It is a little about neutrino, cosmology and domestic projects
Inspired by the article about IceCube and the first direct-generation neutrinos caught .
Undoubtedly, this is a great achievement for neutrino astrophysics, and in general for the whole of physics in general. An event comparable in scale to the discovery of the Higgs boson, and no less interesting. However, I would like to clarify a few points described, both in the article itself and in the comments to it.
As stated in the original article, a huge number of detecting elements (photomultipliers) were frozen in the Antarctic ice to detect high-energy neutrinos. The problem is that the neutrino has no charge, has almost no mass and almost does not interact with matter. As Wolfgang Pauli said, predicting his existence:
However, later in 1934, Vavilov-Cherenkov radiation was discovered, describing the emission of a particle moving at a speed greater than the speed of light in the medium (precisely in the medium, since the speed of light in vacuum in classical physics cannot be exceeded). It is this process that is used to detect neutrinos, which, although extremely rare, but it still interacts with the substance, giving rise to related leptons. These particles take the lion's share of the neutrino energy, which allows them to move in the medium at the same speed, which is greater than the speed of light in the medium, and they glow. This light propagates freely (at short distances) in the water, or ice, and this light detects the PMT.
At the moment, physics knows three types of neutrinos - three types of known leptons: electron, muon and tau neutrinos. It is also predicted (and to some extent confirmed) the existence of the so-called sterile neutrino - neutrino with zero lepton number. Such a particle will not participate in the weak interaction, and therefore will not interact with the substance and give rise to related leptons. Sterile neutrinos cannot be caught with the help of the Cherenkov detector. These sterile neutrinos are investigated in the processes of neutrino oscillations — spontaneous transformations of one type of neutrino into another type of neutrino. However, to investigate such a process, we need to know which neutrinos flew out and which ones hit the destination. The difference is that the content has changed in the process. For such experiments, neutrinos born in accelerators are used, since we know which particles we have created and which were then caught.
The detector is directed downwards and, accordingly, uses the whole Earth as a filter. This is necessary because the flux of cosmic rays "from above" creates a huge number of background events, beyond which it is impossible to discern extreme rare neutrino interactions. A neutrino that has flown through the earth interacts with matter, giving rise to a muon, or an electron (depending on the type of neutrino). The electron interacts well with matter and creates a powerful but short downpour, which is visible as a bright dot in the detector. Muon is able to fly some distance in the earth’s thickness and, using the Vavilov-Cherenkov effect, creates a long track by which one can determine the direction of movement of the particle. Muon also ionizes a substance in its path, creating small showers throughout the track. The intensity of these showers can determine the energy of the neutrino.
As for neutrinos born in the center of the Earth, they are born as a result of beta decay, which means that in the overwhelming majority are electronic (excluding oscillations). Also, these neutrinos have much lower energy, which is easy to distinguish in the detector. And since the interaction cross section grows with energy, then they interact much less frequently.
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IceCube, of course, is not the first neutrino telescope. Their story stretches from the DUMAND detector, placed off the Hawaiian Islands, and work on it was discontinued in 1994. Next was Baikal NT200, American Amanda (predecessor of IceCube), European Antares, Greek Nestor, 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 its natural environment. And this was not achieved in the light of the Soviet Union. The telescope was built in spite of everything by a small group of people in the period from 1993 to 1998.
Modern physics needs scale detectors with an effective volume of the order of cubic kilometers. Such a detector in the world there is one - American IceCube. There is also a project of a joint European detector KM3NeT and a project of a new Baikal detector NT1000. However, there is a crisis in Europe, and in Russia ... in Russia - FANO.
A uniqueness in energy. There is the Graisen-Zatsepin-Kuzmin effect, which prohibits the detection of cosmic radiation on Earth with an energy above 10 19 eV. Neutrino is our only chance to cross this boundary when exploring the universe. And also the neutrino of such energy is a direct-generation neutrino, born somewhere far away, with some kind of, possibly, process that is still completely unknown to us. And the neutrino from a supernova is a neutrino from a known source, and these 28 events themselves will show us new sources that we could never see by other means (at least within the framework of the modern development of science and technology).
Despite the fact that the creation of a neutrino telescope is difficult, expensive and, in modern reality, almost impossible, the Baikal collaboration continues to create its installation. The problem is that the detector is looking through the Earth, and in this respect we have an advantage that the Americans will never have. The center of the galaxy is the most interesting part of our universe located in the southern hemisphere. In this regard, the European project is a competitor to the Russian project, and IceCube complements it, because it looks the other way. In addition, the pure (still) Baikal water has some optical advantages over the Antarctic ice. And if the Baikal project is completed, then it is likely that the data obtained will be even more interesting than the Americans.
I will not say anything about parallel measurements and the theory of superstrings, if anyone is interested, read Brian Green's “Elegant Universe”, 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 profile higher education, I understood very little from there.
Undoubtedly, this is a great achievement for neutrino astrophysics, and in general for the whole of physics in general. An event comparable in scale to the discovery of the Higgs boson, and no less interesting. However, I would like to clarify a few points described, both in the article itself and in the comments to it.
To start the process
As stated in the original article, a huge number of detecting elements (photomultipliers) were frozen in the Antarctic ice to detect high-energy neutrinos. The problem is that the neutrino has no charge, has almost no mass and almost does not interact with matter. As Wolfgang Pauli said, predicting his existence:
I did something awful today. A theoretical physicist should never do this. I suggested something that could never be tested experimentally.
However, later in 1934, Vavilov-Cherenkov radiation was discovered, describing the emission of a particle moving at a speed greater than the speed of light in the medium (precisely in the medium, since the speed of light in vacuum in classical physics cannot be exceeded). It is this process that is used to detect neutrinos, which, although extremely rare, but it still interacts with the substance, giving rise to related leptons. These particles take the lion's share of the neutrino energy, which allows them to move in the medium at the same speed, which is greater than the speed of light in the medium, and they glow. This light propagates freely (at short distances) in the water, or ice, and this light detects the PMT.
Types of neutrinos
At the moment, physics knows three types of neutrinos - three types of known leptons: electron, muon and tau neutrinos. It is also predicted (and to some extent confirmed) the existence of the so-called sterile neutrino - neutrino with zero lepton number. Such a particle will not participate in the weak interaction, and therefore will not interact with the substance and give rise to related leptons. Sterile neutrinos cannot be caught with the help of the Cherenkov detector. These sterile neutrinos are investigated in the processes of neutrino oscillations — spontaneous transformations of one type of neutrino into another type of neutrino. However, to investigate such a process, we need to know which neutrinos flew out and which ones hit the destination. The difference is that the content has changed in the process. For such experiments, neutrinos born in accelerators are used, since we know which particles we have created and which were then caught.
Filtration system
The detector is directed downwards and, accordingly, uses the whole Earth as a filter. This is necessary because the flux of cosmic rays "from above" creates a huge number of background events, beyond which it is impossible to discern extreme rare neutrino interactions. A neutrino that has flown through the earth interacts with matter, giving rise to a muon, or an electron (depending on the type of neutrino). The electron interacts well with matter and creates a powerful but short downpour, which is visible as a bright dot in the detector. Muon is able to fly some distance in the earth’s thickness and, using the Vavilov-Cherenkov effect, creates a long track by which one can determine the direction of movement of the particle. Muon also ionizes a substance in its path, creating small showers throughout the track. The intensity of these showers can determine the energy of the neutrino.
As for neutrinos born in the center of the Earth, they are born as a result of beta decay, which means that in the overwhelming majority are electronic (excluding oscillations). Also, these neutrinos have much lower energy, which is easy to distinguish in the detector. And since the interaction cross section grows with energy, then they interact much less frequently.
')
Neutrino telescopes
IceCube, of course, is not the first neutrino telescope. Their story stretches from the DUMAND detector, placed off the Hawaiian Islands, and work on it was discontinued in 1994. Next was Baikal NT200, American Amanda (predecessor of IceCube), European Antares, Greek Nestor, 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 its natural environment. And this was not achieved in the light of the Soviet Union. The telescope was built in spite of everything by a small group of people in the period from 1993 to 1998.
Modern physics needs scale detectors with an effective volume of the order of cubic kilometers. Such a detector in the world there is one - American IceCube. There is also a project of a joint European detector KM3NeT and a project of a new Baikal detector NT1000. However, there is a crisis in Europe, and in Russia ... in Russia - FANO.
What is the uniqueness of these 28 neutrinos?
A uniqueness in energy. There is the Graisen-Zatsepin-Kuzmin effect, which prohibits the detection of cosmic radiation on Earth with an energy above 10 19 eV. Neutrino is our only chance to cross this boundary when exploring the universe. And also the neutrino of such energy is a direct-generation neutrino, born somewhere far away, with some kind of, possibly, process that is still completely unknown to us. And the neutrino from a supernova is a neutrino from a known source, and these 28 events themselves will show us new sources that we could never see by other means (at least within 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 reality, almost impossible, the Baikal collaboration continues to create its installation. The problem is that the detector is looking through the Earth, and in this respect we have an advantage that the Americans will never have. The center of the galaxy is the most interesting part of our universe located in the southern hemisphere. In this regard, the European project is a competitor to the Russian project, and IceCube complements it, because it looks the other way. In addition, the pure (still) Baikal water has some optical advantages over the Antarctic ice. And if the Baikal project is completed, then it is likely that the data obtained will be even more interesting than the Americans.
I will not say anything about parallel measurements and the theory of superstrings, if anyone is interested, read Brian Green's “Elegant Universe”, 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 profile higher education, I understood very little from there.
Source: https://habr.com/ru/post/203984/
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