A muon is an elementary particle that is very similar to an electron, but much more massive than it. It was discovered in the first half of the twentieth century and was initially suspected to be something else entirely. Let’s understand everything in order to understand why muons are so important to us.
In search of a meson
Some time ago there was news that scientists had solved the mystery of the excess of muons that reach the Earth’s surface. However, for many people, even the existence of this type of particles turned out to be a sensation. Therefore, let’s see where they come from.
After Niels Bohr proposed his planetary model of the atom at the very beginning of the twentieth century, scientists were convinced that all matter in the universe could be reduced to three components: protons, neutrons and electrons. However, in 1935, a young Japanese physicist Hideki Yukawa pondered the question of why protons and neutrons in the nucleus of an atom stick together.
Yukawa concluded that there must be some unknown particle that ensures the existence of a strong interaction between the constituents of the nucleus. The physicist suggested that such a particle should have a mass intermediate between a very heavy proton and a light electron. Because of this, the new particle was named meson, which comes from the Latin word for ‘middle’.
The search for the meson did not last long. Already in 1937, physicists Carl Anderson and Seth Neddermeyer, while studying cosmic rays, discovered a particle that was deflected by a magnetic field stronger than protons but weaker than electrons. At first glance, this was the very particle that Yukawa had predicted.
However, already in 1942 three of his countrymen doubted that it was really so, and in 1947 a completely different particle of intermediate mass was found, which was much better suited to the role of a carrier of the strong interaction. To avoid confusion, it was called a pi meson, and what Anderson and Neddermeyer found in 1937 was called a mu meson.
Later, physicists began to discover more and more mesons in accelerators, and a strange thing was discovered. They had a lot in common with each other and with the pi meson, while the one discovered in 1937 seemed to be a completely different particle.
The main difference between what eventually came to be called not a mu meson, but simply a muon, and the other particles was that it did not interact with the particles of the nucleus at all. That is, it was not exactly what Yukawa had predicted, but rather a heavy type of electron.
Properties of a muon
In the middle of the twentieth century, what is now known as the Standard Model was formed. All matter of the Universe in it is reduced to 17 particles, and mesons are not among them. The carriers of the strong interaction are gluons. Pi-, K- and other discovered ‘mesons’ turned out not to be fundamental particles at all. They consist of quarks – the same components as protons and neutrons.
But the muon turned out to be a real elementary particle. However, its name has been preserved rather for historical reasons and does not carry any meaning for a long time. In the Standard Model, quarks and leptons are divided into three generations, each of which is a heavier version of the previous one. The electron is a lepton of the first generation. The muon is the second and is 207 times heavier than its ‘little brother’. There is also a third generation particle, the taon, and each of the generations corresponds to its own neutrino.
The muon, like the electron, has a single negative charge and behaves the same way in most cases, but it is an unstable particle. It is mostly born from even less stable pi- and K-mesons, which are formed when molecules in the upper atmosphere are bombarded by high-energy photons.
The muon has a lifetime of only 2.2 microseconds. After that it decays into a muon neutrino and a W-boson, and the latter into an electron and an electron antineutrino. In about one per cent of cases, together with these particles, a photon is also formed, and once in 10,000 cases, an electron and a positron.
At the same time, muons formed in the upper atmosphere move towards the Earth with a speed close to light speed and not only reach it, but can go hundreds of metres deep before they decay. Every minute, up to 10,000 muons pass through 1 m2 of our planet’s surface, but we do not notice it because they interact very weakly with matter.
It is with the birth of muons in the upper atmosphere that a recent study has made quite a stir. There are more muons being produced there than previously predicted by theory. And now scientists have found an explanation.
However, humanity has long received muons not only from this natural source. In some cases, some heavy baryons, bosons, and even protons and neutrons can decay into muons and other particles. However, mostly they are still obtained from the decay of tau particles, which are the only leptons heavier than them.
Muon material
The reason why muons interact so weakly with the rest of matter and because of this can pass relatively freely through very dense materials is that they, despite their rather large mass, unlike protons and neutrons, do not participate in strong interactions.
However, all this does not mean that they cannot be part of it. Muons can, like electrons, occupy positions in the orbitals of atoms. However, because of their mass, these must be very special orbitals, which are about 200 times closer to the nucleus than electron orbitals.
As a consequence, muons may at some point react with protons to form neutrons and muon neutrinos. However, even if this does not happen, the time of their existence in the orbital is limited by the same 2.2 µs. That is, all materials obtained in this way are short-lived.
A material based on muons, or rather their antiparticles, can be formed in a completely different way. In this case, an additionally charged antimuon plays the role of a proton. And an ordinary electron falls into its orbital. Something similar to a lighter version of the hydrogen atom is formed. Such a material is called muonium.
Finally, there may exist such an amazing form of matter as dimuonium, or true muonium. These exotic atoms are a pair of muon and antimuon joined together. Although the possibility of the existence of such objects has been confirmed theoretically, so far, they have never been observed anywhere.
Muon catalysis
All the above described is very interesting. But the question arises: do muons have any practical application? And the main answer is thermonuclear fusion. It is this type of reactions that promises mankind a sea of relatively inexpensive and clean energy, the fuel reserves for the extraction of which will run out very soon.
The only problem is that protons, in order to merge together, need to overcome the repulsive forces that exist between charged particles of the same name. On the Sun, this is achieved by very high temperature and pressure, which cannot be recreated on Earth for decades.
And one of the main candidates for solving this problem is muon catalysis. As already mentioned, a muon can replace a single electron in a hydrogen atom. It is large, negatively charged, so the repulsive forces between such an atom and another, normal one, are much lower, and the temperature that is needed is quite possible to achieve.
More interestingly, the muon does not take part in the fusion reactions themselves. It simply flies to the next atom and the process repeats again. Theoretically, in its 2.2 microseconds of existence, a muon could catalyse one million times. This would mean releasing energy far in excess of the cost of its synthesis.
In practice, however, in about 1% of fusion acts, the muon ‘sticks’ to the nucleus, so this figure is 100-150 times during the lifetime of one particle. As a consequence, the energy yield is much weaker. Accordingly, such a reaction is energetically unfavourable.
However, recently a scientific paper has appeared which proves that previous studies did not take into account the heat carried by the muon flux. If it is taken into account, it is possible to achieve a much higher number of fissions per muon, and thus this reaction becomes energetically favourable.
Other applications
However, the application of muons is not limited to catalysing thermonuclear reactions. As already mentioned, those produced in the upper atmosphere pass through the top layer of the earth and everything on it quite easily. But they do interact with obstacles strongly enough for this effect to be measurable.
It is on this phenomenon that the muon detector effect is based. It is capable of detecting the hidden structure of objects through which particles pass much more efficiently and safely than any X-ray device. Earlier this technique was used to search for cameras in the Egyptian pyramids, and now they want to use it to study the condition of the Paton Bridge.
There is also the muon collider project. The current accelerators work by colliding different particles with each other and thus reach high energies at which something can be born that we have not seen so far.
Electron collisions are particularly valued in this process, as they are considered to have no internal structure and thus the results can be interpreted more or less clearly. However, the increasing power of these devices is limited by energy losses to synchrotron radiation.
And this is where very similar to electrons but much heavier muons, in which these losses will be relatively low, come into play. It is quite possible that it is on such devices that new important results for physics will be obtained in the future.
The muon is a very unusual particle that does have a set of characteristics that are common to both electrons and protons. Perhaps to some extent it really is a meson, although not in the sense that Yukawa had in mind. It may well find new and interesting applications in the future.
