There are three classes of common hadrons.
- One class is the baryons with three extra quarks, which include protons, neutrons and a host of other short-lived particles; a famous example is called the Lambda and contains an extra up quark, an extra down quark and an extra strange quark.
- The anti-baryons, with three extra anti-quarks, are just the anti-particles of the baryons, with all quarks replaced with corresponding anti-quarks and vice versa.
- Then there are mesons, with one extra quark and one extra anti-quark; a simple example is the charged pion with an extra up quark and an extra down anti-quark. No mesons exist much longer than a billionth of a second, so they have a limited role outside of particle physics experiments and astrophysics.
The reasons these are the most common, and that various other combinations (such as two extra quarks and no extra anti-quarks) are disallowed, re discussed in this post, using simple math (algebra, trigonometry, and complex numbers). The basic idea is that mesons are like dot-products and baryons are like cross-products. I admit that’s rather cryptic, but explaining it is what the above-mentioned post is all about.
Recently, other hadrons with four extra quarks and one extra antiquark or with two extra quarks and two extra antiquarks have been identified, most by the LHCb experiment at the LHC.
