Just a year ago, Peter Higgs and François Engler received the Nobel Prize for their work on subatomic particles. It may seem ridiculous, but scientists made their discoveries half a century ago, but until now they have not been given any great importance.
In 1964, two more talented physicists also came forward with their innovative theory. At first, she also attracted almost no attention. This is strange, since she described the structure of hadrons, without which no strong interatomic interaction is possible. It was the quark theory.
What is this?
By the way, what is a quark? This is one of the most important components of the hadron. Important! This particle has a "half" spin, in fact being a fermion. Depending on the color (more on that below), the charge of a quark can be equal to one-third or two-thirds of that of a proton. As for colors, there are six of them (generations of quarks). They are needed so that the Pauli principle is not violated.
Basicdetails
In the composition of hadrons, these particles are located at a distance not exceeding the confinement value. This is explained simply: they exchange vectors of the gauge field, that is, gluons. Why is the quark so important? Gluon plasma (saturated with quarks) is the state of matter in which the entire universe was located immediately after the big bang. Accordingly, the existence of quarks and gluons is a direct confirmation that he really was.
They also have their own color, and therefore, during the movement, they create their virtual copies. Accordingly, as the distance between quarks increases, the force of interaction between them increases significantly. As you might guess, at a minimum distance, the interaction practically disappears (asymptotic freedom).
Thus, any strong interaction in hadrons is explained by the transition of gluons between quarks. If we talk about interactions between hadrons, then they are explained by the transfer of pi-meson resonance. Simply put, indirectly, everything again comes down to the exchange of gluons.
How many quarks are in nucleons?
Each neutron consists of a pair of d-quarks, and even a single u-quark. Each proton, on the contrary, is made up of a single d-quark and a pair of u-quarks. By the way, letters are assigned depending on quantum numbers.
Let's explain. For example, beta decay is explained precisely by the transformation of one of the same type of quarks in the composition of the nucleon into another. To make it clearer, this process can be written as a formula like this: d=u + w (this is neutron decay). Respectively,proton is written by a slightly different formula: u=d + w.
By the way, it is the latter process that explains the constant flow of neutrinos and positrons from large star clusters. So, on the scale of the universe, there are few particles as important as the quark: gluon plasma, as we have already said, confirms the fact of the big bang, and studies of these particles allow scientists to better understand the very essence of the world in which we live.
What is smaller than a quark?
By the way, what do quarks consist of? Their constituent particles are preons. These particles are very small and poorly understood, so that even today not much is known about them. That's what's smaller than a quark.
Where did they come from?
To date, the most common two hypotheses of the formation of preons: string theory and the Bilson-Thompson theory. In the first case, the appearance of these particles is explained by string oscillations. The second hypothesis suggests that their appearance is caused by an excited state of space and time.
Interestingly, in the second case, the phenomenon can be fully described using the matrix of parallel transfer along the curves of the spin network. The properties of this very matrix predetermine those for the preon. This is what quarks are made of.
Summing up some results, we can say that quarks are a kind of "quanta" in the composition of hadrons. Impressed? And now we will talk about how the quark was discovered in general. This is a very interesting story, which, in addition, fully reveals some of the nuances described above.
Strange particles
Immediately after the end of World War II, scientists began to actively explore the world of subatomic particles, which until then looked primitively simple (according to those ideas). Protons, neutrons (nucleons) and electrons form an atom. In 1947, pions were discovered (and their existence was predicted back in 1935), which were responsible for the mutual attraction of nucleons in the nucleus of atoms. More than one scientific exhibition was devoted to this event at one time. Quarks had not yet been discovered, but the moment of attack on their "trace" was getting closer.
Neutrinos had not yet been discovered by that time. But their apparent importance in explaining the beta decay of atoms was so great that scientists had little doubt of their existence. In addition, some antiparticles have already been detected or predicted. The only thing that remained unclear was the situation with muons, which were formed during the decay of pions and subsequently passed into the state of a neutrino, electron, or positron. Physicists did not understand what this intermediate station was for at all.
Alas, such a simple and unpretentious model did not survive the moment of discovery of peonies for long. In 1947, two English physicists, George Rochester and Clifford Butler, published an interesting article in the scientific journal Nature. The material for it was their study of cosmic rays by means of a cloud chamber, during which they obtained curious information. On one of the photographs taken during the observation, a pair of tracks with a common beginning was clearly visible. Since the discrepancy resembled the Latin V, it immediately became clear– the charge of these particles is definitely different.
Scientists immediately assumed that these tracks indicate the fact of the decay of some unknown particle, which left no other traces. Calculations have shown that its mass is about 500 MeV, which is much larger than this value for an electron. Of course, the researchers called their discovery the V-particle. However, it was not yet a quark. This particle was still waiting in the wings.
It's just getting started
It all started with this discovery. In 1949, under the same conditions, a trace of a particle was discovered, which gave rise to three pions at once. It soon became clear that she, as well as the V-particle, are completely different representatives of a family consisting of four particles. Subsequently, they were called K-mesons (kaons).
A pair of charged kaons have a mass of 494 MeV, and in the case of a neutral charge - 498 MeV. By the way, in 1947, scientists were lucky enough to capture just the same very rare case of the decay of a positive kaon, but at that time they simply could not interpret the image correctly. However, to be completely fair, in fact, the first observation of the kaon was made back in 1943, but information about this was almost lost against the backdrop of numerous post-war scientific publications.
New weirdness
And then more discoveries awaited scientists. In 1950 and 1951, researchers from the University of Manchester and Melnburg managed to find particles much heavier than protons and neutrons. It again had no charge, but decayed into a proton and a pion. The latter, as can be understood,negative charge. The new particle was named Λ (lambda).
The more time passed, the more questions scientists had. The problem was that new particles arose exclusively from strong atomic interactions, quickly decaying into the known protons and neutrons. In addition, they always appeared in pairs, there were never single manifestations. That is why a group of physicists from the USA and Japan suggested using a new quantum number - strangeness - in their description. According to their definition, the strangeness of all other known particles was zero.
Further research
The breakthrough in research happened only after the emergence of a new systematization of hadrons. The most prominent figure in this was the Israeli Yuval Neaman, who changed the career of an outstanding military man to an equally brilliant path of a scientist.
He noticed that the mesons and baryons discovered by that time decay, forming a cluster of related particles, multiplets. The members of each such association have exactly the same strangeness, but opposite electric charges. Since really strong nuclear interactions do not depend at all on electric charges, in all other respects the particles from the multiplet look like perfect twins.
Scientists suggested that some natural symmetry is responsible for the appearance of such formations, and soon they managed to find it. It turned out to be a simple generalization of the SU(2) spin group, which scientists around the world used to describe quantum numbers. Hereonly by that time 23 hadrons were already known, and their spins were equal to 0, ½ or an integer unit, and therefore it was not possible to use such a classification.
As a result, two quantum numbers had to be used for classification at once, due to which the classification was significantly expanded. This is how the group SU(3) appeared, which was created at the beginning of the century by the French mathematician Elie Cartan. To determine the systematic position of each particle in it, scientists have developed a research program. The quark subsequently easily entered the systematic series, which confirmed the absolute correctness of the experts.
New quantum numbers
So scientists came up with the idea of using abstract quantum numbers, which became hypercharge and isotopic spin. However, strangeness and electric charge can be taken with the same success. This scheme was conventionally called the Eightfold Path. This captures the analogy with Buddhism, where before reaching nirvana, you also need to go through eight levels. However, all this is lyrics.
Neeman and his colleague, Gell-Mann, published their work in 1961, and the number of mesons known then did not exceed seven. But in their work, the researchers were not afraid to mention the high probability of the existence of the eighth meson. In the same 1961, their theory was brilliantly confirmed. The found particle was named eta meson (Greek letter η).
Further findings and experiments with brightness confirmed the absolute correctness of the SU(3) classification. This circumstance has become powerfulan incentive for researchers who have found that they are on the right track. Even Gell-Mann himself no longer doubted that quarks exist in nature. Reviews about his theory were not too positive, but the scientist was sure that he was right.
Here are the quarks
Soon the article "Schematic model of baryons and mesons" was published. In it, scientists were able to further develop the idea of systematization, which turned out to be so useful. They found that SU(3) quite allows the existence of whole triplets of fermions, the electric charge of which ranges from 2/3 to 1/3 and -1/3, and in the triplet one particle always has non-zero strangeness. Gell-Mann, already well-known to us, called them “quark elementary particles.”
According to the charges, he designated them as u, d and s (from the English words up, down and strange). In accordance with the new scheme, each baryon is formed by three quarks at once. Mesons are much simpler. They include one quark (this rule is unshakable) and an antiquark. Only after that did the scientific community become aware of the existence of these particles, to which our article is devoted.
A little more background
This article, which largely predetermined the development of physics for years to come, has a rather curious background. Gell-Mann thought about the existence of this kind of triplets long before its publication, but did not discuss his assumptions with anyone. The fact is that his assumptions about the existence of particles with a fractional charge looked like nonsense. However, after talking with the eminent theoretical physicist Robert Serber, he learned that his colleaguemade exactly the same conclusions.
Besides, the scientist made the only correct conclusion: the existence of such particles is possible only if they are not free fermions, but are part of hadrons. Indeed, in this case, their charges form a single whole! At first, Gell-Mann called them quarks and even mentioned them at MTI, but the reaction of students and teachers was very restrained. That is why the scientist thought for a very long time about whether he should submit his research to the public.
The very word "quark" (a sound reminiscent of the cry of ducks) was taken from the work of James Joyce. Oddly enough, but the American scientist sent his article to the prestigious European scientific journal Physics Letters, as he seriously feared that the editors of the American edition of Physical Review Letters, similar in terms of level, would not accept it for publication. By the way, if you want to look at least at a copy of that article, you have a direct road to the same Berlin Museum. There are no quarks in his exposition, but there is a complete history of their discovery (more precisely, documentary evidence).
Start of the Quark Revolution
To be fair, it should be noted that almost at the same time, a scientist from CERN, George Zweig, came to a similar idea. First, Gell-Mann himself was his mentor, and then Richard Feynman. Zweig also determined the reality of the existence of fermions that had fractional charges, only called them aces. Moreover, the talented physicist also considered baryons as a trio of quarks, and mesons as a combination of quarks.and antiquark.
Simply put, the student completely repeated the conclusions of his teacher, and completely separate from him. His work appeared even a couple of weeks before Mann's publication, but only as a "home-made" institute. However, it was the presence of two independent works, the conclusions of which were almost identical, that immediately convinced some scientists of the correctness of the proposed theory.
From rejection to trust
But many researchers accepted this theory far from immediately. Yes, journalists and theorists quickly fell in love with it for its clarity and simplicity, but serious physicists accepted it only after 12 years. Don't blame them for being too conservative. The fact is that initially the theory of quarks sharply contradicted the Pauli principle, which we mentioned at the very beginning of the article. If we assume that a proton contains a pair of u-quarks and a single d-quark, then the former must be strictly in the same quantum state. According to Pauli, this is impossible.
That's when an additional quantum number appeared, expressed as a color (which we also mentioned above). In addition, it was completely incomprehensible how elementary particles of quarks interact with each other in general, why their free varieties do not occur. All these secrets were greatly helped to unravel by the Theory of Gauge Fields, which was “brought to mind” only in the mid-70s. Around the same time, the quark theory of hadrons was organically included in it.
But the complete absence of at least some experimental experiments hindered the development of the theory most of all,which would confirm both the very existence and the interaction of quarks with each other and with other particles. And they gradually began to appear only from the end of the 60s, when the rapid development of technology made it possible to conduct an experiment with the "transmission" of protons by electron streams. It was these experiments that made it possible to prove that some particles really “hidden” inside the protons, which were originally called partons. Subsequently, nevertheless, they became convinced that this was nothing more than a true quark, but this happened only at the end of 1972.
Experimental confirmation
Of course, much more experimental data was needed to finally convince the scientific community. In 1964, James Bjorken and Sheldon Glashow (the future Nobel Prize winner, by the way) suggested that there might also be a fourth kind of quark, which they called charmed.
It was thanks to this hypothesis that already in 1970 scientists were able to explain many of the oddities that were observed during the decay of neutrally charged kaons. Four years later, two independent groups of American physicists at once managed to fix the decay of the meson, which included just one "charmed" quark, as well as its antiquark. Not surprisingly, this event was immediately dubbed the November Revolution. For the first time, the theory of quarks received more or less "visual" confirmation.
The importance of the discovery is evidenced by the fact that the project leaders, Samuel Ting and Barton Richter, are already throughaccepted their Nobel Prize for two years: this event is reflected in many articles. You can see some of them in the original if you visit the New York Museum of Natural Science. Quarks, as we have already said, are an extremely important discovery of our time, and therefore a lot of attention is paid to them in the scientific community.
Final argument
It wasn't until 1976 that researchers did find one particle with non-zero charm, the neutral D meson. This is a rather complex combination of one charmed quark and a u-antiquark. Here, even hardened opponents of the existence of quarks were forced to admit the correctness of the theory, first stated more than two decades ago. One of the most famous theoretical physicists, John Ellis, called charm “the lever that turned the world around.”
Soon the list of new discoveries included a pair of especially massive quarks, top and bottom, which could easily be correlated with the SU(3) systematization already accepted at that time. In recent years, scientists have been talking about the existence of so-called tetraquarks, which some scientists have already dubbed "hadron molecules."
Some conclusions and conclusions
You need to understand that the discovery and scientific justification for the existence of quarks can indeed be safely considered a scientific revolution. The year 1947 (in principle, 1943) can be considered its beginning, and its end falls on the discovery of the first "enchanted" meson. It turns out that the duration of the last opening of this level to date is, no less, as much as 29 years (or even 32 years)! And all thistime was spent not only in order to find the quark! As the primordial object in the universe, gluon plasma soon attracted much more attention from scientists.
However, the more complex the area of study becomes, the more time it takes to make really important discoveries. As for the particles we are discussing, no one can underestimate the importance of such a discovery. By studying the structure of quarks, a person will be able to penetrate deeper into the secrets of the universe. It is possible that only after a complete study of them will we be able to find out how the big bang happened and according to what laws our Universe develops. In any case, it was their discovery that made it possible to convince many physicists that the reality surrounding us is much more complicated than former ideas.
So you have learned what a quark is. This particle at one time made a lot of noise in the scientific world, and today researchers are full of hopes to finally reveal all its secrets.
