There are several conservation laws that help us predict the outcome of the particle reactions that are observed in accelerator experiments.
Charge (Q) is conserved in all interactions.
Leptons are assigned a Lepton number (L). L = 1 for leptons and L = -1 for antileptons. Lepton number 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. Strangeness is conserved in all but the weak interaction (this is because the weak interaction involves one type of quark changing into another as we have seen).
Consider the following reaction:
| Q: | 1 | + | -1 | = | 0 | + | 0 |
| B: | 1 | + | 0 | = | 1 | + | 0 |
| L: | 0 | + | 1 | = | 0 | + | 1 |
This is in fact ''electron capture''. It is allowed because Q, B and L are all conserved.
Look now at the following: .
| Q: | 1 | + | -1 | = | 1 | + | -1 |
| B: | 1 | + | -1 | = | 0 | + | 0 |
| L: | 0 | + | 0 | = | 0 | + | 1 |
This reaction is not allowed because Lepton number is not conserved.