Baryon number and conservation

The simple statement of “baryon number conservation” is the baryon number is conserved in all particle decays, collisions and interactions.

I have to explain what the baryon number is and for which particles that is important but first let’s quickly check out some particle names and connections.

Fermions are the particles that make up the universe.

Within this group of particles we are especially looking at hadrons. There are two groups of hadrons, these are baryons and mesons. Hadrons are made up from quarks and the baryon number is all about quarks.

There are six different quarks all with slightly silly names and for each one of these there can be an antiquark. Baryon numbers are useful when looking at interactions involving hadrons.The definition of the baryon number is an equation, which looks much more complicated than it is.

where nq is the number of quarks and it makes no difference what kind of quark it is, and nq (with a little bar over the top. Sorry the bar won’t display on the page)  is the number of antiquarks and again it makes no difference what kind of antiquark. There are no complications to using this equation, B turns out to be either zero or 1 for hadrons but the key point is that this does not change in an interaction. Let’s take a couple of examples of the baryon number:

Baryons have three quarks, the baryons most important to us are protons and neutrons A proton contains two up and one down quark, so three in all. Applying the equation above, there are three quarks, no antiquarks, so nq = 3 and the baryon number is 1

A neutron contains one up and two down quarks, so again three in all. Applying the equation, there are again three quarks, no antiquarks, so nq = 3 and the baryon number is 1   The type of quark makes no difference.

Mesons have equal numbers of quarks and antiquarks, usually one of each. They are very unstable with very brief half-lives. In the baryon number equation the two terms nq and nq bar  will always exactly cancel each other out so the baryon number of a meson is zero.