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Hadrons, Baryons, and Mesons

Like social elephants, quarks only exist in groups with other quarks and are never found alone. Composite particles made of quarks are called

Although individual quarks have fractional electrical charges, they combine such that hadrons have a net integer electric charge. Another property of hadrons is that they have no net color charge even though the quarks themselves carry color charge (we will talk more about this later).

There are two classes of hadrons (try putting your mouse on the elephants):

...are any hadron which is made of three quarks (qqq).

Because they are made of two up quarks and one down quark (uud), protons are baryons. So are neutrons (udd).

...contain one quark (q) and one antiquark ().

One example of a meson is a pion (+), which is made of an up quark and a down anitiquark. The antiparticle of a meson just has its quark and antiquark switched, so an antipion (-) is made of a down quark and an up antiquark.

Because a meson consists of a particle and an antiparticle, it is very unstable. The K meson lives much longer than most mesons, which is why it was called "strange" and gave this name to the strange quark, one of its components.

A weird thing about hadrons is that only a very very very small part of the mass of a hadron is due to the quarks in it.