Everyone has heard of atoms and the fact that these small particles of matter make up the matter around us. However, not all people know that the atom is not an elementary "brick" of the universe. What is it? There is no definite answer yet. Nevertheless, considering that this is a hadron will help clarify the problem.
Ambient matter and its structure
We will begin to consider the question that this is a hadron from above. All the substance that a person encounters every day, which can feel, evaluate its color and other properties, consists of sets of molecules and atoms. The latter, in turn, are formed by electrons and nuclei. This fact was established about a century ago thanks to the work of Ernest Rutherford.
Now let us ignore the electron and consider the atomic nucleus. As you know, it is formed by two types of particles: neutrons and protons. And here we finally got to the bottom of the matter, since the neutron and proton are hadrons.
The concept of hadron
In the general case, a hadron is a particle that is formed by quarks and can take part in strong interactions. This definition does not sound completely clear, because you need to know what quarks and strong fields are, which will be discussed below. What is the meaning of the word hadron? It has a Greek root and translates as "massive, dense." That is, we are talking about a dense particle of matter having a large mass.
As mentioned above, hadrons are a proton and a neutron, each of them consists of three quarks.
What is a quark?
Closer to the middle of the 20th century, physicists from around the world in various experiments began to observe more and more "elementary" particles. The experiments were first limited to studying the natural radioactivity of some chemical elements, and then the first particle accelerators were built, which allowed their high-energy beams to collide, which significantly increased the number of particles. The latter had a different charge, spin, mass, lifetime, and behaved differently in various interactions (weak, strong, electromagnetic).
All this huge layer of information led to the need for a theory that would bring together all the particles. Such a theoretical conjecture was the quark. This name was first used by Murray Gell-Mans, an American physicist, in 1963. It is interesting to note that the word "quark" he spied in one of the literary works, it meant an imitation of the cry of gulls.
Thanks to the introduction of a new “brick” in elementary particle physics, all detected clumps of matter harmoniously fell within the framework of the new concept. Note that only hadrons are formed by quarks, particles such as a neutrino or an electron belong to the class of leptons, they are considered elementary, and quarks have nothing to do with them.
How many quarks exist and what characteristics are they described?
Hadrons are made up of quarks. But what is a quark? This is a real object, the size of which is within 10 -18 -10 -15 meters. There are 3 generations of quarks that differ in taste. In reality, only the first generation of quarks is involved in the formation of stable hadrons. The other two generations have a large mass (energy), so they quickly pass into the "base" quarks.
Only two particles belong to the first generation: u or upper and d or lower quarks. They differ in isospin (u has +1/2, d has -1/2), charge and mass. We give the spin specifically to show that we are talking about fermions whose behavior at high densities of matter differs from bosons (integer spin). An example of the latter can be photons, gluons, and any other "carriers" of interaction.
Let's say a few words about the taste and color of quarks, so as not to keep readers at a loss. Taste is a combination of properties (isospin, "strangeness", "miraculous", "bottom", "peak") of a quark, which determines the type of its interaction with the Z and W bosons, that is, it determines the nature of the transition between quarks (weak interactions). The taste of particles u and d is determined exclusively by isospin.
As for color, this is a completely different property of quarks, such as their electric charge or mass. Naturally, it has no physical connection with the usual word “color”, and it was named so because it can take one of 3 meanings (“blue”, “red”, “green”). Color is associated with the three-dimensionality of space. Roughly one can say that color is a vector directed in one of 3 directions (x, y, z). The introduction of color for quarks made it possible to explain why they can be in the same state (the Pauli prohibition principle, which all fermions follow).
If we take into account the above two quarks (u, d), as well as the fact that each of them can have one of 3 colors, then we get 6 different "bricks" for building hadrons. This number needs to be multiplied by 2, because for each of them there is its antiparticle.
Hadron Classification
When the reader got acquainted with the meaning of the word hadron and with the concept of quarks, one can give a generally accepted classification of elementary particles. So, they are all divided into two large classes: hadrons and leptons.
Hadrons are represented by baryons and mesons. The former are formed by three quarks or three aniquarks, the latter are a collection of only 2 particles: a quark-antiquark, so all mesons (peonies, kaons) have a short lifetime and annihilate quickly. Baryons are stable hadron particles having a spin obtained (fermions). The proton and neutron are bright representatives of baryons, they are often called nucleons, since they form atomic nuclei.
Thus, the importance of hadrons in the Universe is great, because all the matter around us is baryon-lepton (an electron is a lepton). However, modern science has come to the threshold of the discovery of a different type of substance, that is, not baryon-lepton (dark matter, the substance of black holes).
Nucleons: proton and neutron
These elementary hadron particles are formed by 2 types of quarks: u and d. The composition of the proton is described as uud, neutron - udd. In them, quarks are connected by strong interactions, the carriers of which are gluons. The farther the quarks are from each other, the more their attractive forces increase. This fact explains that it is not possible to detect a single quark in nature.
As for the mass of the proton and neutron, it is impossible to determine it by a simple summation of three quarks, since it is much larger than this sum. The fact is that not only a quark at rest, but also in motion (kinetic energy) contributes to the mass of these hadrons.
The proton and neutron can transform into each other as a result of weak interactions leading to a transformation between the quarks u and d.
Note that both quarks in hadrons and hadrons interact with each other through the same mechanism - the gluon field.
The current state of particle physics
Quarks appeared in physical theory in the early 1960s, and already in the 1970s it was suggested that they, too, are not elementary "bricks" and consist of so-called preons. The latter are not yet open, however, if this happens, this should greatly simplify the existing theory of the elementary world.
In addition to the problem above, there are still a number of unresolved issues:
- the description of gravity and dark matter does not fit into the standard model of the Universe;
- why do three quarks in a proton give an exact modulus charge of an elementary particle of a completely different class - an electron (lepton);
- there was evidence of the existence of hadrons, consisting not of 2, like mesons, or 3, like baryons, but of 5 quarks.
All the problems mentioned are not simple. It is enough to say that Albert Einstein devoted the last 30 years of his life to the solution of some of them and did not come to any result. He had an IQ of 160!