Why is it so hard to study neutrinos and what this particle will tell you about the history of the Universe

Why is it so hard to study neutrinos and what this particle will tell you about the history of the Universe

Neutrino is one of the most abundant particles in the Universe and is incredibly difficult to detect. It is important to study neutrinos because they contain information about the phenomena and processes that generate them: this means that with the help of a particle, you can learn about the origin of the universe. Let's talk about all the secrets that neutrinos keep in themselves.

What are neutrinos?

Neutrinos are ultra-light particles formed during nuclear reactions. Most of those found on Earth come from the Sun, which converts hydrogen into helium. But in the 1930s it was predicted that the sun should also produce a different type of neutrino through reactions involving carbon, nitrogen and oxygen - the so-called "CNO neutrinos." It was only almost a century later that the Borexino detector first detected these particles.

Until recently, it was not at all clear if she had a lot. In recent years, it has become clear that there is, but very small. Its exact value is unknown at this time, and the available estimates in general boil down to the fact that neutrinos are about 10 orders of magnitude lighter than a proton. The weight of a grasshopper (about 1 gram) correlates in approximately the same way with the displacement of the modern nuclear aircraft carrier George Bush (about 100 thousand tons).

A particle has no or almost no electric charge - experiments have not yet given an unambiguous answer, and of all fundamental physical interactions, it reliably participates only in weak and gravitational ones.

Neutrinos are classified into three generations: electron, muon, and tau neutrinos. They are usually listed in that order, and this is no coincidence: this is how the sequence of their opening is displayed. In addition, there are also antineutrinos - these are antiparticles of three different types, corresponding to "ordinary" ones. Neutrinos of different generations can spontaneously transform into each other. Scientists call this neutrino oscillations, and they were awarded the 2015 Nobel Prize in Physics for their discovery.

Neutrinos are the result of nuclear (and thermonuclear, we will not separate them further) reactions. There are a lot of them, elusive. According to the calculations of theoretical physicists, there are about 10 9 neutrinos for every nucleon (that is, a proton or neutron) in the Universe. Nevertheless, we do not notice it at all: the particles pass through us.

How are scientists looking for neutrinos?

Modern detectors do not register neutrinos themselves - this is still impossible. The object of registration is the results of the interaction of the particle with the substance filling the detector. It is chosen so that neutrinos of certain energies of interest to developers react with it. Since the energy of neutrinos depends on the mechanism of their formation, we can assume that the detector is designed for particles of a certain origin.

As soon as it became clear that neutrinos, although difficult, but still possible to register, scientists began to try to catch neutrinos of extraterrestrial origin. Their most obvious source is the Sun. Nuclear reactions constantly occur in it, and it can be calculated that about 90 billion solar neutrinos per second pass through each square centimeter of the earth's surface.

At that time, the most effective method for catching solar neutrinos was the radiochemical method. Its essence is as follows: the solar neutrino arrives at the Earth, interacts with the nucleus; it turns out, say, a 37Ar nucleus and an electron (it was this reaction that was used in the experiment of Raymond Davis, for which he was later awarded the Nobel Prize).

After that, by counting the number of argon atoms, we can say how many neutrinos interacted in the volume of the detector during the exposure. In practice, of course, things are not so simple. It should be understood that it is required to count single argon atoms in a target weighing hundreds of tons. The mass ratio is about the same as between the mass of an ant and the mass of the earth. It was found that ⅔ solar neutrinos were stolen (the measured flux turned out to be three times less than the predicted one).

In the middle of Baikal covered with ice tractors, caterpillar equipment drive, winches are placed around. And so every winter. Who would have thought that all this is needed for a telescope.

Very Large Volume Neutrino Telescopes Baikal. Photo © baikalgvd. inr. u

The Baikal-GVD Observatory - Baikal Gigaton Volume Detector - is located in the middle of the lake, about 3.5-4 kilometers from the coast, and looks very peculiar: these are garlands 525 meters long, which immersed in water to a depth of more than a kilometer. To date, there are already more than fifty such garlands hung there, and in the coming years they plan to add about 150 more to them. And 36 identical balls are fixed on each. In total, there are more than two thousand of them in the waters of Lake Baikal.

These spheres are called optical modules. There are whole complexes of complex equipment inside. They pick up radiation. Unusual, amazing radiation. It is this. Faster than light in a vacuum, nothing can move. But through any - any - matter, he flies a little slower than in a vacuum. And it so happens that some charged particles rush through this substance, overtaking photons. That is, we can say that inside some kind of environment, they rush with superluminal speed. And at the same time they emit photons, that is, they glow. This radiation follows the particle and forms a cone, like sound waves coming from a supersonic aircraft.

And if the substance is transparent, then this phenomenon can be admired. The result is a mysterious blue glow. It is he who is observed when starting nuclear reactors.

It is called the Cherenkov glow, or the Vavilov-Cherenkov effect. For the discovery of this natural phenomenon, Soviet physicist Pavel Cherenkov and his colleagues Igor Tamm and Ilya Frank were awarded the Nobel Prize in 1958. Scientists decided that the name of Sergei Ivanovich Vavilov should also be immortalized in the name of the effect - it was in his laboratory and under his leadership in 1934 that this light was first seen.

But the most interesting thing is that it can be observed not only in a laboratory or at a nuclear power plant. The phenomenon is also found in nature, namely in the ocean depths. In the darkness at the very bottom, sometimes there are dim flashes - this is the decay of radioactive isotopes of potassium and other natural radionuclides that enter the water from the depths of the earth as a result of natural processes. The result of this decay is the same: electrons fly out, and with them photons.

Space as nobody saw it The best sky map in the Universe - photo

And it also happens that neutrinos crash into the atoms of water. These are such ubiquitous particles that arise as a result of nuclear reactions both inside stars, and on Earth, and in the atmosphere due to its bombardment by cosmic rays. "Neutrino" is an Italian word translated as "neutron". The term was coined by the physicist Enrico Fermi, the creator of the world's first nuclear reactor and the man who formulated the famous "Fermi paradox" ("Where are the aliens?"). This word indicated that the particle is neutral and very small - much smaller than a neutron. The difference between these two particles is that the neutron consists of three even smaller particles - quarks, while the neutrino has no constituent parts. The Universe is teeming with these crumbs, but no one notices them - they practically do not interact or, in any case, very weakly interact with other particles. For them, almost everything is transparent.

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