A bit about neutrinos, cosmology and domestic projects

Inspired by an article about IceCube and the first neutrinos caught directly generated .
Undoubtedly, this is a great achievement for neutrino astrophysics, and in general physics as a whole. An event comparable in scale to the discovery of the Higgs boson, and no less interesting. However, I would like to clarify several points described both in the article itself and in the comments to it.

First, the essence of the process


As mentioned in the original article, a huge number of detecting elements (photomultiplier tubes) were frozen into Antarctic ice to detect high-energy neutrinos. The problem is that the neutrino does not have a charge, practically does not have a mass and almost does not interact with matter. As Wolfgang Pauli, who predicted his existence, said:
I did something terrible today. A theoretical physicist should never do this. I suggested something that could never be verified experimentally.

However, later in 1934, the Vavilov-Cherenkov radiation was discovered , which describes the glow of a particle moving with a speed greater than the speed of light in a medium (in a medium, since the speed of light in vacuum cannot be exceeded in classical physics). It is this process that is used to detect neutrinos, which, although extremely rare, still interact with matter, giving rise to related leptons. These particles take away the lion's share of the neutrino energy, which allows them to move in the medium with the same speed greater than the speed of light in the medium, and they glow. This light freely spreads (over short distances) in water or ice, and this light is detected by PMTs.

Types of neutrinos


Physics currently knows three types of neutrinos — three types of known leptons: electron, muon, and tau neutrinos. The existence of the so-called sterile neutrino - neutrino with zero lepton number is also predicted (and to some extent confirmed). Such a particle will not participate in weak interaction, which means it will not interact with matter and give rise to related leptons. Sterile neutrinos cannot be caught using a Cherenkov detector. These sterile neutrinos are being studied in neutrino oscillation processes.- spontaneous transformations of a neutrino of one type into a neutrino of another type. However, to study such a process, we need to know which neutrinos flew out and which got to their destination. By difference, we can say that the content has changed in the process. For such experiments, neutrinos generated in accelerators are used, since we know which particles we created and which we later caught.

Filtration system


The detector is directed downward and, accordingly, uses the entire Earth as a filter. This is necessary because the stream of cosmic rays "from above" creates a huge number of background events, beyond which it is impossible to discern the extreme rare neutrino interactions. A neutrino flying through the earth interacts with matter, giving rise to a muon, or electron (depending on the type of neutrino). The electron interacts well with the substance and creates a powerful, but short shower, which is visible as a bright dot in the detector. The muon is able to fly a certain distance in the thickness of the earth and, using the Vavilov-Cherenkov effect, creates a long track by which it is possible to determine the direction of motion of the particle. The muon also ionizes the substance in its path, creating small showers throughout the track. The intensity of these showers can be used to determine the neutrino energy.
As for neutrinos born in the center of the Earth, they are born as a result of beta decay, which means that the vast majority are electronic (excluding oscillations). These neutrinos also have much lower energy, which is easy to distinguish in the detector. And since the interaction cross section grows together with energy, then they interact much less orders of magnitude.

Neutrino telescopes


IceCube, of course, is not the first neutrino telescope. Their story stretches from the DUMAND detector, delivered to the Hawaiian Islands, work on it was discontinued in 1994. Next was the Baikal NT200, the American Amanda (the predecessor of IceCube), the European Antares, the Greek Nestor, the Italian Nemo. But all these are small detectors, on which very important results were obtained, but their potential has already been exhausted. Yes, once the Baikal neutrino telescope was the world's first large (at that time) neutrino detector operating in the natural environment. And this was not achieved in the bright Soviet Union. The telescope was built against the odds of a small group of people in the period from 1993 to 1998.
But modern physics needs large-scale detectors with an effective volume of the order of a cubic kilometer. There is only one such detector in the world - the American IceCube. There is also a project for the integrated European detector KM3NeT and a project for the new Baikal detector NT1000. However, there is a crisis in Europe, and in Russia ... in Russia - FANO.

What is unique about these 28 neutrinos?


And uniqueness in energy. There is such a Graisen-Zatsepin-Kuzmin effect that prohibits the detection of cosmic radiation with an energy above 10 19 eV on Earth . The neutrino is our only chance to overcome this boundary when exploring the universe. And a neutrino of such energy is a direct-generated neutrino, born somewhere far away, with some process, possibly still completely unknown to us. And the neutrino from the supernova is neutrino from a well-known source, and these 28 events themselves will show us new sources that we could never see by other means (at least in the framework of the modern development of science and technology).

Russians don't give up


Despite the fact that the creation of a neutrino telescope is difficult, expensive and, in modern realities, practically impossible, the Baikal collaboration continues to create its own facility. The problem is that the detector looks through the Earth, and in this regard we have an advantage that the Americans will never have. The center of the galaxy - the most interesting part of our universe is located in the southern hemisphere. In this regard, the European project is a competitor to the Russian project, and IceCube complements it, as it looks in the other direction. In addition, pure (as yet) Baikal water has some optical advantages over Antarctic ice. And if the Baikal project is completed, then it is likely that the data obtained will be even more interesting than that of the Americans.

I will not say anything about parallel measurements and the theory of superstrings, if anyone is interested, read the "Elegant Universe" by Brian Green, this is the most popular, understandable and very high-quality book from one of the creators of the theory. I can only say that having a specialized higher education, I understood little from there.

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