S-Cool Revision Summary

S-Cool Revision Summary

Particle families

There are two main families of particle, the leptons and the hadrons.


These are the lightest particles. Their name is derived from Greek: Lepton is small.

There are 3 types of lepton: the electron, muon and tau. In addition, each of these particles has an associated neutrino, and corresponding antiparticle, making a total of 12 family members. Leptons are truly fundamental.

Leptons are assigned a Lepton number (L). L = 1 for leptons and L = -1 for antileptons.


Hadrons are generally more massive than leptons. They are sub-divided into baryons (the most massive) such as protons and neutrons, and mesons (somewhat less massive) such as the pion and kaon.

Particle Numbers

In order to understand the interactions of these particles better they are assigned numbers as described below:

Charge (Q) is conserved in all interactions.

Baryons are assigned a Baryon number (B). B = 1 for baryons and B = -1 for antibaryons. Baryon number is conserved in all interactions.

Strange quarks possess a property called Strangeness (S). S = -1 for strange quarks and S = 1 for antistrange quarks.

Particle interactions

The 4 fundamental forces can be thought of as interactions between particles arising as a result of the exchange of virtual particles.

The four fundamental interactions are:

Interaction Exchange particles Range (m)


(between quarks)

mesons (between hadrons)

electromagnetic virtual photon
weak W+, W-, Z0 10-18
gravitational (graviton?)

Such exchanges can be represented on a Feynman diagram:

Fundamental Forces and Exchange Particles

Beta decay

Beta decay is a result of the weak interaction. A down quark in the neutron emits a W- and changes into an up quark. The W- decays into an electron and an antineutrino.

Reactions, Exchange Particles and Feynman Diagrams

Conservation Laws

In any interaction charge (Q), lepton number (L) and baryon number (B) are conserved. (L = 1 for leptons, L = -1 for antileptons. B = 1 for baryons, B = -1 for antibaryons)