Summary
Highlights
Feynman diagrams are pictorial representations of particle interactions. They are named after Richard Feynman, a brilliant US scientist. The first convention states that time is along the bottom axis and space is up the side. Particles like quarks and leptons are represented by straight lines with arrows indicating the direction of time. Anti-particles are shown with arrows pointing backward in time. Gauge bosons, which mediate forces, are represented by curly lines. There are five main gauge bosons: the gluon, W+, W- bosons, Z boson, and the photon, though gluons are not typically used in the diagrams for this syllabus.
Interactions in Feynman diagrams occur at a 'vertex', a dot where lines meet. An incoming electron is a line with an arrow pointing towards the vertex, while an outgoing electron has an arrow pointing away. Anti-particles have arrows reversed. Gauge bosons are represented by curly lines. Crucially, there are always three lines at a vertex: two matter particles (quarks or leptons, or composite particles like protons/neutrons) and one gauge boson (W+, W-, Z, or photon). The video then demonstrates simple interactions like an electron absorbing a photon (electron coming in, photon coming in, electron going out) and an electron emitting a photon (electron coming in, photon going out, electron going out). It also shows an electron and positron annihilating into a photon.
The first of the five required diagrams is the electron-electron interaction (scattering). Two electrons approach each other. As negatively charged particles, they repel. This repulsion is mediated by the exchange of a 'virtual photon', represented by a curly line connecting the two electron lines. After exchanging the virtual photon, the electrons scatter away from each other. The explanation emphasizes that while it resembles billiard balls, the interaction occurs via particle exchange, not direct contact. All conservation laws (momentum, energy, charge, lepton number, baryon number) must apply.
The second type is electron-positron scattering. An electron and a positron approach each other. Similar to electron-electron scattering, they exchange a virtual photon and then scatter away. The key difference is the positron's arrow pointing backward in time. The third type of interaction is electron-positron annihilation. An electron and a positron come together, annihilate, and produce a virtual photon. This virtual photon then undergoes 'pair production', decaying into another electron and positron pair. This diagram shows the incoming electron and positron meeting, forming a virtual photon, which then splits into an outgoing electron and positron.
The fourth diagram illustrates neutron decay. A neutron decays into a proton, an electron, and an electron anti-neutrino. In the Feynman diagram at the hadron level, a neutron transforms into a proton, with a W- boson mediating this change. The W- boson then decays into an electron and an electron anti-neutrino. The diagram shows the neutron line splitting into a proton and a W- boson, and the W- boson then splitting into an electron and an electron anti-neutrino (with its arrow going backward in time). This decay is governed by the weak nuclear force. The explanation emphasizes the importance of mastering this specific reaction due to its applicability in other physics concepts.
The fifth diagram shows neutron decay at the quark level. A neutron is composed of an up-down-down (udd) quark triplet, and a proton is an up-up-down (uud) triplet. In this decay, one of the down quarks in the neutron changes into an up quark, while the other two quarks act as spectators. This 'flavor change' of the quark results in the emission of a W- boson. The W- boson then decays into an electron and an electron anti-neutrino. The diagram clearly illustrates the initial udd configuration, the down quark becoming an up quark, and the subsequent decay of the W- boson. The video concludes by reiterating the importance of these five diagrams and how to approach questions regarding them for exams, emphasizing proper labeling and explanation of processes.