Approximate Conservation Laws

The term "approximate law" sounds like an oxymoron. Physicists use this term to describe a law that is exact for some types of interactions, but does not apply at all for another type of interaction that proceeds more slowly. Below we list the laws known to apply only for certain types of interactions.

Laws BROKEN ONLY by Weak Interactions

Processes involving W bosons are the only ones that can change a quark or lepton of a given flavor into a different flavor. Thus, in strong and electromagnetic interactions, the general laws of quark and lepton number conservation can be subdivided into separate flavor conservation laws (number of particles minus number of antiparticles) for each of the six quark types and each of the six lepton types. For example, the number of up-type quarks minus the number of anti-up-type quarks is a conserved quantity in strong and in electromagnetic processes, but not in weak  processes.

In addition, there are three more technical  invariance properties and related conservation laws that apply in strong and electromagnetic processes, but not in weak processes. These are called parity, charge conjugation, and time-reversal invariance.  

 

Laws that APPLY ONLY to Strong Interactions
but are broken by both weak and electromagnetic interactions

The strong interactions do not distinguish between the different flavors of quark. That is, all quark flavors have the same strong interaction properties. Since different quarks have different charges, this is clearly not the case for electromagnetic interactions. The strong interactions therefore exhibit flavor invariance. However, since different quarks do have different masses, the possible allowed processes can be very different for different quark types -- just because of energy and momentum conservation. But  the masses of the up and down quarks are quite small compared to the masses of all hadrons. For example, the mass of the proton is very close to the mass of the neutron even though they contain different numbers of up and down quarks.

This leads to relationships between processes where some number of protons are replaced by an equal number of neutrons, and more generally between processes where some number of up quarks are replaced by an equal number of down quarks. This goes by the name of isospin invariance and leads to some  conservation laws that apply only for strong interactions.