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We used to use the prefix anti to denote opposite entities. For example, the hero and the antihero in the adventure film are irreconcilable battle. However, in the microcosm, the particle and the antiparticle are not completely opposed to each other. A particle and an antiparticle have the same mass, lifetime, spin, only the charge differs. But it is not so simple.
As a rule, from school, most people under the charge understand only the electric charge. Indeed, if we consider the electron and its antiparticle — positron, they differ in electric charge: the electron electric charge is negative, and the positron-positive. However, in addition to the electromagnetic, there are also gravitational, strong and weak interactions, each of which also has its own charges. For example, a proton having a positive electric charge and an antiproton having a negative electric charge in a strong interaction acquire a baryon charge (or baryon number) equal to +1 for the proton and -1 for the antiproton. Therefore, if there is no electric charge, for example, as in the neutron and antineutron, the strongly interacting particles still differ in the baryon number, which is +1 for the neutron and -1 for the antineutron.
Are there situations when both baryon and electric charges are zero? Yes, for example, in the case of mesons. They are composed of a quark and an antiquark, and by definition their baryon charge is zero. Consider, for example, electrically neutral K-mesons — amazing particles in which a violation of the combined spatial and charge parity has been discovered. There is a K0-meson and an anti-K0-meson. The electric and baryon charges of both particles are zero. Why are they then considered a particle and an antiparticle? In this case, the quark composition of mesons differs. The K0-meson consists of an anti-s-quark and a d-quark. The anti-K0 meson consists, on the contrary, of an s-quark and an anti-d-quark. A strange quark-s-has a new quantum number or charge-strangeness. Strangeness is different for s-and anti-s-quarks just as the baryon charge is different for protons and antiprotons; d-quarks and d-antiquarks have their quantum number analogous to strangeness. These charges allow us to distinguish between Brionne and electrically neutral K0 and anti-K0 mesons.
However, it happens that particles and antiparticles are identical. For example, φ is a meson, which consists of an anti-s-quark and an s-quark, and its antiparticle, on the contrary, is an s-quark and an anti-s-quark. It turns out that φ-meson is its own antiparticle. In fact, there are many particles like φ-meson. The most famous of these is probably the J/ψ — meson, which consists of a charming quark and an antiquark. Photons are also identical to themselves. And the carriers of the weak force-Z0 bosons-too. But there is one elementary particle for which the answer to the question whether it is identical with itself is not yet clear. This particle is a neutrino. It is involved only in the weak and gravitational interactions. However, the gravitational interaction at the energy scales available at the present time, does not play any role. Therefore, we can say that neutrino is involved only in weak interactions. In quantum field theory, there are two approaches to describe neutrino States. The first is the so-called Dirac approach, in which neutrinos and antineutrinos are considered to be non-identical to each other. In other words, from the point of view of theorists, neutrinos and antineutrinos are similar to electrons and positrons. The second is the Majorana approach, in which neutrinos and antineutrinos are considered identical to each other. The choice in favor of the concept of Marjorana can give experimental observation of double neutrinoless beta decay of nuclei. This decay is one of the most difficult to observe experimentally. Currently, this process is still undetected.
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