# How to make a neutrino beam and how to detect a neutrino

## How to make a neutrino ray

A simple explanation for non-professionals is how to make a neutrino beam (details depend on the specific experimental lab).

First, create a beam of protons - just as if you were loading the Large Hadron Collider (this is a separate story, but for now let's take, for granted, the existence of a beam of protons).

Then push the proton beam with the target - a thin sheet of material. The protons will collide with the nuclei of the atoms of the material and break them down - not only separating into protons and neutrons, but also generating many other particles, including pions (example of hadrons) with both positive and negative electric charge. All these particles will fly from the back of a sheet of material, with the result that we get a beam of protons, neutrons, pions and some other particles.

Now align the beam and the magnet. Magnet will twist the path of charged particles. The direction of the curvature depends on the charge of the particle; the degree of curvature depends on the particle energy. So neutrons will go straight, negatively charged pions will go one way, and protons and positively charged peonies will go to another. Let the majority of the particles go to the wall; where you leave the passage, particles passing through it will have approximately equal energies and electric charges. Thus, by placing the passage in the right place, you can get a beam consisting mainly of positively charged pions with the same energies.

Peonies will begin to decay, turning into an anti-muon and neutrino. Soon, your beam will already consist of positively charged muons, several not yet decayed pions and protons, and also neutrinos.

Now align the beam with another magnet. Neutrinos, as electrically neutral, will pass on. Positively charged particles - muons, and the remaining pions with protons, will deviate in one direction. Let them go to the wall. And what will remain? Ray of neutrino. Not particularly narrow, of course, but if you started with a large number of protons, it will be very powerful.

By controlling the motion of the initial protons and intermediate pions, this beam can be directed in any direction. For example, it can be created at CERN and directed towards the Gran Sasso d'Italia mountain, where the OPERA experiment is taking place . This beam will not be narrow - by the time it passes 730 km to Gran Sasso, it will be 2 km across. But it will be enough for our purposes.

## How to detect a neutrino

A simple explanation for non-professionals on how to detect neutrinos.

Neutrinos constantly pass through your body. Their flow comes from the Sun, from its central furnace, and even if you are on the night side of the planet, these neutrinos pass through the Earth and through your body as if no Earth is there. Cosmic rays (high-energy particles arriving from space) often hit atoms in the upper atmosphere and produce several neutrinos. They pass through you too.

Almost always. But a very, very small part of the neutrino crashes into something.

If a neutrino enters the nucleus of an atom, passes inside one of the protons or neutrons and (roughly speaking) turns out to be too close to a quark (or antiquark) that is inside a proton or neutron, then there is a good chance that the neutrino and quark (or antiquark) will collide. The same can be said about neutrinos colliding with an electron on the outskirts of an atom. But this process occurs infrequently, because it involves a weak nuclear interaction, and (especially for low-energy neutrinos) the weakness of this interaction ensures the rarity of such collisions.

Suppose that a neutrino collides with a quark or an antiquark inside the atomic nucleus: what happens next? If a neutrino has enough energy, it breaks the nucleus into separate protons and neutrons, and often, if its energy is high, it leads to the appearance of pions (another type of hadrons: particles consisting of quarks, antiquarks and gluons, like a proton with a neutron). The neutrino continues its path unregistered, but the resulting protons, neutrons and pions can be observed, because they in turn collide with other atomic nuclei, and break them into pieces. The specific features of the observation methods depend on the detectors.

There is another possibility. Sometimes during a collision with a quark or an antiquark, a neutrino can turn into a charged lepton, for example, an electron, muon, or tau. The type of lepton depends on what type of neutrino was, and may even depend on what the neutrino did before it arrived.

The possibility of this option dictates the feature of a weak nuclear interaction that implements this transformation through the W-field, the waves of which are W-particles. In this case, it is possible to detect not only protons, neutrons and pions scattering from the first and subsequent collisions, but also an electron, muon, or tau decay products, into which the neutrino has turned. In the latter case, Tau decay products include an electron, muon, or pion with several photons — and all this can be fixed.

It turns out that although we cannot easily and reliably fix the presence of neutrinos in the way that can be done with electrons or muons (colliding with atoms when passing through matter) or protons and neutrons (colliding with a large number of atomic nuclei when passing through matter), we yet we can sometimes observe them. If you have enough neutrinos, for example, after a not very distant star has turned into a supernova, or at the center of a neutrino beam, or even just a constant stream of neutrinos from the Sun, we can detect these neutrinos when one of them collides with atomic nucleus inside the detector. This is due to the fact that even a single collision with one unfortunate nucleus can create a cascade of protons, neutrons and pions (which we can easily detect), and possibly electrons and muons (which we can also easily detect).

It turns out that one of the ways to study neutrinos is to create powerful neutrino rays, build a detector capable of capturing protons, neutrons, pions, muons and / or electrons flying from a nucleus broken by neutrinos, and be patient with (OPERA experiment took three years to detect 16000 neutrinos - only a half dozen per day). There are many other neutrino detectors in the world, they use different materials and different strategies. A common way is to build a huge detector filled with water or another clean fluid, located deep underground to protect against cosmic rays, and patiently wait for some random neutrino from the Sun or one of the cosmic rays, or from a supernova that can generate ". And the splashes are tangible - in recent times, several important discoveries have already been made with the help of neutrinos. Perhaps the most important of these was done at OPERA. [Shortly before writing the article in September 2011, data were obtained in this experiment, according to which some neutrinos showed movement exceeding the speed of light. After thorough checks, it turned out that the reason for this was the experiment error - approx. transl.]

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