Mesons and Baryons

Mesons and Baryons

Now, note that the hadrons in the table given above are divided into two subgroups, called mesons (originally for medium mass) and baryons (the name originally meaning large mass). The division between mesons and baryons is actually based on their observed decay modes and is not strictly associated with their masses. Mesons are hadrons that can decay to leptons and leave no hadrons, which implies that mesons are not conserved in number.

Baryons are hadrons that always decay to another baryon. A new physical quantity called baryon number \(B\) seems to always be conserved in nature and is listed for the various particles in the table given above. Mesons and leptons have \(B=0\) so that they can decay to other particles with \(B=0\). But baryons have \(B\text{=+}1\) if they are matter, and \(B=-1\) if they are antimatter. The conservation of total baryon number is a more general rule than first noted in nuclear physics, where it was observed that the total number of nucleons was always conserved in nuclear reactions and decays. That rule in nuclear physics is just one consequence of the conservation of the total baryon number.