If you spread your arms and turn your palms towards the sky, about 500 muons created by cosmic radiation will pass through them in one minute. Until recently, the beams of these interesting, almost miraculous elementary particles – 10,000 of which land on a square meter of the Earth’s surface – could not be “cooled” and used in experiments as electrons or protons. Now, an international experiment in which several researchers from our Institute of Physics in Belgrade also participated has shown that this is feasible. Namely, the MICE collaboration published the result of an experiment in the prestigious journal “Nature” which shows that it is possible to cool muons.

The illustration shows a sketch of a shower of muons falling on one of the many detectors of these particles, at the Pierre Auger Observatory in Argentina. The muons arrive on Earth due to cosmic radiation, and at one time, in 1936, they were discovered by the American physicists Carl Anderson and Seth Nedermeyer. They then observed unknown particles from cosmic radiation in the magnetic field that behaved like electrons, but their mass was too large. With the mass between electrons and protons, they were initially classified in the group of “medium-heavy” particles, mesons, so they were called “we mesons” for a long time.

In the middle of the 20th century, the ancient idea of ​​Democritus, beautiful and simple, about a world made up of elementary particles, turned into a nightmare (as is the case with beautiful and simple ideas). New experiments from year to year revealed new particles with new properties – in addition to neutrons and protons, the existence of pi, K, eta mesons, then lambda, sigma, omega and other baryons – it was a whole forest of particles that was difficult and enumerate, and aspired to the elementary status of the basic gradients of matter.

It will be shown that all these newly discovered hadrons (a family that includes both a proton and a neutron) are not actually elementary at all, and that “within them” are more elementary particles, forever intertwined, which Marie Gel-Man will call quarks in the mid-1960s. James Joyce. This allowed all those numerous baryons recorded in the bubble chambers to be understood as particles that are some of the combinations of 3 quarks, while the slightly lighter mesons are made up of 2 quarks. Most of the known world consists of only two combinations of quarks, those that build a stable proton and a neutron, but there are many unstable ones.

We mesons, however, did not fit into this picture of hadrons made up of quarks because they proved to be a significantly different, awkward member of the meson family. They looked more like electron brothers – they were almost like them, only with a mass about 200 times larger and the ability to penetrate deeper into matter. As they are not affected by a strong interaction, it became clear that they are not made of quarks at all and that they are not actually mesons. That is why their name will be changed to mioni.

A standard model that describes the structure of matter by two groups of particles, leptons and quarks (and particles that carry interactions), will classify muons, along with electrons, into leptons. There are six of them – the electron, the muon and the taon, and next to them, three of their neutrinos, each of which has its own antiparticles.

Muons are naturally formed by the collision of high-energy rays, cosmic radiation, with particles in the higher layers of the atmosphere. In a series, pawns are formed first, which break down into muons and neutrinos. The muons then fall to the ground in a downpour from a height of about 10 kilometers. It all doesn’t sound strange – particles constantly collide this way and that – if it weren’t for one completely confusing detail.

Muons, like most particles, are unstable. Their average lifespan is about two millionths of a second, after which they decay into an electron and two neutrinos. However, the average lifespan of muons is so short that at the speed at which they move, they should not cover a distance of more than a few hundred meters, and yet they fall from a height of 10 kilometers. The muons must disintegrate long before they touch your fists, and experiments show that they not only reach the ground, but also penetrate more than 100 meters deep into the Earth. How is that possible?

Namely, muons are living proof of the correctness of Einstein’s special theory of relativity. They move extremely fast, almost at the speed of light, so it becomes evident that time does not flow equally in their system and in the system of observers waiting for them on Earth with open arms. This so-called time dilation causes their short inherent time to decay in a dormant system on Earth, lasting much, much longer, quite enough for the muons to cross their path.

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