Lesson No. 67.

Lesson topic: Problems of elementary particles

Lesson objectives:

Educational: to introduce students to the concept of an elementary particle, with the classification of elementary particles, to generalize and consolidate knowledge about fundamental types of interactions, to form a scientific worldview.

Educational: to form a cognitive interest in physics, instilling love and respect for the achievements of science.

Educational: development of curiosity, ability to analyze, independently formulate conclusions, development of speech and thinking.

Equipment: interactive whiteboard (or projector with screen).

Lesson type: learning new material.

Lesson type: lecture

During the classes:

Organizational stage

Studying a new topic.

In nature, there are 4 types of fundamental (basic) interactions: gravitational, electromagnetic, strong and weak. According to modern ideas, interaction between bodies is carried out through the fields surrounding these bodies. The field itself in quantum theory is understood as a collection of quanta. Each type of interaction has its own interaction carriers and comes down to the absorption and emission of corresponding light quanta by particles.

Interactions can be long-range (manifest at very long distances) and short-range (manifest at very short distances).

Gravitational interaction occurs through the exchange of gravitons. They have not been detected experimentally. According to the law discovered in 1687 by the great English scientist Isaac Newton, all bodies, regardless of shape and size, attract each other with a force directly proportional to their mass and inversely proportional to the square of the distance between them. Gravitational interaction always leads to the attraction of bodies.

Electromagnetic interaction is long-range. Unlike gravitational interaction, electromagnetic interaction can result in both attraction and repulsion. The carriers of electromagnetic interaction are quanta of the electromagnetic field - photons. As a result of the exchange of these particles, electromagnetic interaction arises between charged bodies.

Strong interaction is the most powerful of all interactions. It is short-range, the corresponding forces decrease very quickly as the distance between them increases. The radius of action of nuclear forces is 10 -13 cm

The weak interaction occurs at very short distances. The range of action is approximately 1000 times less than that of nuclear forces.

The discovery of radioactivity and the results of Rutherford's experiments convincingly showed that atoms are composed of particles. They have been found to consist of electrons, protons and neutrons. At first, the particles from which atoms are built were considered indivisible. That's why they were called elementary particles. The idea of ​​a “simple” structure of the world was destroyed when in 1932 the antiparticle of the electron was discovered - a particle that had the same mass as the electron, but differed from it in the sign of the electric charge. This positively charged particle was called a positron... according to modern concepts, every particle has an antiparticle. The particle and antiparticle have the same mass, but opposite signs of all charges. If the antiparticle coincides with the particle itself, then such particles are called truly neutral, their charge is 0. For example, a photon. When a particle and antiparticle collide, they annihilate, that is, they disappear, turning into other particles (often these particles are a photon).

All elementary particles (which cannot be divided into components) are divided into 2 groups: fundamental (structureless particles, all fundamental particles at this stage of development of physics are considered structureless, that is, they do not consist of other particles) and hadrons (particles with a complex structure).

Fundamental particles, in turn, are divided into leptons, quarks and carriers of interactions. Hadrons are divided into baryons and mesons. Leptons include the electron, positron, muon, taon, and three types of neutrinos.

Quarks are the particles that make up all hadrons. Participate in strong interactions.

According to modern concepts, each of the interactions arises as a result of the exchange of particles, called carriers of this interaction: a photon (a particle that carries the electromagnetic interaction), eight gluons (particles that carry the strong interaction), three intermediate vector bosons W + , W− and Z 0, carrying the weak interaction, graviton (carrier of gravitational interaction). The existence of gravitons has not yet been proven experimentally.

Hadrons participate in all types of fundamental interactions. They consist of quarks and are divided, in turn, into: baryons, consisting of three quarks, and mesons, consisting of two quarks, one of which is an antiquark.

The strongest interaction is the interaction between quarks. A proton consists of 2 u quarks, one d quark, a neutron consists of one u quark and 2 d quarks. It turned out that at very short distances none of the quarks notice their neighbors, and they behave like free particles that do not interact with each other. When quarks move away from each other, an attraction arises between them, which increases with increasing distance. To split hadrons into individual isolated quarks would require a lot of energy. Since there is no such energy, the quarks turn out to be eternal prisoners and forever remain locked inside the hadron. Quarks are held inside the hadron by the gluon field.

III. Consolidation

Option 1.

Option 2.

3.. How long does a neutron live outside an atom nucleus? A. 12 min B. 15 min

Lesson summary. During the lesson we got acquainted with the particles of the microworld and found out which particles are called elementary.

D/z§ 9.3

Particle name Mass (in electronic masses) Electric charge Life time (s) Antiparticle Stable Neutrino electron Stable Neutrino muon Stable Electron Stable Pi mesons ≈ 10 –10 –10 –8 Eta-null-meson Stable Lambda hyperon Sigma hyperons Xi-hyperons Omega-minus-hyperon

III. Consolidation

Name the main interactions that exist in nature

What is the difference between a particle and an antiparticle? What do they have in common?

Which particles participate in gravitational, electromagnetic, strong and weak interactions?

Option 1.

1. One of the properties of elementary particles is the ability……… A. to transform into each other B. to spontaneously change

2. Particles that can exist in a free state for an unlimited time are called..... A. unstable B. stable.

3. Which particle is stable? A. proton B. meson

4. A long-lived particle. A. neutrino B. neutron

5. Neutrinos are produced as a result of the decay of..... A. electron B. neutron

Option 2.

What is the main factor in the existence of elementary particles?

A. their mutual penetration B. their mutual transformation.

2. Which of the elementary particles is not isolated into a free particle. A. pion B. quarks

3. How long does a neutron live outside an atom nucleus? A. 12 min B. 15 min

Which particle is not stable? A. photon B. lepton

Are there immutable particles in nature? A. yes B. no

Aristotle believed that matter in the Universe consists of four basic elements - earth, air, fire and water, which are acted upon by two forces: the force of gravity, which draws earth and water down, and the force of lightness, under the influence of which fire and air tend upward. This approach to the structure of the Universe, when everything is divided into matter and forces, continues to this day.

According to Aristotle, matter is continuous, that is, any piece of matter can be endlessly crushed into smaller and smaller pieces, never reaching such a tiny grain that would no longer be divided. However, some other Greek philosophers, such as Democritus, were of the opinion that matter is granular in nature and everything in the world is made up of a large number of different atoms (the Greek word "atom" means indivisible). Centuries passed, but the dispute continued without any real evidence that would confirm the rightness of one side or the other. Finally, in 1803, the English chemist and physicist John Dalton showed that the fact that chemical substances always combine in certain proportions can be explained by assuming that atoms are combined into groups called molecules. However, until the beginning of our century, the dispute between the two schools was never resolved in favor of the atomists. Einstein made a very important contribution to resolving this dispute. In a paper written in 1905, a few weeks before his famous paper on special relativity, Einstein pointed out that a phenomenon called Brownian motion—the irregular, chaotic movement of tiny particles suspended in water—could be explained by impacts atoms of the liquid about these particles.

By that time, there was already some reason to think that atoms were also not indivisible. A few years earlier, J. J. Thomson of Trinity College, Cambridge, had discovered a new particle of matter, the electron, whose mass was less than one-thousandth the mass of the lightest atom. Thomson's experimental setup was a bit like a modern television picture tube. A red-hot metal thread served as a source of electrons. Since the electrons are negatively charged, they were accelerated in the electric field and moved towards the screen covered with a layer of phosphor. When the electrons hit the screen, flashes of light appeared on it. It soon became clear that these electrons must fly out of the atoms, and in 1911 the English physicist Ernst Rutherford finally proved that the atoms of matter actually have an internal structure: they consist of a tiny positively charged nucleus and electrons rotating around it. Rutherford came to this conclusion by studying how alpha particles (positively charged particles emitted by radioactive atoms) are deflected when they collide with atoms.

At first, it was thought that the nucleus of an atom consisted of electrons and positively charged particles, which were called protons (from the Greek word "protos" - primary), because protons were considered to be the fundamental blocks of which matter is composed. However, in 1932, James Chadwick, Rutherford's colleague at the University of Cambridge, discovered that there are also other particles in the nucleus - neutrons, whose mass is almost equal to the mass of the proton, but which are not charged. For this discovery, Chadwick was awarded the Nobel Prize and was chosen to head Conville and Caius College, Cambridge University (the college where I now work). Then he had to give up this post due to disagreements with employees. The college had been the subject of constant bitter disputes since, after the war, a group of returning youth voted against keeping the old staff in the positions they had already held for many years. All this happened before me; I started working at the college in 1965 and saw the end of the struggle when the other head of the college, Nobel laureate Neville Mott, was forced to also resign.

Just twenty years ago, protons and neutrons were considered “elementary” particles, but experiments on the interaction of protons and electrons moving at high speeds with protons showed that in fact protons consist of even smaller particles. Murray Gell-Mann, a theorist at the California Institute of Technology, called these particles quarks. In 1969, Gell-Mann was awarded the Nobel Prize for his research on quarks. The name "quark" is taken from James Joyce's clever line of poetry: "Three quarks for Master Mark!" The word quark is supposed to be pronounced like quart, with the t at the end replaced by a k, but it is usually pronounced so that it rhymes with lark.

Several varieties of quarks are known: it is believed that there are at least six “flavors”, which correspond to the u-quark, d-quark, strange quark, charm quark, b-quark and t-quark. A quark of each “flavor” can also be of three “colors” - red, green and blue. (It should be emphasized that these are just notations, since the size of quarks is much smaller than the wavelength of visible light and therefore they do not have color in the usual sense of the word. The point is simply that modern physicists like to come up with names for new particles and phenomena, without further limiting their fantasy in the Greek alphabet). A proton and a neutron are made up of three quarks of different “colors.” A proton contains two u-quarks and one d-quark, a neutron contains two d-quarks and one u-quark. Particles can be built from other quarks (strange, charm, b and t), but all these quarks have much greater mass and very quickly decay into protons and neutrons.

We already know that neither atoms nor the protons and neutrons inside an atom are indivisible, and therefore the question arises: what are real elementary particles - those initial bricks from which everything consists? Because the wavelengths of light are substantially larger than the size of an atom, we have no hope of “seeing” the constituent parts of an atom in the usual way. For this purpose, much shorter wavelengths are required. In the previous chapter, we learned that, according to quantum mechanics, all particles are actually waves, and the higher the energy of a particle, the shorter the corresponding wavelength. Thus, our answer to this question depends on how high the energy of the particles at our disposal is, because it determines how small the scale of the lengths that we can observe. The units in which particle energy is usually measured are called electronvolts. (In his experiments, Thomson used an electric field to accelerate electrons. An electronvolt is the energy that an electron acquires in an electric field of 1 volt). In the 19th century, when they could only use particles with energies of several electron volts released in chemical reactions such as combustion, atoms were considered the smallest parts of matter. In Rutherford's experiments, the energies of alpha particles amounted to millions of electron volts. Then we learned, using electromagnetic fields, to accelerate particles, first to energies of millions, and then of thousands of millions of electron volts. This is how we learned that particles that were thought to be elementary twenty years ago are actually made up of smaller particles. What if, during the transition to even higher energies, it turns out that these smaller particles, in turn, consist of even smaller ones? Of course, this is a completely probable situation, but we now have some theoretical reasons to believe that we already have, or almost have, information about the initial “bricks” from which everything in nature is built.

Everything that exists in the Universe, including light and gravity, can be described based on the idea of ​​particles, taking into account the particle-wave dualism that we discussed in the previous chapter. Particles have a certain rotational characteristic - spin.

Let's imagine particles in the form of small tops rotating around their axis. True, such a picture can be misleading, because in quantum mechanics particles do not have a well-defined axis of rotation. In fact, the spin of a particle tells us what that particle looks like when viewed from different angles. A particle with spin 0 is like a point: it looks the same from all sides (Fig. 5.1, I). A particle with spin 1 can be compared to an arrow: it looks different from different sides (Fig. 5.1, II) and takes the same form only after a full rotation of 360 degrees. A particle with spin 2 can be compared to an arrow sharpened on both sides: any of its positions is repeated after a half-turn (180 degrees). Likewise, a particle with a higher spin returns to its original state when rotated through an even smaller fraction of a full rotation. This is all quite obvious, but what is surprising is that there are particles that, after a full rotation, do not take their previous form: they need to be completely rotated twice! Such particles are said to have spin 1/2.

All known particles in the Universe can be divided into two groups: particles with spin 1/2, of which matter in the Universe is made, and particles with spin 0, 1 and 2, which, as we will see, create forces acting between particles of matter. Particles of matter obey the so-called Pauli exclusion principle, discovered in 1925 by the Austrian physicist Wolfgang Pauli. In 1945, Pauli was awarded the Nobel Prize for his discovery. He was an ideal example of a theoretical physicist: they say that his mere presence in the city disrupted the progress of all experiments! The Pauli principle states that two identical particles cannot exist in the same state, that is, they cannot have the same coordinates and velocities with the accuracy specified by the uncertainty principle. The Pauli principle is extremely important, since it made it possible to explain why, under the influence of forces created by particles with spin 0, 1, 2, particles of matter do not collapse into a state with a very high density: if particles of matter have very close coordinate values, then their the velocities must be different, and, therefore, they will not be able to stay at points with these coordinates for a long time. If the Pauli principle had not participated in the creation of the world, quarks could not have combined into individual, well-defined particles - protons and neutrons, which in turn could not have combined with electrons to form individual, well-defined atoms. Without the Pauli principle, all these particles would collapse and turn into a more or less homogeneous and dense “jelly”.

There was no proper understanding of the electron and other spin-1/2 particles until 1928, when Paul Dirac proposed a theory to describe these particles. Subsequently, Dirac received the chair of mathematics at Cambridge (which Newton once held and which I now hold). Dirac's theory was the first theory of its kind to be consistent with both quantum mechanics and special relativity. It gave a mathematical explanation of why the electron spin is equal to 1/2, i.e., why when the electron rotates once, it does not take its previous form, but when it rotates twice, it does. Dirac's theory also predicted that the electron should have a partner - an antielectron, or, in other words, a positron. The discovery of the positron in 1932 confirmed Dirac's theory, and in 1933 he received the Nobel Prize in Physics. We now know that every particle has an antiparticle with which it can annihilate. (In the case of particles that provide interaction, the particle and antiparticle are one and the same). There could be entire antiwords and antipeople consisting of antiparticles. But if you meet an anti-self, don’t even think about shaking his hand! There will be a blinding flash of light and you will both disappear. An extremely important question is why there are so many more particles around us than antiparticles. We will return to it later in this chapter.

In quantum mechanics, all forces, or interactions, between particles of matter are assumed to be carried by particles with an integer spin of 0, 1, or 2. A particle of matter, such as an electron or quark, emits a particle that carries the force. As a result of recoil, the speed of a particle of matter changes. Then the carrier particle collides with another particle of the substance and is absorbed by it. This collision changes the speed of the second particle, as if a force were acting between the two particles of matter.

Interaction carrier particles have one important property: they do not obey the Pauli exclusion principle. This means that there are no restrictions on the number of particles exchanged, so the resulting interaction force can be large. But if the mass of carrier particles is large, then at large distances their creation and exchange will be difficult. Thus, the forces they carry will be short-range. If the carrier particles do not have their own mass, long-range forces will arise. Carrier particles exchanged between particles of matter are called virtual because, unlike real ones, they cannot be directly detected using a particle detector. However, we know that virtual particles exist because they create measurable effects: virtual particles create forces between particles of matter. Under certain conditions, particles with spins 0, 1, 2 also exist as real ones; then they can be registered directly. From the point of view of classical physics, such particles occur to us in the form of waves, say light or gravitational. They are sometimes emitted during the interaction of particles of a substance, which occurs due to the exchange of interaction carrier particles. (For example, the electrical force of mutual repulsion between two electrons arises from the exchange of virtual photons, which cannot be directly detected. But if the electrons fly past each other, real photons can be emitted, which will be detected as light waves.)

Carrier particles can be divided into four types depending on the magnitude of the interaction they carry and what particles they interact with. We emphasize that such a division is completely artificial; This is a scheme convenient for developing particular theories; there is probably nothing more serious in it. Most physicists hope that eventually it will be possible to create a unified theory in which all four forces would be variations of the same force. Many even see this as the main goal of modern physics. Recently, attempts to unite the three forces were crowned with success. I'm going to talk about them more in this chapter. We will talk a little later about how things stand with the inclusion of gravity in such a unification.

So, the first type of force is gravitational force. Gravitational forces are universal. This means that every particle is under the influence of a gravitational force, the magnitude of which depends on the mass or energy of the particle. Gravity is much weaker than each of the remaining three forces. This is a very weak force that we would not notice at all if not for two of its specific properties: gravitational forces act over large distances and are always attractive forces. Consequently, very weak gravitational forces of interaction between individual particles in two large bodies, such as the Earth and the Sun, can add up to a very large force. The other three types of interaction either act only at short distances, or are either repulsive or attractive, which generally leads to compensation. In the quantum mechanical approach to the gravitational field, the gravitational force between two particles of matter is considered to be carried by a spin-2 particle called a graviton. The graviton does not have its own mass, and therefore the force it carries is long-range. The gravitational interaction between the Sun and the Earth is explained by the fact that the particles that make up the Earth and the Sun exchange gravitons. Despite the fact that only virtual particles participate in the exchange, the effect they create is certainly measurable, because this effect is the rotation of the Earth around the Sun! Real gravitons propagate in the form of waves, which in classical physics are called gravitational waves, but they are very weak and so difficult to register that no one has yet succeeded in doing this.

The next type of interaction is created by electromagnetic forces that act between electrically charged particles, such as electrons and quarks, but are not responsible for the interaction of uncharged particles such as gravitons. Electromagnetic interactions are much stronger than gravitational ones: the electromagnetic force acting between two electrons is about a million million million million million million million (one followed by forty-two zeros) times greater than the gravitational force. But there are two types of electric charge - positive and negative. Between two positive charges, just as between two negative charges, there is a repulsive force, and between positive and negative charges there is an attractive force. In large bodies, such as the Earth or the Sun, the content of positive and negative charges is almost equal, and therefore the forces of attraction and repulsion almost cancel each other, and very little pure electromagnetic force remains. However, on the small scale of atoms and molecules, electromagnetic forces dominate. Due to the electromagnetic attraction between the negatively charged electrons and the positively charged protons in the nucleus, the electrons in the atom rotate around the nucleus in exactly the same way that the gravitational attraction causes the Earth to rotate around the Sun. Electromagnetic attraction is described as the result of the exchange of large numbers of virtual massless spin-1 particles called photons. As with gravitons, the photons carrying out the exchange are virtual, but when an electron moves from one allowed orbit to another, located closer to the nucleus, energy is released and as a result a real photon is emitted, which, at a suitable wavelength, can be observed by the human eye as visible light, or using some kind of photon detector, such as photographic film. Similarly, when a real photon collides with an atom, an electron can move from one orbit to another, more distant from the nucleus. This transition occurs due to the energy of the photon, which is absorbed by the atom. The third type of interaction is called weak interaction. It is responsible for radioactivity and exists between all particles of matter with spin 1/2, but particles with spin 0, 1, 2 - photons and gravitons - do not participate in it. Before 1967, the properties of weak forces were poorly understood, and in 1967 Abdus Salam, a theorist from Imperial College London, and Steven Weinberg from Harvard University simultaneously proposed a theory that combined the weak force with the electromagnetic force in the same way as a hundred years earlier Maxwell combined electricity and magnetism. Weinberg and Salam proposed that in addition to the photon, there are three more spin-1 particles, collectively called heavy vector bosons, that carry the weak force. These bosons were designated W+, W–, and Z0, and each had a mass of 100 GeV (GeV stands for gigaelectronvolt, i.e., a thousand million electronvolts). The Weinberg-Salam theory has the property of so-called spontaneous symmetry breaking. It means that particles that are completely different at low energies turn out to be actually the same particle at high energies, but in different states. This is in some ways similar to the behavior of a ball when playing roulette. At all high energies (i.e., with rapid rotation of the wheel), the ball always behaves almost the same - it rotates non-stop. But as the wheel slows down, the ball's energy decreases and it eventually falls into one of the thirty-seven grooves on the wheel. In other words, at low energies the ball can exist in thirty-seven states. If for some reason we could only observe the ball at low energies, we would think that there were thirty-seven different types of balls!

The Weinberg-Salam theory predicted that at energies well above 100 GeV, the three new particles and the photon should behave identically, but at lower particle energies, that is, in most ordinary situations, this “symmetry” should break down. The masses of the W+, W– and Z0 bosons were predicted to be large so that the forces they create would have a very short range of action. When Weinberg and Salam put forward their theory, few people believed them, and with the low-power accelerators of those times it was impossible to achieve the energy of 100 GeV required for the production of real W+, W– and Z0 particles. However, ten years later, the predictions obtained in this theory at low energies were so well confirmed experimentally that Weinberg and Salam were awarded the 1979 Nobel Prize together with Sheldon Glashow (also from Harvard), who proposed a similar unified theory of electromagnetic and weak nuclear interactions . The Nobel Prize Committee was spared the embarrassment that might have arisen if it had been shown to have made a mistake by the 1983 discovery at CERN of three massive partners of the photon with the correct masses and other predicted characteristics. Carlo Rubbia, who led the team of several hundred physicists who made this discovery, received the 1984 Nobel Prize, awarded to him jointly with CERN engineer Simon Van der Meer, the author of the antiparticle storage ring used in the experiment. (It's extremely difficult to make your mark in experimental physics these days unless you're already at the top!).

The strong nuclear force is a type 4 force that keeps quarks inside the proton and neutron, and protons and neutrons inside the atomic nucleus. The carrier of the strong interaction is considered to be another particle with spin 1, which is called a gluon.

Gluons interact only with quarks and other gluons. Strong interaction has one extraordinary property - it has confinement (confinement - restriction, retention (English). - Ed.).

Confinement is that particles are always held in colorless combinations. A single quark cannot exist on its own, because then it would have to have a color (red, green or blue). Therefore, the red quark must be coupled to the green and blue via a gluon “jet” (red + green + blue = white). Such a triplet turns out to be a proton or neutron. There is another possibility, when a quark and an antiquark are paired (red + anti-red, or green + anti-green, or blue + anti-blue = white). Such combinations make up particles called mesons. These particles are unstable because a quark and an antiquark can annihilate each other to form electrons and other particles. Likewise, a single gluon cannot exist on its own due to confinement, because gluons also have color. Therefore, gluons must group in such a way that their colors add up to white. The described group of gluons forms an unstable particle - a glueball.

We cannot observe an individual quark or gluon due to confinement. Doesn't this mean that the very idea of ​​quarks and gluons as particles is somewhat metaphysical? No, because the strong interaction is characterized by another property called asymptotic freedom. Thanks to this property, the concept of quarks and gluons becomes completely definite. At ordinary energies, the strong interaction is indeed strong and presses the quarks tightly together. But, as experiments at powerful accelerators show, at high energies the strong interaction noticeably weakens and quarks and gluons begin to behave almost like free particles. In Fig. Figure 5.2 shows a photograph of a high-energy proton-antiproton collision. We see that several almost free quarks, born as a result of the interaction, formed the “jets” of tracks that are visible in the photograph.

The successful unification of the electromagnetic and weak interactions resulted in attempts to combine these two types of interactions with the strong interaction, resulting in the so-called grand unified theory. There is some exaggeration in this name: firstly, grand unified theories are not that great, and secondly, they do not completely unify all forces because they do not include gravity. In addition, all these theories are in fact incomplete, because they contain parameters that cannot be predicted theoretically and which must be calculated by comparing theoretical and experimental results. Nevertheless, such theories can be a step towards a complete unification theory covering all interactions. The main idea behind constructing grand unified theories is as follows: as already mentioned, strong interactions at high energies become weaker than at low energies. At the same time, electromagnetic and weak forces are not asymptotically free, and at high energies they increase. Then, at some very large value of energy - at the energy of the great unification - these three forces could become equal to each other and become simply varieties of the same force. Grand unification theories predict that at this energy, different particles of spin-1/2 matter, such as quarks and electrons, would also cease to be different, which would be another step towards unification.

The grand unified energy value is not very well known, but it must be at least a thousand million million GeV. In current generation accelerators, particles with energies of about 100 GeV collide, and in future projects this value should increase to several thousand GeV. But accelerating particles to grand unified energy requires an accelerator the size of the solar system. It is unlikely that in the current economic situation anyone would decide to finance it. This is why direct experimental testing of grand unified theories is impossible. But here, as with the electroweak unified theory, there are low-energy consequences that can be tested.

The most interesting of these consequences is that protons, which make up most of the mass of ordinary matter, can spontaneously decay into lighter particles such as antielectrons. The reason is that at grand unified energy there is no significant difference between a quark and an antielectron. Three quarks inside a proton usually do not have enough energy to transform into antielectrons, but one of the quarks may, completely by chance, one day receive enough energy for this transformation, because due to the uncertainty principle it is impossible to accurately record the energy of the quarks inside a proton. Then the proton must decay, but the probability that the quark will have sufficient energy is so small that the wait for this will have to be at least a million million million million million (one followed by thirty zeros) years, which is much longer than the time that has passed since the big bang. which does not exceed ten thousand million years or something like that (one followed by ten zeros). This suggests the conclusion that the possibility of spontaneous proton decay cannot be verified experimentally. It is possible, however, to increase the probability of observing proton decay by studying a very large number of protons. (By observing, for example, 1 with thirty-one zero protons over the course of a year, one can hope to detect, according to one of the simplest grand unification theories, more than one proton decay).

Several such experiments have already been carried out, but they did not provide definite information about the decays of the proton or neutron. One of the experiments, which used eight thousand tons of water, was carried out in a salt mine in Ohio (in order to eliminate cosmic interference that could be mistaken for proton decay). Since no proton decays were detected during the entire experiment, it can be calculated that the proton lifetime must be greater than ten million million million million million (one followed by thirty-one zeros) years. This result exceeds the predictions of the simplest grand unified theory, but there are more complex theories that give a higher estimate. To verify them, even more precise experiments with even larger quantities of the substance will be required.

Despite the difficulties of observing proton decay, it is possible that our very existence is a consequence of the reverse process - the formation of protons or, even more simply, quarks at the very initial stage, when there were no more quarks than antiquarks. This picture of the beginning of the Universe seems to be the most natural. Earth's matter consists largely of protons and neutrons, which in turn are made of quarks, but there are no antiprotons or antineutrons, which are made of antiquarks, except for the few that have been produced in large accelerators. Experiments with cosmic rays confirm that the same is true for all matter in our Galaxy: there are no antiprotons or antineutrons, except for the small number of antiparticles that arise as a result of the creation of particle-antiparticle pairs in particle collisions at high energies . If there were large areas of antimatter in our Galaxy, then one would expect strong radiation at the interfaces between matter and antimatter, where many collisions of particles and antiparticles would occur, which, annihilating, would emit high-energy radiation.

We have no direct indication whether the matter of other galaxies consists of protons and neutrons or of antiprotons and antineutrons, but it must consist of particles of the same type: within one galaxy there cannot be a mixture of particles and antiparticles, because as a result of their annihilation powerful radiation would be emitted. Therefore we believe that all galaxies are made of quarks, not antiquarks; It is unlikely that some galaxies consisted of matter and others of antimatter.

But why should there be so many more quarks than antiquarks? Why are their numbers not the same? We are very lucky that this is so, because if there were an equal number of quarks and antiquarks, then almost all the quarks and antiquarks would have annihilated each other in the early Universe, filling it with radiation, but hardly leaving any matter. There would be no galaxies, no stars, no planets on which human life could develop. Grand unified theories can explain why there should now be more quarks in the Universe than antiquarks, even if at the very beginning there were equal numbers. As we already know, in grand unified theories at high energies, quarks can turn into antielectrons. Reverse processes are also possible, when antiquarks turn into electrons, and electrons and antielectrons turn into antiquarks and quarks. Once upon a time, at a very early stage in the development of the Universe, it was so hot that the energy of particles was sufficient for such transformations. But why did this result in more quarks than antiquarks? The reason lies in the fact that the laws of physics are not exactly the same for particles and antiparticles.

Until 1956, it was believed that the laws of physics were invariant under three symmetry transformations - C, P and T. Symmetry C means that all laws are the same for particles and antiparticles. P symmetry means that the laws of physics are the same for any phenomenon and for its mirror reflection (the mirror image of a particle rotating clockwise will be a particle rotating counterclockwise). Finally, the meaning of T symmetry is that when the direction of motion of all particles and antiparticles is reversed, the system will return to the state in which it was before; in other words, the laws are the same whether moving forward or backward in time.

In 1956, two American physicists, Tzundao Li and Zhenning Yang, suggested that the weak interaction is in fact not invariant under P transformations. In other words, as a result of weak interaction, the development of the Universe may proceed differently than the development of its mirror image. That same year, Jinxiang Wu, a colleague of Li and Yang, was able to prove that their assumption was correct. By arranging the nuclei of radioactive atoms in a magnetic field so that their spins were in the same direction, she showed that more electrons were emitted in one direction than in the other. The following year, Lee and Yang were awarded the Nobel Prize for their discovery. It turned out that weak interactions do not obey C symmetry either. This means that a Universe consisting of antiparticles will behave differently than our Universe. However, it seemed to everyone that the weak interaction should still obey the combined CP symmetry, that is, the development of the Universe should occur in the same way as the development of its mirror reflection, if, having reflected it in the mirror, we also replace each particle with an antiparticle! But in 1964, two more Americans, James Cronin and Vel Fitch, discovered that even CP symmetry is broken in the decay of particles called K mesons.

As a result, in 1980, Cronin and Fitch received the Nobel Prize for their work. (What a huge number of prizes have been awarded for works that show that the Universe is not as simple as we think).

There is a mathematical theorem that states that any theory that obeys quantum mechanics and relativity must always be invariant under the combined CPT symmetry. In other words, the behavior of the Universe will not change if you replace particles with antiparticles, reflect everything in a mirror, and also reverse the direction of time. But Cronin and Fitch showed that if you replace particles with antiparticles and produce a mirror image, but do not reverse the direction of time, the Universe will behave differently. Consequently, when time is reversed, the laws of physics must change, i.e. they are not invariant with respect to the symmetry of T.

It is clear that in the early Universe the symmetry T was broken: when time flows forward, the Universe expands, and if time went backward, the Universe would begin to contract. And since there are forces that are not invariant with respect to the symmetry T, it follows that as the Universe expands under the influence of these forces, antielectrons should turn into quarks more often than electrons into antiquarks. Then, as the Universe expanded and cooled, the antiquarks and quarks would have annihilated, but since there would have been more quarks than antiquarks, there would have been a slight excess of quarks. And they are the very quarks that make up today’s matter that we see and from which we ourselves are created. Thus, our very existence can be considered as confirmation of the grand unification theory, although only as a qualitative confirmation. The uncertainties arise because we cannot predict how many quarks will remain after annihilation, or even whether the remaining particles will be quarks or antiquarks. (True, if there were a surplus of antiquarks left, we would simply rename them quarks, and quarks - antiquarks).

Grand unified theories do not include gravitational interaction. This is not so significant, because gravitational forces are so small that their influence can simply be neglected when we

Presentation on the topic "Elementary particles" in physics in powerpoint format. This presentation for 11th grade schoolchildren explains the physics of elementary particles and systematizes knowledge on the topic. The goal of the work is to develop abstract, ecological and scientific thinking of students based on ideas about elementary particles and their interactions. Author of the presentation: Popova I.A., physics teacher.

Fragments from the presentation

How many elements are in the periodic table?

• Only 92.
• How? Is there more?
• True, but all the rest are artificially obtained; they do not occur in nature.
• So - 92 atoms. Molecules can also be made from them, i.e. substances!
• But the fact that all substances consist of atoms was stated by Democritus (400 BC).
• He was a great traveler, and his favorite saying was:
• "Nothing exists except atoms and pure space, everything else is a view"

Timeline of particle physics

• Theoretical physicists faced the most difficult task of ordering the entire discovered “zoo” of particles and trying to reduce the number of fundamental particles to a minimum, proving that other particles consist of fundamental particles
• All these particles were unstable, i.e. decayed into particles with lower masses, eventually becoming stable protons, electrons, photons and neutrinos (and their antiparticles).
• The third one is this. M. Gell-Mann and independently J. Zweig proposed a model of the structure of strongly interacting particles from fundamental particles - quarks
• This model has now turned into a coherent theory of all known types of particle interactions.

How to detect an elementary particle?

Usually, traces (trajectories or tracks) left by particles are studied and analyzed using photographs.

Classification of elementary particles

All particles are divided into two classes:

• Fermions, which make up matter;
• Bosons through which interaction occurs.

Quarks

• Quarks participate in strong interactions, as well as weak and electromagnetic ones.
• Gell-Mann and Georg Zweig proposed the quark model in 1964.
• The Pauli principle: in one system of interconnected particles there never exist at least two particles with identical parameters if these particles have half-integer spin.

What is spin?

• Spin demonstrates that there is a state space that has nothing to do with the movement of a particle in ordinary space;
• Spin (from English to spin - to spin) is often compared to the angular momentum of a “rapidly rotating top” - this is not true!
• Spin is an internal quantum characteristic of a particle that has no analogue in classical mechanics;
• Spin (from the English spin - twirl, rotation) is the intrinsic angular momentum of elementary particles, which has a quantum nature and is not associated with the movement of the particle as a whole

Four types of physical interactions

• gravitational,
• electromagnetic,
• weak,
• strong.

• Weak interaction- changes the internal nature of particles.
• Strong interactions- determine various nuclear reactions, as well as the emergence of forces that bind neutrons and protons in nuclei.

Properties of quarks

• Quarks have a property called color charge.
• There are three types of color charge, conventionally designated as
• blue,
• green
• Red.
• Each color has a complement in the form of its own anti-color - anti-blue, anti-green and anti-red.
• Unlike quarks, antiquarks do not have color, but anticolor, that is, the opposite color charge.

Properties of quarks: mass

• Quarks have two main types of masses, which differ in size:
• current quark mass, estimated in processes with significant transfer of squared 4-momentum, and
• structural mass (block, constituent mass); also includes the mass of the gluon field around the quark and is estimated from the mass of hadrons and their quark composition.

Properties of quarks: flavor

• Each flavor (type) of a quark is characterized by such quantum numbers as
• isospin Iz,
• strangeness S,
• charm C,
• charm (bottomness, beauty) B′,
• truth (topness) T.

Tasks

• What energy is released during the annihilation of an electron and a positron?
• What energy is released during the annihilation of a proton and antiproton?
• What nuclear processes produce neutrinos?
  • A. During α - decay.
  • B. During β - decay.
  • B. When γ - quanta are emitted.
• What nuclear processes produce antineutrinos?
  • A. During α - decay.
  • B. During β - decay.
  • B. When γ - quanta are emitted.
  • D. During any nuclear transformations
• A proton is made up of...
  • A. . . .neutron, positron and neutrino.
  • B. . . .mesons.
  • IN. . . .quarks.
  • D. A proton has no constituent parts.
• A neutron is made up of...
  • A. . . .proton, electron and neutrino.
  • B. . . .mesons.
  • IN. . . . quarks.
  • D. The neutron has no constituent parts.
• What was proven by the experiments of Davisson and Germer?
  • A. Quantum nature of energy absorption by atoms.
  • B. Quantum nature of energy emission by atoms.
  • B. Wave properties of light.
  • D. Wave properties of electrons.
• Which of the following formulas determines the de Broglie wavelength for an electron (m and v are the mass and speed of the electron)?

Test

• What physical systems are formed from elementary particles as a result of electromagnetic interaction? A. Electrons, protons. B. Atomic nuclei. B. Atoms, molecules of matter and antiparticles.
• From the point of view of interaction, all particles are divided into three types: A. Mesons, photons and leptons. B. Photons, leptons and baryons. B. Photons, leptons and hadrons.
• What is the main factor in the existence of elementary particles? A. Mutual transformation. B. Stability. B. The interaction of particles with each other.
• What interactions determine the stability of nuclei in atoms? A. Gravitational. B. Electromagnetic. B. Nuclear. D. Weak.
• Are there immutable particles in nature? A. There are. B. They don’t exist.
• The reality of the transformation of matter into an electromagnetic field: A. Confirmed by the experience of annihilation of an electron and a positron. B. Confirmed by the experiment of annihilation of an electron and a proton.
• Reaction of transformation of matter into a field: A. e + 2γ→e+ B. e + 2γ→e- C. e+ +e- =2γ.
• What interaction is responsible for the transformation of elementary particles into each other? A. Strong interaction. B. Gravitational. B. Weak interaction D. Strong, weak, electromagnetic.

The answer to the ongoing question: what is the smallest particle in the Universe that evolved with humanity.

People once thought that grains of sand were the building blocks of what we see around us. The atom was then discovered and thought to be indivisible until it was split to reveal the protons, neutrons and electrons within. They also did not turn out to be the smallest particles in the Universe, since scientists discovered that protons and neutrons consist of three quarks each.

So far, scientists have not been able to see any evidence that there is anything inside the quarks and that the most fundamental layer of matter or the smallest particle in the Universe has been reached.

And even if quarks and electrons are indivisible, scientists don't know if they are the smallest bits of matter in existence or if the Universe contains objects that are even smaller.

The smallest particles in the Universe

They come in different flavors and sizes, some have amazing connections, others essentially evaporate each other, many of them have fantastic names: quarks made up of baryons and mesons, neutrons and protons, nucleons, hyperons, mesons, baryons, nucleons, photons, etc. .d.

The Higgs boson is a particle so important to science that it is called the “God particle.” It is believed that it determines the mass of all others. The element was first theorized in 1964 when scientists wondered why some particles were more massive than others.

The Higgs boson is associated with the so-called Higgs field, which is believed to fill the Universe. Two elements (the Higgs field quantum and the Higgs boson) are responsible for giving the others mass. Named after the Scottish scientist Peter Higgs. With the help of March 14, 2013, the confirmation of the existence of the Higgs Boson was officially announced.

Many scientists argue that the Higgs mechanism has solved the missing piece of the puzzle to complete the existing “standard model” of physics, which describes known particles.

The Higgs boson fundamentally determined the mass of everything that exists in the Universe.

Quarks

Quarks (meaning quarks) are the building blocks of protons and neutrons. They are never alone, existing only in groups. Apparently, the force that binds quarks together increases with distance, so the further you go, the more difficult it will be to separate them. Therefore, free quarks never exist in nature.

Quarks are fundamental particles are structureless, pointy approximately 10−16 cm in size.

For example, protons and neutrons are made up of three quarks, with protons containing two identical quarks, while neutrons have two different ones.

Supersymmetry

It is known that the fundamental “building blocks” of matter, fermions, are quarks and leptons, and the guardians of the force, bosons, are photons and gluons. The theory of supersymmetry says that fermions and bosons can transform into each other.

The predicted theory states that for every particle we know, there is a related one that we have not yet discovered. For example, for an electron it is a selectron, a quark is a squark, a photon is a photino, and a higgs is a higgsino.

Why don't we observe this supersymmetry in the Universe now? Scientists believe they are much heavier than their regular cousins ​​and the heavier they are, the shorter their lifespan. In fact, they begin to collapse as soon as they arise. Creating supersymmetry requires quite a large amount of energy, which only existed shortly after the big bang and could possibly be created in large accelerators like the Large Hadron Collider.

As for why the symmetry arose, physicists theorize that the symmetry may have been broken in some hidden sector of the Universe that we cannot see or touch, but can only feel gravitationally.

Neutrino

Neutrinos are light subatomic particles that whistle everywhere at close to the speed of light. In fact, trillions of neutrinos are flowing through your body at any moment, although they rarely interact with normal matter.

Some originate from the sun, while others come from cosmic rays interacting with Earth's atmosphere and astronomical sources such as exploding stars in the Milky Way and other distant galaxies.

Antimatter

All normal particles are thought to have antimatter with the same mass but opposite charge. When matter meets, they destroy each other. For example, the antimatter particle of a proton is an antiproton, while the antimatter partner of an electron is called a positron. Antimatter is one of the most expensive substances in the world that people have been able to identify.

Gravitons

In the field of quantum mechanics, all fundamental forces are transmitted by particles. For example, light is made up of massless particles called photons, which carry an electromagnetic force. Likewise, the graviton is a theoretical particle that carries the force of gravity. Scientists have yet to detect gravitons, which are difficult to find because they interact so weakly with matter.

Threads of Energy

In experiments, tiny particles such as quarks and electrons act as single points of matter with no spatial distribution. But point objects complicate the laws of physics. Since it is impossible to approach infinitely close to a point, since the acting forces can become infinitely large.

An idea called superstring theory could solve this problem. The theory states that all particles, instead of being pointlike, are actually small threads of energy. That is, all objects in our world consist of vibrating threads and membranes of energy. Nothing can be infinitely close to the thread, because one part will always be a little closer than the other. This "loophole" appears to solve some of the problems with infinity, making the idea attractive to physicists. However, scientists still have no experimental evidence that string theory is correct.

Another way of solving the point problem is to say that space itself is not continuous and smooth, but is actually made up of discrete pixels or grains, sometimes called space-time structure. In this case, the two particles will not be able to approach each other indefinitely, because they must always be separated by a minimum grain size of space.

Black hole point

Another contender for the title of smallest particle in the Universe is the singularity (a single point) at the center of a black hole. Black holes form when matter condenses into a space small enough that gravity grabs, causing matter to be pulled inward, eventually condensing into a single point of infinite density. At least according to the current laws of physics.

But most experts don't think black holes are truly infinitely dense. They believe that this infinity is the result of an internal conflict between two current theories - general relativity and quantum mechanics. They suggest that when the theory of quantum gravity can be formulated, the true nature of black holes will be revealed.

Planck length

Threads of energy and even the smallest particle in the Universe can be the size of a “planck length”.

The length of the bar is 1.6 x 10 -35 meters (the number 16 is preceded by 34 zeros and a decimal point) - an incomprehensibly small scale that is associated with various aspects of physics.

The Planck length is a “natural unit” of length that was proposed by the German physicist Max Planck.

Planck's length is too short for any instrument to measure, but beyond this, it is believed to represent the theoretical limit of the shortest measurable length. According to the uncertainty principle, no instrument should ever be able to measure anything less, because in this range the universe is probabilistic and uncertain.

This scale is also considered the dividing line between general relativity and quantum mechanics.

The Planck length corresponds to the distance where the gravitational field is so strong that it can begin to make black holes from the energy of the field.

Apparently now, the smallest particle in the Universe is approximately the size of a plank: 1.6 x 10 −35 meters

conclusions

From school it was known that the smallest particle in the Universe, the electron, has a negative charge and a very small mass, equal to 9.109 x 10 - 31 kg, and the classical radius of the electron is 2.82 x 10 -15 m.

However, physicists are already operating with the smallest particles in the Universe, the Planck size which is approximately 1.6 x 10 −35 meters.

One of the main properties of particles is their ability to transform into each other, to be born and destroyed as a result of interaction.
The discovery of the positron, a particle similar in characteristics to an electron, but unlike an electron, has a positive unit charge, was an extremely important event in physics. Back in 1928, P. Dirac proposed an equation to describe the relativistic quantum mechanics of the electron. It turned out that the Dirac equation has two solutions, both with positive and negative energy. A negative energy state describes a particle similar to an electron, but with a positive electrical charge. The positron was the first particle discovered from a whole class of particles called antiparticles. Before the discovery of the positron, the unequal role of positive and negative charges in nature seemed inexplicable. Why is there a heavy, positively charged proton, but not a heavy particle with the mass of a proton and a negative charge? But there was a light negatively charged electron. The discovery of the positron in 1932 essentially restored charge symmetry for light particles and confronted physicists with the problem of finding an antiparticle for the proton. Another surprise is that the positron is a stable particle and can exist in empty space indefinitely. However, when an electron and a positron collide, they annihilate. The electron and positron disappear, and instead of them two γ quanta are born

e + + e - → 2γ

m(e -) = m(e +) = 0.511 MeV.

There is a transformation of particles with a rest mass different from zero into particles with zero rest mass (photons), i.e. rest mass is not conserved, but is converted into kinetic energy.
Along with the process of annihilation, the process of creation of an electron-positron pair was also discovered. Electron-positron pairs were easily produced by -quanta with an energy of several MeV in the Coulomb field of the atomic nucleus. In classical physics, the concepts of particles and waves are sharply differentiated - some physical objects are particles, while others are waves. The transformation of electron-positron pairs into photons provided additional confirmation of the idea that there is much in common between radiation and matter. The processes of annihilation and the birth of pairs forced us to rethink what particles, which were previously called elementary, are. The particle has ceased to be an unchanging “brick” in the structure of matter. A new, extremely profound concept of the mutual transformation of particles has emerged. It turned out that particles can be born and disappear, turning into other particles.
In the theory of -decay created by E. Fermi, it was shown that the electrons emitted during the process of -decay do not exist in the nucleus, but are born as a result of the decay of a neutron. As a result of this decay, neutron n disappears and proton p, electron e - and electron antineutrino e are born.

n p + e - + e

m(n) = 939.6 MeV. m(p) = 938.3 MeV. m(e) = ? τ(n) = 887c.

As a result of reactions between an antiproton and a proton p, depending on the energy of the colliding particles, various particles can be born

p+	→ n + + π + + π -	m() = m(p), m() = m(n) m(π +) = m(π -) = 140 MeV. τ (π +) = τ (π -) = 2.6∙ 10 -8 s.
	→π + + π - + π 0
	→ K + + K -

A positively charged K + meson, whose average lifetime is 1.2∙10 -8 s, decays in one of the following ways (the relative probabilities of decays are shown on the right.

Λ -hyperon and Δ 0 -resonance have approximately the same masses and decay into the same particles - proton and π - meson. The large difference in their lifetime is due to the decay mechanism. Λ -hyperon decays as a result of weak interaction, and Δ 0 -resonance - as a result of strong interaction.

Λ → p + π	m(Λ ) = 1116 MeV. τ (Λ ) = 2.6∙ 10 -10 s.
Δ 0 → p + π	m(Δ ) = 1232 MeV. τ(Δ) = 10 -23 s

During the decay of a negative muon (-) in the final state, two neutral particles appear along with the electron - a muon neutrino ν μ and electron antineutrino e. This decay occurs as a result of weak interaction.