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Particles and Antiparticles - Interaction and Laws of Conservation Revision Notes

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21.2Particles and Antiparticles - Interaction and Laws of Conservation


In these revision notes for Particles and Antiparticles - Interaction and Laws of Conservation, we cover the following key points:

  • What are mesons and anti-mesons? What are some of their basic properties?
  • What is antiproton? How is can be obtained?
  • The same for antineutron and anti-photon.
  • How neutrino and antineutrino are produced?
  • What is baryon charge? What happens when baryon charge is conserved?
  • The same for lepton charge.
  • What is strangeness? Where is it observed?
  • What does the law of strangeness conservation indicate in regard to a nuclear reaction?

Particles and Antiparticles - Interaction and Laws of Conservation Revision Notes

Muons are unstable elementary particles; their lifespan is 2.2 microseconds approximately. The electric charge of muons (and pions) is the same as that of electron. The two types of muons (μ+ and μ- are antiparticles of each other. Each of them has a spin of 1/2 and a mass of 106 me = 54 MeV/c2.

Muons are unstable particles; their lifespan is 2.2 microseconds approximately. Muons transform into an electron (positron), a neutrino and an antineutrino according the scheme

μ+ → e+ + νe + vμ

and

μ- → e- + νμ + ve

There are three types of pions, all of them having a zero spin. Two of them, π+ and π- have equal mass (273 me or 140 MeV/c2). These particles are also unstable; their lifespan is 26 nanoseconds (about 85 times shorter than the lifespan of muons).

The three possible reactions in which a pion splits in two other elementary particles are:

π+ → μ+ + νμ
π- → μ- + vμ
π0 → γ + γ

Antiproton (p) is a particle with the same mass and spin as proton but with opposite charge. It was found that the energy needed to give the protons in order to obtain antiprotons is about 6 GeV (6 × 109 eV).

One years after the discovery of antiproton, the discovery of antineutron (n) was made possible as well. From these three antiparticles (antielectron, antiproton and antineutron) the antiatom and hence antimatter is obtained. This is a possible form of the existence of matter composed by antiparticles. Following this logic, the photon has its antiparticle (anti-photon) as well. We express the anti-photon with the symbol (γ).

Neutrino and antineutrino are two other elementary particles that are produced during beta decay processes. According to theory, an antineutrino with sufficient energy can bring the following change when interacting with proton:

v + p → n + e+

In order to explain the stability of proton and antiproton from the experimental point of view, scientists associated a nucleonic number otherwise known as baryon charge B to each particle, in analogy with the electric charge e. Baryon charge represents the amount of nucleons attraction inside the atomic nucleus, i.e. it characterizes the strength of nucleons as sources of nuclear field.

The conservation of baryon charge implies the conservation of the number of baryons in a nucleus. Baryons are not only neutrons and protons but also some heavier elementary particles discovered recently (Λ, Σ, Ξ and Ω - hyperons) which contain nuclear charge. In other words, the conservation of baryon charge also implies the conservation of nuclear material.

Each baryon bears a baryon number B as follows:

  • For every nucleon (proton or neutron), B = +1
  • For every antinucleon (antiproton or antineutron), B = -1
  • For every meson, neutrino, electron or photon (and their corresponding antiparticles), B = 0

It is impossible to find neutrino and antineutrino alone in space. This fact brought scientists in the conclusion that there exist another charge besides the electric and baryon ones in elementary particles as well as the law governing its behavior. This is known as a lepton charge - a type of charge that is also conserved in various nuclear reactions and radioactive decay processes.

The category of leptons (lightweight particles) includes electrons and positrons (e- and e+), muons and antimuon (μ- and μ+), tau particles - elementary particles similar to the electron, with negative electric charge and a spin of 1/2 (τ+ and τ-) and the three types of neutrinos (νμ, νe and μτ) each of them having in correspondence an antineutrino as well. In total, there are six leptons and six antileptons. The tau particles and muons are unstable; each tau particle splits into one muon and two neutrinos while one muon splits into one electron and two neutrinos. Tau particles have a large mass (1784 MeV/c2, i.e. about 3491 times heavier than electron). They belong to the category of leptons only because they don't produce a strong interaction.

Leptons obey to the laws of conservation. There are 3 lepton numbers according to the type of corresponding leptons (Le, Lμ and Lτ). Thus, electron e- and electronic neutrino νe have L = +1 as well as μ-meson μ- and muon neutrino νμ (L = +1) while their antiparticles have L = -1. In all types of interaction, the lepton charge must be conserved in order to have a valid reaction.

Some elementary particles don't need the structure of matter and fields known in the explanation of theory or the known interactions involved. For this reason, scientists called these elementary particles "strange". The distinctive feature of such particles is their generation in pairs. For example, Σ-hyperon is produced together with K-mesons (K+, K-, K0) otherwise known as kaons. Like all the other particles that are produced in pairs, the strange particles have a specific charge associated with, the law of conservation of which, allows us to determine whether a specific reaction involving such particles can occur or not. This new type of charge is known as Strangeness, S.

Nucleons, muons and π-mesons do not manifest any strange behavior or property. Therefore their strangeness is S = 0. Two strange elementary particles that belong to the same pair have their strangeness equal and opposite. The algebraic sum of strangeness before and after the reaction takes place must be the same. Thus, the Λ0, Σ+ , Σ- and Σ0 hyperons have S = -1 whereas their corresponding accompanying particles K0 and K+ have S = +1. The corresponding antiparticles of elementary particles mentioned above, have an opposite strangeness. Thus, Λ0, Σ+, Σ-,Σ0 have S = + 1 while K0 and K- have S = -1.

The law of strangeness conservation is applicable only in strong interaction; it is not applied in weak interaction observed during radioactive decay processes.

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