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.
As far as we know, there are
nine exact conservation laws that govern all particle decays.
They are Conservation of:
| 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 new 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.
