The carriers of the weak interaction are the vector bosons W + , W− and Z 0 . In this case, the interaction of the so-called charged weak currents and neutral weak currents is distinguished. Interaction of charged currents (with the participation of charged bosons W± ) leads to a change in particle charges and the transformation of some leptons and quarks into other leptons and quarks. Interaction of neutral currents (with the participation of a neutral boson Z 0 ) does not change the particle charges and transforms leptons and quarks into the same particles.
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Using the Pauli hypothesis, Enrico Fermi developed in 1933 the first theory of beta decay. Interestingly, his work was refused to be published in the journal Nature, referring to the excessive abstractness of the article. Fermi's theory is based on the use of the secondary quantization method, similar to that which had already been applied by that time to the processes of emission and absorption of photons. One of the ideas voiced in the work was also the assertion that the particles emitted from the atom were not initially contained in it, but were born in the process of interaction.
For a long time it was believed that the laws of nature are symmetrical with respect to mirror reflection, that is, the result of any experiment should be the same as the result of an experiment conducted on a mirror-symmetric installation. This symmetry with respect to spatial inversion (which is usually denoted as P) is related to the law conservation parity . However, in 1956, while theoretically considering the process of K-meson decay, Yang Zhenning and Li Zongdao suggested that the weak interaction might not obey this law. As early as 1957, Wu Jiansong's group confirmed this prediction in a beta decay experiment, which earned Yang and Li the 1957 Nobel Prize in Physics. Later, the same fact was confirmed in the decay of the muon and other particles.
To explain the new experimental facts, in 1957 Murray Gell-Mann, Richard Feynman, Robert Marshak and George Sudarshan developed a universal theory of the four-fermion weak interaction, called V − A-theory.
In an effort to preserve the maximum possible symmetry of interactions, L. D. Landau suggested in 1957 that although P-symmetry is broken in weak interactions, combined symmetry must be preserved in them CP- a combination of mirror reflection and replacement of particles by antiparticles. However, in 1964, James-Cronin and Wahl-Fitch found a weak violation in the decays of neutral kaons CP-parity. It was the weak interaction that also turned out to be responsible for this violation, moreover, the theory in this case predicted that in addition to the two generations of quarks and leptons known by that time, there should be at least one more generation. This prediction was confirmed first in 1975 with the discovery of the tau lepton, and then in 1977 with the discovery of the b quark. Cronin and Fitch received the 1980 Nobel Prize in Physics.
Properties
All fundamental fermions (leptons and quarks) take part in the weak interaction. This is the only interaction in which neutrinos participate (apart from gravity, which is negligible in the laboratory), which explains the colossal penetrating power of these particles. Weak interaction allows leptons, quarks and their antiparticles to exchange energy, mass, electric charge and quantum numbers - that is, to turn into each other.
The weak force gets its name from the fact that its characteristic intensity is much lower than that of electromagnetism. In elementary particle physics, the intensity of an interaction is usually characterized by the rate of processes caused by this interaction. The faster the processes proceed, the higher the intensity of interaction. At energies of interacting particles of the order of 1 GeV, the characteristic rate of processes due to weak interaction is about 10 −10 s, which is approximately 11 orders of magnitude higher than for electromagnetic processes, that is, weak processes are extremely slow processes.
Another characteristic of the intensity of interaction is the length free path of particles in a substance. So, in order to stop a flying hadron due to the strong interaction, a plate of iron several centimeters thick is required. And a neutrino, which only participates in the weak interaction, can fly through a plate billions of kilometers thick.
Among other things, the weak interaction has a very small radius of action - about 2·10 -18 m (this is approximately 1000 times smaller than the size of the nucleus). It is for this reason that, despite the fact that the weak interaction is much more intense than the gravitational one, the range of which is unlimited, it plays a noticeably smaller role. For example, even for nuclei located at a distance of 10 −10 m, the weak interaction is weaker not only electromagnetic, but also gravitational.
In this case, the intensity of weak processes strongly depends on the energy of the interacting particles. The higher the energy, the higher the intensity. For example, due to the weak interaction, the neutron, whose energy release during beta decay is approximately 0.8 MeV, decays in about 10 3 s, and the Λ-hyperon, with an energy release of about a hundred times more, already in 10 −10 s. The same is true for energetic neutrinos: the cross section for interaction with a nucleon of a neutrino with an energy of 100 GeV is six orders of magnitude larger than that of a neutrino with an energy of about 1 MeV. However, at energies of the order of several hundred GeV (in the center-of-mass system of colliding particles), the intensity of the weak interaction becomes comparable to the energy of the electromagnetic interaction, as a result of which they can be described in a unified way as the electroweak interaction.
The weak interaction is the only one of the fundamental interactions for which the law conservation parity does not hold, which means that the laws that weak processes obey change when the system is mirrored. Violation of the parity conservation law leads to the fact that only the left particles (whose spin is directed opposite to the momentum) are subject to weak interaction, but not the right ones (whose spin is co-directed with the momentum), and vice versa: the right antiparticles interact in a weak way, but the left ones are inert.
In addition to spatial parity, the weak interaction also does not preserve the combined space-charge parity, that is, the only known interaction violates the principle CP-invariance .
Theoretical description
Fermi theory
The first theory of the weak interaction was developed by Enrico Fermi in the 1930s. His theory is based on a formal analogy between the process of β-decay and the electromagnetic processes of photon emission. Fermi's theory is based on the interaction of the so-called hadron and lepton currents. In this case, unlike electromagnetism, it is assumed that their interaction is of a contact nature and does not imply the presence of a carrier similar to a photon. In modern notation, the interaction between the four main fermions (proton, neutron, electron, and neutrino) is described by an operator of the form
G F 2 p ¯ ^ n ^ ⋅ e ¯ ^ ν ^ (\displaystyle (\frac (G_(F))(\sqrt (2)))(\hat (\overline (p)))(\hat (n) )\cdot (\hat (\overline (e)))(\hat (\nu ))),where G F (\displaystyle G_(F))- the so-called Fermi constant, numerically equal to approximately 10 −48 J/m³ or 10 − 5 / m p 2 (\displaystyle 10^(-5)/m_(p)^(2)) (m p (\displaystyle m_(p))- proton mass) in units, where ℏ = c = 1 (\displaystyle \hbar =c=1); p ¯ ^ (\displaystyle (\hat (\overline (p))))- proton creation operator (or antiproton annihilation), n ^ (\displaystyle (\hat(n)))- neutron annihilation operator (antineutron creation), e ¯ ^ (\displaystyle (\hat (\overline (e))))- operator of electron creation (positron annihilation), ν ^ (\displaystyle (\hat (\nu )))- neutrino annihilation operator (antineutrino generation).
Work p ¯ ^ n ^ (\displaystyle (\hat (\overline (p)))(\hat (n))), responsible for the conversion of a neutron into a proton, was called the nucleon current, and e ¯ ^ ν ^ , (\displaystyle (\hat (\overline (e)))(\hat (\nu )),) converting an electron into a neutrino - lepton. It is postulated that these currents, similarly to electromagnetic currents, are 4-vectors p ¯ ^ γ μ n ^ (\displaystyle (\hat (\overline (p)))\gamma _(\mu )(\hat (n))) and e ¯ ^ γ μ ν ^ (\displaystyle (\hat (\overline (e)))\gamma _(\mu )(\hat (\nu ))) (γ μ , μ = 0 … 3 (\displaystyle \gamma _(\mu ),~\mu =0\dots 3)- Dirac matrices). Therefore, their interaction is called vector.
The essential difference between the weak currents introduced by Fermi and the electromagnetic ones is that they change the charge of the particles: a positively charged proton becomes a neutral neutron, and a negatively charged electron becomes a neutral neutrino. In this regard, these currents are called charged currents.
Universal V-A Theory
The universal theory of the weak interaction, also called V−A-theory, was proposed in 1957 by M. Gell-Mann, R. Feynman, R. Marshak and J. Sudarshan. This theory took into account the recently proved fact of parity violation ( P-symmetries) in the case of weak interaction. For this, weak currents were represented as the sum of the vector current V and axial A(hence the name of the theory).
The vector and axial currents behave in exactly the same way under Lorentz transformations. However, during spatial inversion, their behavior is different: the vector current remains unchanged during such a transformation, while the axial current changes sign, which leads to parity violation. In addition, currents V and A differ in the so-called charge parity (violate C-symmetry).
Similarly, the hadronic current is the sum of the quark currents of all generations ( u- top, d- bottom, c- enchanted s- strange, t- true, b- lovely quarks):
u ¯ ^ d ′ ^ + c ¯ ^ s ′ ^ + t ¯ ^ b ′ ^ . (\displaystyle (\hat (\overline (u)))(\hat (d^(\prime )))+(\hat (\overline (c)))(\hat (s^(\prime ))) +(\hat (\overline (t)))(\hat (b^(\prime ))).)Unlike the lepton current, however, here the operators d ′ ^ , (\displaystyle (\hat (d^(\prime ))),) s ′ ^ (\displaystyle (\hat (s^(\prime )))) and b ′ ^ (\displaystyle (\hat (b^(\prime )))) are a linear combination of operators d ^ , (\displaystyle (\hat (d)),) s ^ (\displaystyle (\hat(s))) and b ^ , (\displaystyle (\hat (b)),) that is, the hadron current contains a total of not three, but nine terms. These terms can be combined into a single 3×3 matrix called the Cabibbo - Kobayashi - Maskawa matrix. This matrix can be parameterized with three angles and a phase factor. The latter characterizes the degree of violation CP-invariance in the weak interaction.
All terms in the charged current are the sum of the vector and axial operators with multipliers equal to one.
L = G F 2 j w ^ j w † ^ , (\displaystyle (\mathcal (L))=(\frac (G_(F))(\sqrt (2)))(\hat (j_(w)))(\ hat (j_(w)^(\dagger ))),)where j w ^ (\displaystyle (\hat (j_(w)))) is the charged current operator, and j w † ^ (\displaystyle (\hat (j_(w)^(\dagger ))))- conjugate to it (obtained by replacing e ¯ ^ ν e ^ → ν e ¯ ^ e ^ , (\displaystyle (\hat (\overline (e)))(\hat (\nu _(e)))\rightarrow (\hat (\overline (\ nu _(e))))(\hat (e)),) u ¯ ^ d ^ → d ¯ ^ u ^ (\displaystyle (\hat (\overline (u)))(\hat (d))\rightarrow (\hat (\overline (d)))(\hat (u ))) etc.)
Theory of Weinberg - Salam
In the modern form, the weak interaction is described as part of a single electroweak interaction in the framework of the Weinberg-Salam theory. This is a quantum field theory with a gauge group SU(2)× U(1) and the spontaneously broken symmetry of the vacuum state caused by the action of the Higgs boson field. The proof of the renormalizability of such a model by Martinus Veltman and Gerard "t Hooft was awarded the 1999 Nobel Prize in Physics.
In this form, the theory of the weak interaction is included in the modern Standard Model, and it is the only interaction that breaks symmetries P and CP .
According to the theory of the electroweak interaction, the weak interaction is not a contact, but has its own carriers - vector bosons W + , W− and Z 0 with non-zero mass and spin equal to 1. The mass of these bosons is about 90 GeV / s², which causes a small range of weak forces.
In this case, charged bosons W± are responsible for the interaction of charged currents, and the existence of a neutral boson Z 0 means the existence of neutral currents as well. Such currents, indeed, were discovered experimentally. An example of interaction with their participation is, in particular, the elastic scattering of a neutrino by a proton. In such interactions, both the type of particles and their charges are preserved.
To describe the interaction of neutral currents, the Lagrangian must be supplemented with a term of the form
L = G F ρ 2 2 f 0 ^ f 0 ^ , (\displaystyle (\mathcal (L))=(\frac (G_(F)\rho )(2(\sqrt (2))))(\hat ( f_(0)))(\hat (f_(0))),)where ρ is a dimensionless parameter, equal to unity in the standard theory (experimentally it differs from unity by no more than 1%), f 0 ^ = ν e ¯ ^ ν e ^ + ⋯ + e ¯ ^ e ^ + ⋯ + u ¯ ^ u ^ + … (\displaystyle (\hat (f_(0)))=(\hat (\overline ( \nu _(e))))(\hat (\nu _(e)))+\dots +(\hat (\overline (e)))(\hat (e))+\dots +(\hat (\overline (u)))(\hat (u))+\dots )- self-adjoint neutral current operator.
Unlike charged currents, the neutral current operator is diagonal, that is, it translates particles into themselves, and not into other leptons or quarks. Each of the terms of the neutral current operator is the sum of a vector operator with a multiplier and an axial operator with a multiplier I 3 − 2 Q sin 2 θ w (\displaystyle I_(3)-2Q\sin ^(2)\theta _(w)), where I 3 (\displaystyle I_(3))- the third projection of the so-called weak
Weak interaction
Strong interaction
The strong interaction is short-range. Its radius of action is about 10-13 cm.
The particles involved in the strong interaction are called hadrons. In an ordinary stable substance at a not too high temperature, strong interaction does not cause any processes. Its role is to create a strong bond between nucleons (protons and neutrons) in nuclei. The binding energy averages about 8 MeV per nucleon. In this case, during collisions of nuclei or nucleons with a sufficiently high energy (on the order of hundreds of MeV), strong interaction leads to numerous nuclear reactions: fission of nuclei, transformation of some nuclei into others, etc.
Beginning with energies of colliding nucleons on the order of several hundred MeV, the strong interaction leads to the production of P-mesons. At even higher energies, K-mesons and hyperons are born, and many meson and baryon resonances (resonances are short-lived excited states of hadrons).
At the same time, it turned out that not all particles experience strong interaction. So, it is experienced by protons and neutrons, but electrons, neutrinos and photons are not subject to it. Usually only heavy particles participate in the strong interaction.
The theoretical explanation of the nature of the strong interaction has been difficult to develop. A breakthrough was outlined only in the early 1960s, when the quark model was proposed. In this theory, neutrons and protons are considered not as elementary particles, but as composite systems built from quarks.
The strong interaction quanta are eight gluons. Gluons got their name from the English word glue (glue), because they are responsible for the confinement of quarks. The rest masses of gluons are equal to zero. At the same time, gluons have a color charge, due to which they are capable of interacting with each other, as they say, of self-action, which leads to difficulties in describing the strong interaction mathematically due to its nonlinearity.
Its radius of action is less than 10-15 cm. Weak interaction is several orders of magnitude weaker than not only strong, but also electromagnetic. At the same time, it is much stronger than the gravitational one in the microcosm.
The first discovered and most widespread process caused by the weak interaction is the radioactive b-decay of nuclei.
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This type of radioactivity was discovered in 1896 by A.A. Becquerel.em. In the process of radioactive electronic / b - - / decay, one of the neutrons / n/ atomic nucleus turns into a proton / R/ with electron emission / e-/ and electronic antineutrino //:n ® p + e-+
In the process of positron /b + -/ decay, a transition occurs:
p® n + e++
In the first theory of b-decay, created in 1934 by E. Fermi, to explain this phenomenon, it was necessary to introduce a hypothesis about the existence of a special type of short-range forces that cause the transition
n ® p + e-+
Further research showed that the interaction introduced by Fermi has a universal character.
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It causes the decay of all unstable particles, whose masses and selection rules for quantum numbers do not allow them to decay due to strong or electromagnetic interaction. Weak interaction is inherent in all particles, except for photons. The characteristic time of the weak interaction processes at energies of the order of 100 MeV is 13-14 orders of magnitude longer than the characteristic time for the strong interaction.Weak interaction quanta are three bosons - W + , W − , Z°- bosons. Superscripts indicate the sign of the electric charge of these quanta. The quanta of the weak interaction have a significant mass, which leads to the fact that the weak interaction manifests itself at very short distances.
It must be taken into account that today the weak and electromagnetic interactions are already combined into a single theory. There are a number of theoretical schemes in which an attempt is made to create a unified theory of all types of interaction. However, these schemes are not yet developed enough to be tested experimentally.
26. Structural physics. Corpuscular approach to the description and explanation of nature. Reductionism
The objects of structural physics are elements of the structure of matter (for example, molecules, atoms, elementary particles) and more complex formation of them. It:
1) plasma - it is a gas in which a significant part of the molecules or atoms are ionized;
2) crystals- these are solids in which atoms or molecules are arranged in an orderly manner and form a periodically repeating internal structure;
3) liquids- this is the state of aggregation of matter, ĸᴏᴛᴏᴩᴏᴇ combines the features of a solid state (preservation of volume, a certain tensile strength) and gaseous (shape variability).
Fluids are characterized by:
a) short-range order in the arrangement of particles (molecules, atoms);
b) a small difference in the kinetic energy of thermal motion and their potential energy of interaction.
4) stars,ᴛ.ᴇ. glowing gas (plasma) balls.
When highlighting the structural equations of matter, the following criteria are used:
Spatial dimensions: particles of the same level have spatial dimensions of the same order (for example, all atoms have dimensions of the order of 10 -8 cm);
The time of the processes: at one level, it is about the same order;
Objects of the same level consist of the same elements (for example, all nuclei consist of protons and neutrons);
The laws that explain processes at one level are qualitatively different from the laws that explain processes at another level;
Objects of different levels differ in basic properties (for example, all atoms are electrically neutral, and all nuclei are positively electrically charged).
As new levels of structure and states of matter are discovered, the object area of structural physics is expanding.
It should be borne in mind that when solving specific physical problems, the issues related to the elucidation of the structure, interaction, and motion are closely intertwined.
At the root of structural physics is the corpuscular approach to the description and explanation of nature.
For the first time, the concept of an atom as the last and indivisible particle of the body arose in Ancient Greece within the framework of the natural-philosophical teachings of the school of Leucippus-Democritus. According to this view, there are only atoms in the world that move in the void. The ancient atomists considered the continuity of matter to be apparent. Various combinations of atoms form various visible bodies. This hypothesis was not based on experimental data. She was just a brilliant guess. But it determined the entire further development of natural science for many centuries to come.
The hypothesis of atoms as indivisible particles of matter was revived in natural science, in particular, in physics and chemistry to explain some patterns that were established empirically (for example, the laws of Boyle-Mariotte and Gay-Lussac for ideal gases, thermal expansion of bodies, etc.). d.). Indeed, Boyle-Mariotte's law states that the volume of a gas is inversely proportional to its pressure, but it does not explain why this is so. Similarly, when a body is heated, its dimensions increase. But what is the reason for this expansion? In the kinetic theory of matter, these and other regularities established by experience are explained with the help of atoms and molecules.
Indeed, the directly observed and measured decrease in gas pressure with an increase in its volume in the kinetic theory of matter is explained as an increase in the free path of its constituent atoms and molecules. It is as a result of this that the volume occupied by the gas increases. Similarly, the expansion of bodies when heated in the kinetic theory of matter is explained by an increase in the average speed of moving molecules.
Explanations in which the properties of complex substances or bodies are trying to be reduced to the properties of their simpler elements or components are called reductionism. This method of analysis made it possible to solve a large class of problems in natural science.
Until the end of the XIX century. It was believed that the atom is the smallest, indivisible, structureless particle of matter. At the same time, the discoveries of the electron, radioactivity showed that this is not so. Rutherford's planetary model of the atom arises. Then it is replaced by the model N. Bora. But as before, the thought of physicists is aimed at reducing the entire variety of complex properties of bodies and natural phenomena to the simple properties of a small number of primary particles. Subsequently, these particles were named elementary. Now their total number exceeds 350. For this reason, it is unlikely that all such particles can be called truly elementary, not containing other elements. This belief is strengthened in connection with the hypothesis of the existence of quarks. According to it, known elementary particles consist of particles with fractional electric charges. They are called quarks.
According to the type of interaction in which elementary particles participate, all of them, except for the photon, are classified into two groups:
1) hadrons. It is worth saying that they are characterized by the presence of a strong interaction. At the same time, they can also participate in weak and electromagnetic interactions;
2) leptons. Οʜᴎ participate only in electromagnetic and weak interactions;
According to the lifetime are distinguished:
a) stable elementary particles. These are the electron, photon, proton and neutrino;
b) quasi-stable. These are particles that decay due to electromagnetic and weak interactions. For example, to + ® m + +;
c) unstable. Οʜᴎ decay due to strong interaction, for example, neutron.
The electric charges of elementary particles are multiples of the smallest charge inherent in an electron. At the same time, elementary particles are divided into pairs of particle - antiparticle, for example, e - - e + (they all have the same characteristics, and the signs of the electric charge are opposite). Electrically neutral particles also have antiparticles, for example, P -,- .
So, the atomistic concept is based on the idea of the discrete structure of matter. The atomistic approach explains the properties of a physical object on the basis of the properties of its smallest particles, which at a certain stage of cognition are considered indivisible. Historically, such particles were first recognized as atoms, then elementary particles, and now - quarks. The difficulty of this approach is the complete reduction of the complex to the simple, which does not take into account the qualitative differences between them.
Until the end of the first quarter of the 20th century, the idea of the unity of the structure of the macro- and microcosm was understood mechanistically, as the complete identity of laws and the complete similarity of the structure of both.
Microparticles were interpreted as miniature copies of macrobodies, ᴛ.ᴇ. as extremely small balls (corpuscles) moving along precise orbits, which are completely analogous to planetary orbits, with the only difference that celestial bodies are connected by gravitational interaction forces, and microparticles - by electrical interaction forces.
After the discovery of the electron (Thomson, 1897 ᴦ.), the creation of the quantum theory (Planck, 1900 ᴦ.), the introduction of the concept of photon (Einstein, 1905 ᴦ.), the atomic doctrine acquired a new character.
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The idea of discreteness was extended to the field of electrical and light phenomena, to the concept of energy (in the 19th century, the doctrine of energy served as the sphere of representation of continuous quantities and state functions). The most important feature of modern atomic doctrine is the atomism of action. It is connected with the fact that the movement, properties and states of various micro-objects can be quantized, ᴛ.ᴇ. are expressed in the form of discrete quantities and ratios. The new atomistics recognizes the relative stability of each discrete type of matter, its qualitative certainty, its relative indivisibility and irreversibility within certain limits of natural phenomena. For example, being divisible in some physical ways, the atom is chemically indivisible, ᴛ.ᴇ. in chemical processes it behaves as something whole, indivisible. A molecule, being chemically divisible into atoms, in thermal motion (up to certain limits) behaves as a whole, indivisible, etc.Especially important in the concept of new atomistics is the recognition of the interconvertibility of any discrete types of matter.
Different levels of the structural organization of physical reality (quarks, microparticles, nuclei, atoms, molecules, macrobodies, megasystems) have their own specific physical laws. But no matter how the studied phenomena differ from the phenomena studied by classical physics, all experimental data must be described using classical concepts. There is a fundamental difference between the description of the behavior of the microobject under study and the description of the operation of measuring instruments. This is the result of the fact that the operation of measuring instruments, in principle, should be described in the language of classical physics, while the object under study may not be described in this language.
The corpuscular approach in explaining physical phenomena and processes has always been combined with the continuum approach since the emergence of interaction physics. It was expressed in the concept of the field and the disclosure of its role in physical interaction. The representation of the field as a flow of a certain kind of particles (quantum field theory) and the attribution of wave properties to any physical object (Louis de Broglie's hypothesis) combined these two approaches to the analysis of physical phenomena.
Weak interaction - concept and types. Classification and features of the category "Weak interaction" 2017, 2018.
Time is like a river carrying events passing by, and its current is strong; only something will seem to your eyes - and it has already been carried away, and something else is visible, which will also soon be carried away.
Marcus Aurelius
Each of us strives to create a complete picture of the world, including a picture of the Universe, from the smallest subatomic particles to the greatest scales. But the laws of physics are sometimes so strange and counterintuitive that this task can become overwhelming for those who have not become professional theoretical physicists.
The reader asks:
Although this is not astronomy, but maybe you will tell me. The strong force is carried by gluons and binds quarks and gluons together. Electromagnetic is carried by photons and binds electrically charged particles. Gravity is supposedly carried by gravitons and binds all particles to mass. The weak is carried by the W and Z particles, and … is due to decay? Why is the weak force described in this way? Is the weak force responsible for the attraction and/or repulsion of any particles? And what? And if not, why then is this one of the fundamental interactions, if it is not associated with any forces? Thank you.
Let's take a look at the basics. There are four fundamental forces in the universe - gravity, electromagnetism, strong nuclear force and weak nuclear force.And all these are interactions, forces. For particles whose state can be measured, the application of a force changes its momentum - in ordinary life in such cases we speak of acceleration. And for three of these forces, this is true.
In the case of gravity, the total amount of energy (mostly mass, but that includes all energy) warps spacetime, and the motion of all other particles changes in the presence of anything that has energy. This is how it works in the classical (not quantum) theory of gravity. Maybe there is a more general theory, quantum gravity, where there is an exchange of gravitons, leading to what we observe as a gravitational interaction.
Before proceeding, please understand:
- Particles have a property, or something inherent in them, that allows them to feel (or not feel) a certain type of force.
- Other interaction-carrying particles interact with the first
- As a result of interactions, particles change momentum, or accelerate
In electromagnetism, the main property is electric charge. Unlike gravity, it can be positive or negative. A photon, a particle that carries an interaction associated with a charge, leads to the fact that the same charges repel, and the different ones attract.
It is worth noting that moving charges, or electric currents, experience another manifestation of electromagnetism - magnetism. The same thing happens with gravity, and is called gravitomagnetism (or gravitoelectromagnetism). We will not go deep - the point is that there is not only a charge and a carrier of force, but also currents.
There is also a strong nuclear force, which has three types of charges. Although all particles have energy and are all subject to gravity, and although quarks, half of the leptons and a couple of bosons contain electrical charges, only quarks and gluons have a color charge and can experience the strong nuclear force.
There are a lot of masses everywhere, so gravity is easy to observe. And since the strong force and electromagnetism are quite strong, they are also easy to observe.
But what about the last one? Weak interaction?
We usually talk about it in the context of radioactive decay. A heavy quark or lepton decays into lighter and more stable ones. Yes, the weak force has something to do with it. But in this example, it somehow differs from the rest of the forces.
It turns out that the weak force is also a force, just not often talked about. She's weak! 10,000,000 times weaker than electromagnetism at a distance as long as the diameter of a proton.
A charged particle always has a charge, whether it is moving or not. But the electric current created by it depends on its movement relative to other particles. Current determines magnetism, which is just as important as the electrical part of electromagnetism. Composite particles like the proton and neutron have significant magnetic moments, just like the electron.
Quarks and leptons come in six flavors. Quarks - top, bottom, strange, charmed, charming, true (according to their letter designations in Latin u, d, s, c, t, b - up, down, strange, charm, top, bottom). Leptons - electron, electron-neutrino, muon, muon-neutrino, tau, tau-neutrino. Each of them has an electrical charge, but also a flavor. If we combine electromagnetism and the weak force to get the electroweak force, then each of the particles will have some kind of weak charge, or electroweak current, and a weak force constant. All this is described in the Standard Model, but it was quite difficult to verify this because electromagnetism is so strong.
In a new experiment, the results of which have recently been published, the contribution of the weak interaction has been measured for the first time. The experiment made it possible to determine the weak interaction of up and down quarks
And the weak charges of the proton and neutron. The predictions of the Standard Model for weak charges were:
Q W (p) = 0.0710 ± 0.0007,
Q W (n) = -0.9890 ± 0.0007.And according to the scattering results, the experiment gave the following values:
Q W (p) = 0.063 ± 0.012,
Q W (n) = -0.975 ± 0.010.Which agrees very well with the theory, taking into account the error. Experimenters say that by processing more data, they will further reduce the error. And if there are any surprises or discrepancies with the Standard Model, that will be cool! But nothing indicates this:
Therefore, particles have a weak charge, but we do not expand on it, since it is unrealistically difficult to measure. But we did it anyway, and apparently reaffirmed the Standard Model.
This interaction is the weakest of the fundamental interactions experimentally observed in the decays of elementary particles, where quantum effects are fundamentally significant. Recall that quantum manifestations of gravitational interaction have never been observed. Weak interaction is singled out using the following rule: if an elementary particle called a neutrino (or antineutrino) participates in the interaction process, then this interaction is weak.
A typical example of a weak interaction is the neutron beta decay, where n- neutron, p- proton, e- - electron, e+ is an electron antineutrino. However, it should be borne in mind that the above rule does not mean at all that any act of weak interaction must be accompanied by a neutrino or antineutrino. It is known that a large number of neutrinoless decays take place. As an example, we can note the process of decay of the lambda hyperon D into a proton p+ and a negatively charged pion p– . According to modern concepts, the neutron and proton are not truly elementary particles, but consist of elementary particles called quarks.
The intensity of the weak interaction is characterized by the Fermi coupling constant G F. Constant G F dimensional. To form a dimensionless quantity, it is necessary to use some kind of reference mass, for example, the mass of a proton m p. Then the dimensionless coupling constant will be. It can be seen that the weak interaction is much more intense than the gravitational one.
The weak interaction, in contrast to the gravitational one, is short-range. This means that the weak interaction between particles only comes into play if the particles are close enough to each other. If the distance between the particles exceeds a certain value, called the characteristic radius of interaction, the weak interaction does not manifest itself. It has been experimentally established that the characteristic radius of the weak interaction of the order of 10–15 cm, that is, the weak interaction, is concentrated at distances smaller than the size of the atomic nucleus.
Why can we talk about the weak interaction as an independent form of fundamental interactions? The answer is simple. It has been established that there are processes of transformations of elementary particles that cannot be reduced to gravitational, electromagnetic and strong interactions. A good example showing that there are three qualitatively different interactions in nuclear phenomena is related to radioactivity. Experiments indicate the presence of three different types of radioactivity: α-, β- and γ-radioactive decays. In this case, α-decay is due to strong interaction, γ-decay is due to electromagnetic. The remaining β-decay cannot be explained by the electromagnetic and strong interactions, and we are forced to accept that there is another fundamental interaction called the weak one. In the general case, the need to introduce a weak interaction is due to the fact that processes occur in nature in which electromagnetic and strong decays are prohibited by conservation laws.
Although the weak interaction is essentially concentrated inside the nucleus, it has certain macroscopic manifestations. As we have already noted, it is associated with the process of β-radioactivity. In addition, the weak interaction plays an important role in the so-called thermonuclear reactions responsible for the mechanism of energy release in stars.
The most amazing property of the weak interaction is the existence of processes in which mirror asymmetry is manifested. At first glance, it seems obvious that the difference between the concepts of left and right is arbitrary. Indeed, the processes of gravitational, electromagnetic, and strong interactions are invariant with respect to spatial inversion, which implements mirror reflection. It is said that in such processes the spatial parity P is conserved. However, it has been experimentally established that weak processes can proceed with nonconservation of spatial parity and, therefore, seem to feel the difference between left and right. At present, there is solid experimental evidence that parity nonconservation in weak interactions is of a universal nature; it manifests itself not only in the decays of elementary particles, but also in nuclear and even atomic phenomena. It should be recognized that mirror asymmetry is a property of Nature at the most fundamental level.
Parity nonconservation in weak interactions seemed to be such an unusual property that almost immediately after its discovery, theorists attempted to show that in fact there is a complete symmetry between left and right, only it has a deeper meaning than previously thought. Mirror reflection must be accompanied by the replacement of particles by antiparticles (charge conjugation C), and then all fundamental interactions must be invariant. However, later it was found that this invariance is not universal. There are weak decays of the so-called long-lived neutral kaons into pions p + , p – , which are forbidden if the indicated invariance actually takes place. Thus, the distinguishing property of the weak interaction is its CP non-invariance. It is possible that this property is responsible for the fact that the matter in the Universe significantly prevails over antimatter, built from antiparticles. The world and the anti-world are not symmetrical.
The question of which particles are carriers of the weak interaction was unclear for a long time. Understanding was achieved relatively recently within the framework of the unified theory of electroweak interactions - the theory of Weinberg-Salam-Glashow. It is now generally accepted that the carriers of the weak interaction are the so-called W + - and Z 0 -bosons. These are charged W + and neutral Z 0 elementary particles with spin 1 and masses equal in order of magnitude to 100 m p.
Feynman's diagram of the beta decay of a neutron into a proton, an electron, and an electron antineutrino through an intermediate W-boson is one of the four fundamental physical interactions between elementary particles, along with gravitational, electromagnetic, and strong. Its best-known manifestation is beta decay and the associated radioactivity. The interaction is named weak since the intensity of the field corresponding to it is 10 13 less than in the fields that hold together nuclear particles (nucleons and quarks) and 10 10 less than the Coulomb one on these scales, but much stronger than the gravitational one. The interaction has a short range and manifests itself only at distances of the order of the size of the atomic nucleus.
The first theory of the weak interaction was proposed by Enrico Fermi in 1930. When developing the theory, he used the hypothesis of Wolfgang Pauli about the existence of a new elementary particle of the neutrino at that time.
The weak interaction describes those processes of nuclear physics and elementary particle physics that occur relatively slowly, in contrast to the fast processes due to the strong interaction. For example, the half-life of a neutron is about 16 minutes. – Eternity compared to nuclear processes, which are characterized by a time of 10 -23 s.
For comparison charged pions? ± decay through the weak interaction and have a lifetime of 2.6033 ± 0.0005 x 10 -8 s, while the neutral pion? 0 decays into two gamma quanta through electromagnetic interaction and has a lifetime of 8.4 ± 0.6 x 10 -17 s.
Another characteristic of interaction is the mean free path of particles in matter. Particles that interact through electromagnetic interaction - charged particles, gamma quanta, can be held back by an iron plate several tens of centimeters thick. Whereas a neutrino, interacting only weakly, passes, without colliding even once, through a layer of metal a billion kilometers thick.
The weak interaction involves quarks and leptons, including neutrinos. In this case, the aroma of the particles changes, i.e. their type. For example, as a result of the decay of a neutron, one of its d-quarks turns into a u-quark. Neutrinos are unique in that they interact with other particles only behind a weak, and still weak, gravitational interaction.
According to modern concepts formulated in the Standard Model, the weak interaction is carried by gauge W and Z bosons, which were discovered at accelerators in 1982. Their masses are 80 and 90 proton masses. The exchange of virtual W-bosons is called a charged current, the exchange of Z-bosons is called a neutral current.
The vertices of Feynman diagrams describing possible processes involving gauge W and Z bosons can be divided into three types:A lepton can viprominites or absorb a W-boson and turn into a neutrino;
a quark can viprominate or absorb a W-boson and change its flavor, becoming a superposition of other quarks;
lepton or quark can absorb or viprominites Z-bosonThe ability of a particle to interact weakly is described by a quantum number, which is called weak isospin. The possible isospin values for particles that can exchange W and Z bosons are ± 1/2. It is these particles that interact through the weak force. Particles with zero weak isospin do not interact beyond weak mutuality, for which the processes of W and Z exchange by bosons are impossible. Weak isospin is preserved in reactions between elementary particles. This means that the total weak isospin of all particles involved in the reaction remains unchanged, although the types of particles may change.
A feature of the weak interaction is that it violates parity, since only fermions with left chirality and antiparticles of fermions with right chirality have the ability to weak interaction through charged currents. Parity nonconservation in the weak interaction was discovered by Yang Zhenning and Li Zhengdao, for which they received the Nobel Prize in Physics in 1957. The reason for parity nonconservation is seen in spontaneous symmetry breaking. In the framework of the Standard Model, a hypothetical particle, the Higgs boson, corresponds to symmetry breaking. This is the only part of the ordinary model that has not yet been experimentally detected.
In the case of weak interaction, the CP symmetry is also violated. This violation was revealed experimentally in 1964 in experiments with the kaon. The authors of the discovery, James Cronin and Val Fitch, were awarded the Nobel Prize for 1980. CP-symmetry violation occurs much less frequently than parity violation. It also means, since the conservation of CPT-symmetry is based on fundamental physical principles - Lorentz transformations and short-range interactions, the possibility of violation of T-symmetry, i.e. non-invariance of physical processes in terms of changing the direction of time.In 1969, a unified theory of electromagnetic and weak nuclear interactions was constructed, according to which, at energies of 100 GeV, which corresponds to a temperature of 10 15 K, the difference between electromagnetic and weak processes disappears. Experimental verification of the unified theory of the electroweak and strong nuclear interactions requires an increase in the energy of accelerators by a hundred billion times.
The theory of the electroweak interaction is based on the symmetry group SU(2).
Despite its small magnitude and short duration, the weak interaction plays a very important role in nature. If it were possible to “turn off” the weak interaction, then the Sun would go out, since the process of converting a proton into a neutron, a positron and a neutrino would become impossible, as a result of which 4 protons turn into 4 He, two positrons and two neutrinos. This process is the main source of energy for the Sun and most stars (see Hydrogen cycle). Weak interaction processes are important for the evolution of stars, since they cause the energy loss of very hot stars in supernova explosions with the formation of pulsars, etc. If there were no weak interaction in nature, muons, pi-mesons and other particles would be stable and widespread in ordinary matter. Such an important role of the weak interaction is due to the fact that it does not obey a number of prohibitions characteristic of the strong and electromagnetic interactions. In particular, the weak interaction turns charged leptons into neutrinos, and quarks of one flavor into quarks of another.
