2.3 Conservation Laws & Particle Interactions

The virtual photon mediates the electromagnetic force. 

It is called a ‘virtual’ photon because it is not detectable in a laboratory. 

Sketch a Feynman diagram to show electrostatic repulsion between two electrons.

Explain why virtual photons cannot be detected in a laboratory but are nonetheless required by particle physics.

The unfinished Feynman diagram in Figure 1 shows the interaction between a proton and an anti-neutrino.

Complete the Feynman diagram shown in Figure 1.

The neutron-neutrino interaction can be expressed in a similar way to the Feynman diagram in Figure 1. 

Describe the changes to the Feynman diagram in Figure 1 that would show the neutron-neutrino interaction.

Exchange particles are theoretical objects that are used to explain how particles interact with each other. 

One observation that is explained by exchange particles is the fact that the weak nuclear force acts over a much shorter distance than the strong nuclear force.

Explain two differences between the relevant exchange particles that account for this observation.

The strong interaction between a K^– particle, which has a strangeness S = –1, and a proton p is shown below:

Deduce the quark composition of X.

Figure 1 shows a possible mechanism for the interaction between the negative kaon and proton. The exchange particle involved in this reaction is the gluon, g.

Figure 1

Complete the Feynman diagram for the strong interaction in part (b), in terms of the quarks involved.

Do not include any spectator quarks.

There are other possible outcomes for interaction between the K^– and the proton p. Sometimes they interact weakly, with a neutron among only two reaction products.

Discuss this possible interaction, including an appropriate equation.

Annihilation is a type of particle interaction during which a colliding particle  and anti-particle  convert their masses into pure energy. This energy is radiated by at least two photons  . 

The Feynman diagram for such a process is shown in Figure 1

Explain why at least two photons result from annihilation.

Pair production, the opposite process to annihilation, is the creation of a particle and anti-particle pair from the energy carried by a photon.

Sketch a Feynman diagram that represents pair production from a single photon.

State and explain whether the photon in part (b) can be classified as a virtual photon.

During decay of a nucleus, both the nucleon composition and the quark composition change. 

Deduce the change in quark composition and the type of interaction that occurs in this decay. 

Write the  decay equation on a quark level.

For  decay to be possible each of baryon number, lepton number and charge must be conserved. 

Use these rules to show that the decay in part (b) is possible.

State two more conservation laws that are obeyed in all nuclear interactions.

Define what is meant by the term fundamental particle in physics and give one example of a fundamental particle.

A physicist is attempting to analyse an electron capture event of a proton. 

Write down the equation that represents this interaction. 

The proton numbers, nucleon numbers and appropriate symbols of all the particles should be shown.

Show in terms of any three conservation laws, that this interaction is permitted.

State the strangeness of an electron and explain why it has this value.

In alpha decay, only one particle is emitted. 

Explain how baryon number and charge are conserved in alpha decay.

Particle X decays in the following way: 

Deduce the relative charge, baryon number and lepton number of particle X.

Deduce whether X is a meson, baryon or lepton, explaining how you arrive at your answer. 

Particle X is a strange particle. 

Determine the quark combination of particle X. Hence or otherwise, state its name. Clearly show your working.

A proposed particle interaction is:

                  Σ⁰  + K⁺      

 Σ⁰ has a quark structure of .

Use the principles of conservation of charge, baryon number, lepton number and strangeness to determine whether this decay is possible.

Σ⁰  is part of a family of baryons called Sigma baryons. They are all strange particles. 

Determine the quark combination of the Σ⁺ baryon. Clearly show your working.

Determine the charge, baryon number, lepton number and strangeness of a particle with the quark combination . 

Clearly explain your reasoning for each.