Weak Interaction
WEAK
INTERACTION
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Weak interactions manifest themselves as reactions, or decays, in which some particles may disappear, while others appear. No structure is bound together by a ‘weak force’, but weak interactions are vital for understanding the world around us.
Weak interactions were involved in most of the reactions in the very early Universe by which particles changed from one sort to another. They are therefore largely responsible for the overall mixture of particles from which the current Universe is made.
The most common example of a weak interaction is beta-decay and, as you saw earlier, there are three related processes, each of which is a different type of beta-decay. In each of these three processes, the nucleus involved will change from one type of element to another, as a result of either increasing or decreasing its proton content by one. Each process relies on the weak interaction.
In fact, there are two types of W boson, one with a negative electric charge, the W− boson, and one with a positive electric charge, the W+ boson. The two (charged) W bosons each have a mass of about 80 GeV/c2 whereas the (neutral) Z boson has a mass of about 90 GeV/c2. In weak interactions, W and Z bosons interact with each other, as well as with all quarks and leptons.
(GeV - giga electron volt)
(c^2 - (speed of light)^2)
As you know, the beta-minus decay of a nucleus occurs when a neutron turns into a proton, with the emission of an electron and an electron antineutrino. At most, a few MeV of energy is released in this process, corresponding to the difference in mass between the original nucleus and the resultant nucleus. At the quark level, the explanation is that a down quark, d, with a negative electric charge equal to one-third that of an electron is transformed into an up quark, u, with a positive electric charge equal to two-thirds that of a proton.
(MeV - mega electron volt)
A W− boson is emitted with one unit of negative electric charge, so conserving electric charge in the process. The mass-energy of the W− boson is about 80 GeV, so it cannot possibly emerge from the nucleus as there are only a few MeV of energy available. By the energy-time uncertainty principle it therefore rapidly decays to produce an electron and an electron antineutrino, setting the energy accounts straight.
A beta-minus decay process involves the creation and disappearance of a W− boson. A down quark decays into a W− boson and an up quark. The W− boson subsequently decays into an electron and an electron antineutrino.
In weak interactions, the total number of quarks minus the total number of antiquarks is the same both before and after the interaction. The number of leptons is also conserved. In the example of beta-minus decay, there are no leptons initially present, and after the interaction, there is one lepton and one antilepton – a net result of zero again.
This is the explanation for why neutrinos and antineutrinos are produced in beta-decays. If they were not, then the rule of lepton conservation would be violated. Notice also that the production of a charged lepton is always accompanied by the corresponding flavour of neutrino. In all weak interactions:
The third of the quanta involved in weak interactions is the Z boson with zero electric charges. An example of the type of reaction involving the Z boson is a collision between an electron and a positron. This can create a Z boson from the mass-energy of the electron-positron pair, which subsequently decays into a muon neutrino and a muon antineutrino pair. Notice that there is one lepton and one antilepton both before and after the interaction.
An electron-positron pair undergo annihilation, creating a Z boson which subsequently decays to create a muon neutrino and muon antineutrino pair.
Weak interactions were involved in most of the reactions in the very early Universe by which particles changed from one sort to another. They are therefore largely responsible for the overall mixture of particles from which the current Universe is made.
The most common example of a weak interaction is beta-decay and, as you saw earlier, there are three related processes, each of which is a different type of beta-decay. In each of these three processes, the nucleus involved will change from one type of element to another, as a result of either increasing or decreasing its proton content by one. Each process relies on the weak interaction.
W AND Z BOSONS:
In the same way that photons and gluons are the quanta involved in electromagnetic and strong interactions, respectively, weak interactions involve other quanta – known as W bosons and Z bosons.In fact, there are two types of W boson, one with a negative electric charge, the W− boson, and one with a positive electric charge, the W+ boson. The two (charged) W bosons each have a mass of about 80 GeV/c2 whereas the (neutral) Z boson has a mass of about 90 GeV/c2. In weak interactions, W and Z bosons interact with each other, as well as with all quarks and leptons.
(GeV - giga electron volt)
(c^2 - (speed of light)^2)
As you know, the beta-minus decay of a nucleus occurs when a neutron turns into a proton, with the emission of an electron and an electron antineutrino. At most, a few MeV of energy is released in this process, corresponding to the difference in mass between the original nucleus and the resultant nucleus. At the quark level, the explanation is that a down quark, d, with a negative electric charge equal to one-third that of an electron is transformed into an up quark, u, with a positive electric charge equal to two-thirds that of a proton.
(MeV - mega electron volt)
A W− boson is emitted with one unit of negative electric charge, so conserving electric charge in the process. The mass-energy of the W− boson is about 80 GeV, so it cannot possibly emerge from the nucleus as there are only a few MeV of energy available. By the energy-time uncertainty principle it therefore rapidly decays to produce an electron and an electron antineutrino, setting the energy accounts straight.
A beta-minus decay process involves the creation and disappearance of a W− boson. A down quark decays into a W− boson and an up quark. The W− boson subsequently decays into an electron and an electron antineutrino.
In weak interactions, the total number of quarks minus the total number of antiquarks is the same both before and after the interaction. The number of leptons is also conserved. In the example of beta-minus decay, there are no leptons initially present, and after the interaction, there is one lepton and one antilepton – a net result of zero again.
This is the explanation for why neutrinos and antineutrinos are produced in beta-decays. If they were not, then the rule of lepton conservation would be violated. Notice also that the production of a charged lepton is always accompanied by the corresponding flavour of neutrino. In all weak interactions:
- electric charge is conserved
- the number of quarks minus the number of antiquarks is conserved
- the number of leptons minus the number of antileptons is conserved
- flavour changing of quarks or leptons is allowed, as long as these three rules are obeyed.
The third of the quanta involved in weak interactions is the Z boson with zero electric charges. An example of the type of reaction involving the Z boson is a collision between an electron and a positron. This can create a Z boson from the mass-energy of the electron-positron pair, which subsequently decays into a muon neutrino and a muon antineutrino pair. Notice that there is one lepton and one antilepton both before and after the interaction.
An electron-positron pair undergo annihilation, creating a Z boson which subsequently decays to create a muon neutrino and muon antineutrino pair.
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