See also Chapter 1, Topic 1.5, “How particles interact”ģ In decay a neutron decays to a proton, a particle and an antineutrino by the weak force. It is also possible that three gamma ray photons are emitted or, if the electron or positron has high kinetic energy, then other particles can be produced such as W +/W and even Z bosons. (b) The Feynman diagram should show an electron and a positron (lines) rising vertically to meet and emit two photons (wavy lines). 2 (a) The Feynman diagram should show one photon (wavy line) rising to emit an electron and a positron. Similarities: the same particles enter and exit the exchange particle is the same mass and type. In Figure 5 the neutrino emits a W+ particle which is absorbed by the neutron. (h) Differences: in Figure 4 the neutron emits a W particle which is absorbed by the neutrino. Similarities: both involve the change of a neutron to a proton and a beta particle is emitted. In Figure 4 the W particle interacts with a neutrino. (g) Differences: in Figure 2 the W particle decays into the antineutrino and a beta particle. It is emitted from the vertex where n, p and W meet and should be shown moving forwards in time (that is, up the page). (f) The W particle is shown moving backwards in time. The W particle, which is the exchange particle, decays into a β particle and an antineutrino. Students should have access to the textbook so that they can refer to some of the Feynman diagrams that are in it, and to the data table at the back of the textbook (p245).Īnswers 1 (a) A neutron emits a W particle and changes to a proton. Teaching notes Students will need to discuss the particles and obtain familiarity with the processes of beta decay, annihilation and electron capture. To make calculations using rest mass energies of some particles and draw conclusions from the data. To describe what is happening in various Feynman diagrams and explain the similarities and differences of similar looking diagrams. To develop skills in interpreting Feynman diagrams. What are the important differences between a proton and a positron? (e) Compare and comment on the amount of energy produced when an electron meets a positron in annihilation and when an electron meets a proton in electron capture. Explain why the W boson in Figure 2 can only exist for a short time. (c) The rest energy of a W boson is 80 GeV. (b) decay can be represented as p → n + ν Look at the rest energies of the particles involved and make a comment. (a)Ĭalculate the energy in joules released in the decay n → p + + ν Describe, in your own words, the difference between these decays.įigure 6 4 The rest energies of different particles are given in the AQA data booklet and some of them are given in the article. Pair production of an electron and a positron from a photon.Īnnihilation of an electron and a positron into two photons.ģ Look at the Feynman diagrams for and decay in Figure 6. What are the differences and similarities between Figure 2 and Figure 4? (d) What are the differences and similarities between Figure 4 and Figure 5?Ģ Draw the Feynman diagram for the following: (a) Why is it wrong to draw beta decay as shown in Figure 3? Questions 1 The Feynman diagram for beta decay is shown in Figure 2. Since they exist for a very short time they do not violate the law of conservation of energy.ĪQA Physics A AS Level Extension Activity © Nelson Thornes Ltd 2008
W bosons are heavier than a neutron and give rise to the weak force. Virtual, or exchange, particles such as the photon or the W boson (W or W+) are shown as wavy lines.
(Particle physicists show antiparticles moving in the opposite direction.) Note that the lines do not show the actual trajectory of particles in space. (Particle physicists often reverse this orientation.) Particles are shown as lines with arrows that denote the direction of their travel in time. They are a type of space–time diagram the time axis points upwards and the space axis points to the right, as shown in Figure 1. Feynman diagrams were developed to describe the interactions of charged particles in quantum physics and they have found a wide use in describing a variety of particle interactions. You will also consider the rest mass energies of some of the particles, make calculations from the data and draw conclusions from your answers.įeynman diagrams Read the article that follows and then answer the questions. After some explanation you are asked to describe what is happening in various diagrams and to explain the similarities and differences of similar looking diagrams. Matter matters Aims This activity develops your skills in interpreting Feynman diagrams.
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