatory (Iyai RAS). It uses 50 tons of gallium (in a maximum of 60 tons, Gallium was developed as a strategic supply in the 1980s), the detector is located at a depth of one and a half kilometers under the mountain. The second similar experiment was conducted in Italy, about 30 tons of gallium were used there. The difference between the experiments is that pure metal gallium is used on the bucks, and in Italy - Gaul chloride. Both experiments confirmed the shortage of electronic neutrinos by about three times, and it was impossible to write off this shortage to the wrong model of the sun. I had to admit that the only way to explain the shortage is to attribute it to neutrino oscillations. At that time, it was already known that there are three types of neutrinos - electronic, Muonnoye and Tau, which is why the shortage was three times - as a result of neutrino oscillations, they were evenly distributed between three types, while the radiochemical method is sensitive only to electronic.

By the way, the Baksan experiment continues the set of statistics and gives an indication that perhaps there is another type of neutrino: sterile.

* * *

The next step in the sunny-neutrine mystery was the experiment in the Canadian Sadbury (Sudbury Neutrino Experiment, SNO).

The bottom line is that all radiochemical methods used reactions with the transformation of a neutrino into an electron (the so -called charged current). That is why radiochemical experiments are sensitive only to electronic neutrinos-the muonnoye should, in the interaction of this type, give birth to muon, tau-neutrino-tau lepton, and for this they do not have enough energy. But there is another channel of interaction when the neutrino remains itself, but transfers part of the energy to another particle (the so -called neutral current). In particular, any neutrino due to a neutral current can ruin the deine. And the neutron from the collapse then leaves a clear mark-gamma quant-absorbing the deater atom.

Thus, the detector is sensitive to all types of active neutrinos: electronic, muonnoye and tau-neutrino. Moreover, electronic neutrinos are recalculated separately through a charged current. As a result, it was found that the full stream of the neutrino from the sun is in accordance with the standard solar model (SSM), and the flow of electronic neutrinos is about 1/3 of the stream predicted by the SSM. Thus, in the experiment it was shown that the deficiency of electronic neutrinos from the Sun, detected in radiochemical experiments, occurs due to the transformations of one type of neutrino into another, i.e., the phenomenon of oscillations.

And yet there was another question. The fact is that there is the so -called effect of Mikheev - Smirnov - Wolfenstein (Mikheev and Smirnov worked in Iya RAS). The neutrino oscillations in a vacuum may not occur completely: neutrino, born as electronic, can only partially go into other types of neutrino - the so -called mixing angle plays a role here. Thanks to the effect of Mikheev - Smirnov - Wolfenstein, the mixing of neutrino flying in the substance (namely inside the Sun) changes and with a certain density of electrons becomes maximum, so that one neutrino is evenly mixed in all three types, which showed observations. But the way neutrinos in vacuum really mixed up, it still had to measure.

* * *

The next step in the neutrino drama is the Japanese experiment Super-Kamiokande.

Its detector is a huge barrel of stainless steel with a diameter of 39.3 m and a height of 41.4 m, filled with clean water, which is visible 11 thousand photographs installed in the walls of the detector. The total mass of the detector is 50 thousand tons. Photo -enhances record Cherenkovsky radiation of muons born in the detector. The installation, like the above neutrino detectors, is located deep underground to protect against the background. Its main “booty” is neutrinos of medium energies (hundreds of MeV), born in atmospheric showers. The detector distinguishes neutrino, which go from above and below - that is, those that went through the earth through. The result of the experiment is shown in the figure on the right.

The registered stream of muon neutrinos, which passed through the Earth in a vertical direction, that is, from the bottom up, turned out to be twice as smaller than the stream of neutrino flying from the atmosphere, i.e., from top to bottom. The result is explained by the oscillations of the muon neutrinos in Tau-neutrino. Moreover, in this case, oscillations cannot be described by the effect of Mikheev - Smirnov - Wolfenstein - the thickness of the earth with such energies is not enough. This means that the mixing between Muonnoye and Tau-neutrino is great.

The results obtained in experiments with sunny and atmospheric neutrinos were confirmed in reactor and accelerating experiments, in which “man-made” neutrinos were used from beta-dashes in the active zone of the reactor (electronic antineutrino) and from decays born in the proton-nuclear colonies of peonies (muonnic neutrinos and antinerino). So, for example, the measured oscillation parameters in the Kamland reactor experiment (Japan) are in excellent consent with the parameters obtained in experiments with solar neutrinos. The oscillation parameters measured in accelerator experiments K2K and T2K (Japan) and Minos (USA) coincide with the results of Super-Kamiokande. The T2K experiment opened the oscillations of muon neutrinos in electronic neutrinos, and Double Chooz (France), Daya Bay (China) and Reno (Korea) measured the last unknown mixing angle between the first and third mass states. In fact, in these experiments, the oscillation physics undergoes a qualitative transformation - a smooth transition from the stage of discoveries to the stage of precision measurements. This, of course, does not exclude new unexpected results and discoveries, which are so generous neutrine physics. In particular, the fundamental problem of the CP in neutrino oscillations and the experimental detection of this effect, which became possible due to the fact that all three corners of mixing neutrinos are different from zero and have large sizes comes to the fore.

* * *

Now the physics of particles is based on the so -called standard model - it contains strong, weak, electromagnetic and slightly apart gravitational interactions. It contains a gentleman's set of particles, including, among other things, three generations of leptons and quarks. Neutrino is lepton, there are three of them, and in the standard model their mass is equal to zero. If not, then you have to explain why it is so small - at least a million times easier than the next lightest particle - an electron. The standard model simply does not have such a parameter to give the difference in mass in a million. And the difference in the masses is even smaller and requires an even smaller parameter. This does not mean that neutrino oscillations refute the standard model. This means that it should be expanded - beyond its borders there is a new physicist. One of the interesting options-the existence of very heavy particles, somewhere around 1015 GEV-is an effect where the mass of a heavy particle is in the denominator in the expression for the mass of a light particle associated with it.

This is the fourth (or fifth, if you consider the Li and Yang Award in 1957 for the “insightful study of the so -called paisse laws, which led to important discoveries in the physics of elementary particles”) the Nobel Prize associated with the physics of the neutrino. It seems that it is not the last, since the detective neutrine epic is far from finished.