top of page
Interactions Between Particles

Physics (Year 12) - The Standard Model

Dev Lohar


Interactions between subatomic particles are happening every second and come in different forms. A common aspect between all of the interactions is that they can be represented diagrammatically by a Feynman diagram. A Feynman diagram shows the movement of particles in space and time, and the bosons they exchange.


The interactions can be represented similar to how chemical equations are written. The left side of the arrow represents the particles you begin with and the right side represents the particles produced as a result of the interaction.


A common interaction is a particle-antiparticle interaction, or as its named, annihilation. When a particle, such as a proton, collides with an antiparticle, such as an antiproton, they annihilate and produce new particles. The opposite of this interaction can also occur. A particle, such as a proton, can create a particle and an antiparticle pair.


Many properties need to be conserved in interactions. These include mass-energy (you should remember from year 11 that mass is equivalent to energy; ie. E=mc^2), charge, and quantum numbers.

In every interaction, mass-energy needs to be conserved. For example, if a particle of mass m decays into a particle-antiparticle pair with mass less than m, then the particle-antiparticle pair needs to have kinetic energy which is equivalent to the ‘missing mass’, hence allowing mass-energy to be conserved. Similar to this, momentum also needs to be conserved.

Conservation of charge is also necessary in particle interactions. The net charge before the event needs to be equal to the net charge after the event. For example, a neutron decaying into a proton and an electron conserves charge because the net charge before the decay is 0 (neutron is neutral charge) and the net charge after the decay is also 0 (+1 from proton and -1 from electron). If charge is not conserved, then the interaction is not possible.

We learnt previously that quarks and leptons have specific quantum numbers associated with them. Baryons have baryon numbers whilst leptons have lepton numbers. Proton and neutron, which are both baryons, have a baryon number of +1, whilst their antiparticles have a baryon number of -1. Electron, muon, and tau, which are all leptons, have a lepton number of -1 but the neutrinos have a lepton number of 0. Hence, these quantum numbers need to be conserved in particle interactions; the net baryon/lepton number prior the event needs to equal to the net baryon/lepton number after the event.

The above conservation laws are important because they govern if an interaction is possible or impossible. According to the Standard model, all the conservation laws need to be followed for an interaction to be allowed. These laws can also be used to determine missing particles in an interaction.

bottom of page