- Quote: The strong nuclear force is so overwhelming that you will never find a quark or gluon on its own; each is permanently trapped inside a proton or a neutron.
- Endnote: They can also be trapped inside other short-lived cousins of protons and neutrons, which are generically called hadrons and lend their name to the LHC.
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. (Here’s a list of a very few mesons; the “Makeup” column tells you what the extra quark and anti-quark are.)
The reasons these are the most common, and that various other combinations (such as two extra quarks and no extra anti-quarks) are disallowed, are 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 the post I just mentioned explains this remark.
Another post about mesons shows how we can use them to gain insight both into the quarks’ rest masses and into the workings of the strong nuclear force.
Recently, a number of other hadrons with four extra quarks and one extra antiquark, or with two extra quarks and two extra antiquarks, have been identified, most of them by the LHCb experiment at the Large Hadron Collider. The details of these objects are still very poorly known and are the subject of an active area of research.
