Conservation Laws
Conservation laws are empirical laws that we use to "explain" consistent patterns in physical processes. Typically these laws are needed to explain why some otherwise possible process does not occur. For example, the decay of a proton to produce a positron and a photon has not been seen.
Conservation laws are a set of rules that forbid all such non-occurring decays. If the rules are simple, they can be extremely powerful and govern a huge variety of processes.
The risk in setting these laws is that there may be some very rare processes that we have not yet observed that may not respect the laws we state them today. If such decays are found, we will have to downgrade at least one of these laws to an approximate conservation law.
Laws that Apply for All Decays So Far Observed
As far as we know, there are nine exact conservation laws that govern all particle decays. They are Conservation of:
| Law # | Law Title | Description |
| 1 | Energy | |
| 2 | Momentum | |
| 3 | Angular momentum | including particle spin or intrinsic angular momentum. |
| 4 | Electric charge | |
| 5 | Color-charge | quark and gluon color-charge conservation |
| 6 | Quark number | number of quarks minus number
of antiquarks. (For historical reasons, and because we observe baryons and not quarks, this is usually stated as baryon number conservation, where baryon number is the same as quark number divided by 3.) |
| 7 | Electron number | number of electrons plus number of electron-type neutrinos minus anti-particles (positrons plus anti-electron type neutrinos) |
| 8 | Muon number | number of negatively-charged muons plus number of muon-type neutrinos minus number of anti-particles (positively charged muons plus anti-muon type neutrinos) |
| 9 | Tau number | number of negatively-charged taus plus number of tau-type neutrinos minus number of anti-particles (positively charged taus plus anti-tau type neutrinos) |
Laws 7, 8 and 9 can be combined to give one less restrictive law --
| 7, 8, 9 | Lepton number | number of leptons (negative charges plus neutrinos) minus number of anti-lepton (positive charges plus anti-neutrino) |
All of these conservation laws are consequences of the Standard Model of particle interactions. The observation of a process that violates one of these rules would be evidence for additional laws of nature beyond the Standard Model.
Within the Standard Model, the three lepton number laws (7, 8,9) are separate conservation laws if neutrinos have zero mass. Nonzero neutrino masses can potentially violate these laws (7, 8, 9) in any combination. Just now there are experimental results that suggest neutrinos have mass. If these results are confirmed, the lepton number laws may be downgraded to the status of approximate conservation laws.
