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There are four types of basic physical effects between matter known to date, namely gravitational interaction, electromagnetic interaction, strong interaction and weak interaction.
The first two interactions are widely manifested in macroscopic physical processes and have been recognized by people earlier. Electromagnetic action also plays an extremely important role in the microscopic physics process. Whether in atomic physics, molecular physics, condensed matter physics, or in photophysics and photochemistry, the basic interactions of the matter involved are just electromagnetic effects.
strong and weak effects were studied only after the 1930s through the development of nuclear physics. The strong effect causes the proton and the neutron to combine into the nucleus , and the weak effect causes the nucleus β to decay .
1. Four-fermion action model
1957. The universal Fermi-type weak interaction theory (i.e. V-A theory) established in 1958 shows that weak action and electromagnetic action have an important similarity, namely its universal flow-flow action form.
The electromagnetic effect between charged particles can also be represented as a universal flow-stream coupled , but the flow in it is current (the current here refers to the flow in the four-dimensional space-time transformation, and the vector and axis vector are also for the four-dimensional space-time transformation).
, but weak action has different types in some basic properties from electromagnetic action.
First of all, in terms of force range, the flow-flow action between charged particles is transmitted through photon . Since the mass of the photon is zero, the action is a long-range action. In V-A theory, the effect between the weak current and the weak current is direct, that is, the force range is zero. This means that weak effects are not only short-range effects, but also the limit of short-range effects.
is secondly in terms of space-time symmetry. Electromagnetic action is symmetrical to spatial coordinate reflection, while weak action is asymmetrical to spatial coordinate reflection. It is manifested in the flow: the current has only a vector (V) component, while the weak current is a superposition of two components: vector (V) and axis vector (A), and the two components have the same size.
In addition, electromagnetic action is a normative action, while V-A theory is not. Although the
V-A theory has achieved great success in philosophical phenomena, it has serious difficulties in theory. Since it cannot be re-regulated, it will not be avoided when calculating advanced corrections. This shows that it is not a complete theory and can only give results for primary approximations. Although the results of this primary approximation are consistent with experiments in the low energy field (the weak action process studied at that time is just the decay of various particles, and they all belong to the low energy field), once the transition to the high energy field, the theoretical calculated values will inevitably contradict the experimental results.
If in the process of scattering and reaction caused by weak action, when the energy value in the center of mass exceeds a certain value (the order of magnitude is several hundred gigawatts), the calculated cross-sectional value will exceed the limit allowed by unitaryity, which is obviously impossible to be correct.
2. The flow-flow coupling characteristics of the charged intermediate boson action model
V-A theory make people think that weak effects may also be transmitted through a certain vector boson like electromagnetic effects (photons are vector bosons). This vector boson that transmits weak effects is also called intermediate bosons. The extremely short-range properties of weak action can be attributed to the large mass of the intermediate boson. However, the theory of heavy intermediate boson established in this way is still not re-regular.
can be concluded from the properties of the weak flow that the intermediate boson must be charged (usually represented by symbols), which in turn causes a new problem of what form should the coupling between the vector charged boson and the photon be taken so that the corresponding electrodynamics can be reproportionized. This situation leads some physicists to believe that only by combining weak action with electromagnetic action can a complete set of theories be obtained.
electromagnetic action is an Abel normative action. In 1954, Yang Zhenning and R.L. Mills extended the normative action to situations with internal symmetry and proposed the non-Abelian gauge field theory, which created conditions for the expanded application of the gauge theory.
But there is a major obstacle to directly applying this theory to weak effects, that is, the normative theory requires that the vector bosons must be of zero mass, while the intermediate bosons should have a large mass.
In addition, weak-acting parity disconservation also creates difficulties in establishing a strict theory of symmetry. The non-conservation of parity means that if the weak action has internal symmetry, this symmetry must be different from the left and right hands, that is, the left and right hands components of an fermion must have different quantum numbers. But the fermion mass term in the Hamiltonian will destroy this symmetry because it causes the left and right hand components of a fermion to transform each other. Such strict weak internal symmetry requires that the mass of fermions must also be zero, but except for neutrino , all fermions have mass.
1958, G. Feinberg discovered that when a charged vector boson has a specific magnetic moment, a certain type of divergence can be eliminated. This magnetic moment does not equal the value given by "minimum electromagnetic coupling", but corresponds to the magnetism required in some non-Abelian gauge field theory. This sign indicates that non-Abelian gauge field theory may play an important role in solving divergence difficulties.
When applying the non-Abelian norm concept to a weak effect and taking the internal symmetry as a weak isotope, in addition to the charged normative boson, there should be a neutral one.
1957-1959, J.S. Schwenger, S.L. Grazow , Salam and J.C. Ward all imagined that this neutral normative boson was a photon solution, thus obtaining a unified theory of weak action and electromagnetic action. However, the results given here are obviously inconsistent with the experiment.
1961, Grazhao first realized that to describe both weak and electromagnetic effects, the internal symmetry should be expanded, that is, in addition to weak isotope rotation, weak superload should be added. At this time, there are two neutral norm bosons. One is a photon after mixing, and the other has mass, which is called, which is coupled to a very special form of neutral weak current.
In 1964, Salam and Ward also proposed a similar theory. Grazhao's theory is not a strict non-Abelian gauge field theory, because the mass term of the intermediate boson of
is added to this theory. In fact, it is only after the concept of spontaneous symmetry breaking is introduced that it is possible to establish an electroweak unified theory that can be reproducible and make the intermediate bosons have mass.
3. Symmetric spontaneous breaking
The concept of symmetric spontaneous breaking was introduced into particle physics by physicists such as Nanbu Yoichiro from solid physics to particle physics around 1960. It refers to the fact that the physical laws themselves have some precise symmetry, but the ground state is degenerate, and the actual physical ground state is only one of these many possible ground states. Therefore, the physical phenomena occurring on the basis of this particular ground state will not display or only partially display the inherent symmetry of the physical laws.
Here the symmetry is not destroyed by external factors, and its destruction is completely spontaneous. In essence, the symmetry of physical laws is not broken at this time, but it cannot be displayed in a specific context. Therefore, the symmetry of spontaneous destruction is also called implicit symmetry.
Superconductivity in solid physics is an example of spontaneous symmetry failure. Inspired by the superconducting theory, Nanbu et al. proposed a theoretical model to obtain mass by nuclear around 1960. It is assumed that physical laws have chiral symmetry, so that the fermions inside do not have the original mass. The spontaneous breakdown of chiral symmetry in this model causes the original massless two dicomponent fermions to be synthesized into a massed nucleon (with four components). The south was found, at the same time, there were signs of a zero-mass scalar boson present. This scalar particle is considered meon and assumes a small mass is obtained for other reasons.
J. Goldstone clearly revealed through a specific model in 1961 how continuous symmetry spontaneous failure in relativity field theory leads to the emergence of zero-mass particles, and believes that this is a universal conclusion. This result is called the Goldstone theorem, and the above zero mass scalar boson is often called the Goldstone boson (or the Southern-Godstone boson). In 1962, Goldstone, Salam and Weinberg gave general proofs of the theorem.
1964, P.W. Hegers and others pointed out that there is an exception to the Goldstone theorem, that is, the spontaneous breakage is the case of normative symmetry. At this time, the Goldstone boson does not manifest as a physical particle. It can be absorbed into the normative boson through normative transformation and become its longitudinal component, and allows the normative boson to obtain mass. This phenomenon can be regarded as a relativistic variant of the plasmon phenomenon in superconducting.
The mechanism for eliminating the Goldstone boson described above is called the Hagers mechanism. It can not only eliminate undesired Goldstone particles in the theory (but not actually observed) but also enable the gauge boson to obtain mass, solving two major problems encountered in applying gauge theory to weak effects.
4. Electroweak Unified Action Model
1967, Weinberg applied normative theory and the concept of spontaneous symmetry to the electrical weak effect, and proposed a reproducible theory to uniformly handle the electromagnetic and weak effects of lepton . The normative symmetry adopted is the weak isotope rotation and weak supercharge symmetry proposed by Grazow.
In 1968, Salam also proposed a similar theoretical model. This model is therefore called the Grazhao-Weinberg-Salam electroweak unified theoretical model.
In this theoretical model, the fermions are temporarily limited to being leptons, and their weak electric effect Hamiltonian is: in
, it is a weak isotopic rotor operator, it is a corresponding normative boson, it is a weak isotopic rotor coupling constant, it is a weak supercharge operator, it is a corresponding normative boson, it is a weak supercharge coupling constant, and it is a correlation function of complex conjugation.
particle physics ground state is also the vacuum state. The hypothesis that vacuum degenerates in this model is due to the aggregation of a scalar particles in a weak isotope diletto state in the vacuum, which has a weak supercharge of 1.
For the lepton mass question, the model adopts the idea in the southern model, that is, it is assumed that all leptons have no original mass. In this way, the left-hand components and the right-hand components of electrons or other charged leptons are originally two different fermions, each with only two components. It is only because they are coupled with the scalar particles condensed in the vacuum, so they can be converted to each other under this vacuum background. They are called different components of the same particle and combined into mass fermions with four components.
introduces lepton mass into the theory in this way, which can not only not destroy the original symmetry of the left and right hand components, but can adapt to the need for weak effect parity and non-conservation, and ensure that the remaining electromagnetic normative effects after spontaneous breaking are symmetrical to the left and right hand. Because the left and right hand components assembled in the above formula must have the same value for the still conserved quantum number-one charge. As for neutrinos, it maintains mass at zero because it is not coupled with other bicomponent fermions through scalar particles. Although it only has the left-hand component and is not symmetrical left and right, it is not charged, so it has no effect on the left and right symmetry of electromagnetic action.
In this model, the left-hand electron and the corresponding neutrino form a weak isotopic spiral doublet state and carry a weak supercharge-1, while the right-hand electron is a weak isotopic spiral singleton and carry a weak supercharge-2. Other leptons are similar. When the scalar particles condense in vacuum, only one quantum number corresponding norm symmetry is not destroyed, it is the charge, and the corresponding operator is expressed in: in the formula
, it is the third component of the weak isotropic rotation. The normative boson corresponding to the charge remains massless, i.e. photons. Some kind of mixing of sum: in
, it represents the mixing angle, called the Weinberg angle, which can be expressed by the weak isotopic rotation coupling constant and the weak supercharge coupling constant ratio. Another combination of the and, both obtain mass, and the theoretical predicted value is: the neutral weak current coupled to
has the following form: the coupling constant is.Three of the four real components of the scalar particles become Goldstone particles and have been absorbed and summed. The remaining one is a mass neutral scalar particle called the Hagers particle.
Weinberg and Salam once conjectured that this spontaneously broken normative field theory is still reproducible, but failed to give a proof. In 1971, G. Hoft demonstrated its reproducibility. In 1972, B.W. Lee and J. Zinn-Justen, and Hoft and M.J.G. Weitman further gave a detailed proof of the reproducibility of this theory.
There is another problem when the Grazhao-Weinberg-Salam theory was proposed, that is, how to generalize and apply it to strongly acting particles (in the quark theory, that is, to quark ).
The difficulty lies in how to avoid the occurrence of neutral weak currents with singular numbers in theory (experiments show that such weak current does not exist).
However, by 1971, there were actually ready-made solutions to this problem. In 1970, Grazhao, J. Illopoulos and L. Maiani analyzed the neutral weak current problem of singular number changes. Since they didn't know that there was any reproducible theory at that time, they used truncated treatment. In this work, they demonstrated that for various known weak action models (such as the action of four fermion, charged intermediate boson, and electric weak unified action), some effects that were not observed in experiments, such as large mass, decay , etc., unless the strongly acting particles obey some constraints. They pointed out that if the fourth quark ( charm quark ) exists, these undesirable effects can be eliminated in the theory.
With the Grazhao-Illopoulos-Mianni mechanism, it is not difficult to generalize the Grazhao-Weinberg-Salam model to strongly acting particles. This just adds the weak isotopic singlet state of the four right-handed quarks and the weak isotopic doublet state of the two left-handed quarks:
. Where is the Kabi angle. In order to make the quarks carry fractional charges, it should be assumed that the right-hand quarks and the weak supercharge are 4/3, and the weak supercharge is -2/3, while the left-hand quarks have a weak supercharge of 1/3.
Until 1971, a complete unified theoretical model of electroweakness was formed. At the same time, experimental technology has also developed greatly, especially with neutrino beams, neutral weak flow experiments can be performed.
1973 FJ. Hasselt and Diben Venuti and others measured examples of neutral weak current reactions at the European Nuclear Research Center (CERN) and the Fermi National Accelerator Laboratory in the United States. The form and intensity are consistent with the theoretical predictions, and the determined value is about 0.215.
In 1983, particles and particles were discovered at the European Nuclear Research Center, with the determined mass values of about 81.2 gigatron electron volts and 92.5 gigatron electron volts respectively, which is consistent with the prediction of the standard model of electricity-weak uniformity. This shows that the Grazhao Weinberg-Salam theory has achieved great success. The establishment of the unified theory of electroweakness is a major breakthrough in modern physics. The only thing that has not been discovered in the unified standard model of electroweakness is the Heges particles (now discovered). There is no theoretical prediction about the mass value of the Heges particle, so it has not been discovered yet and does not constitute a theoretical difficulty. However, the theory of electroweak unification also has shortcomings.
① In a sense, this is not a truly unified theory, because it contains two norm groups and two independent coupling constants g and g'. ② There is no explanation for why the charge is quantum . ③It contains too many parameters, mainly the various self-acting coupling constants of scalar particles and the coupling constant between scalar particles and each fermions.
This raises new research topics for particle physics, and theoretical physicists are further exploring these problems.
Excerpted from: "China Encyclopedia (2nd Edition)" Volume 5, China Encyclopedia Press , 2009
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