How diverse are elementary particles, their characteristics are just as diverse. It is on the basis of the characteristics that the classification of elementary particles is made.
One of the most important indicators is mass. It is important to take into account precisely the rest mass of the particle, since at the moment of motion, especially at high speeds, it increases significantly. Sometimes, not taking into account this fact, some scientists announced the discovery of new particles, although they dealt with long-known ones. The reference unit is considered to be the rest mass of the electron, since it is the lightest of them. Nowadays, the classification of elementary particles in accordance with their rest masses is as follows:
- photons that have no rest mass, because they constantly move at light speed;
- light particles - leptons, which include neutrinos and electrons;
- medium particles that weigh from one electron mass to a thousand;
- heavy - baryons with a mass of more than a thousand masses of an electron, which include neutrons, protons, hyperions.
The next characteristic by which the classification of elementary particles is created is their electric charge. It is a multiple of the charge of one electron (-1) always. According to this characteristic, all particles are divided into positively charged, negatively charged and with zero charge. The existence of particles having a fractional charge is also assumed.
Another characteristic by which the classification of elementary particles was created is their lifetime:
- stable particles, which include an electron, neutrino, photon and proton, a neutron remains stable as long as it is in the composition of the nucleus of an atom, and in a free state it decays within 15 minutes;
- unstable particles are all the rest. They have a lifetime of 10 (-10) - 10 (-24) seconds, and particles with the shortest life expectancy are resonances that are destroyed even before they manage to leave the atomic nucleus or atom. They live 10 (-25) - 10 (-26) seconds. Although the existence of such particles is theoretically calculated, but practically no one has fixed them. In human life, the most important role belongs, of course, to stable particles, because it is from them that all macrobodies are built.
The following classification characteristic is the type of interaction:
- leptons are particles that participate only in weak and electromagnetic interactions ;
- hadrons are particles that, in addition to the above, also take part in strong interactions.
But the most important characteristic of elementary particles is the spin (description of the particle momentum). Such a quantity in classical mechanics describes the characteristic of the rotation of a body. But physics, considering the properties of elementary particles, characterizes spin differently - as the internal degree of freedom of a given particle. It can take only positive values that are proportional to the Planck constant, in contrast to the spin in mechanics, which can take any values. The coefficient expressing its proportionality is called the quantum spin number. For some particles it can have integer (0, 1, 2), while for others it can have half-integer (1/2, 3/2) values. A particle with a 0-spin looks the same when it is rotated at any angle, with a spin of one - it takes the same form after it is rotated 360 degrees, and from a spin of 2 - after a rotation of 180 degrees, with a spin of ½ - 720 degrees. All particles with non-integer spin values are called fermions. These include all widely known particles, for example, protons, electrons, neutrons. Their backs are 1/2. And with integers - bosons. These substances, in fact, are field quanta, but even having corpuscular properties, they nevertheless act as fields in the classical limit. A vivid example of bosons is a photon with a spin of 1, as well as mesons with zero spin.