Hadrons: baryons and mesons

By the end of this lesson

By the end you can decide whether a particle is a hadron, classify it as a baryon or a meson from its quark content, and check its charge from the quarks.

Definition

Hadron

A hadron is any particle made of quarks bound together. Protons and neutrons are hadrons, and so are many short-lived particles produced in collisions.

Hadrons come in exactly two shapes:

class	quark content	examples
baryon	three quarks	proton $\text{u}\,\text{u}\,\text{d}$ , neutron $\text{u}\,\text{d}\,\text{d}$
meson	one quark and one antiquark	pion $\pi^+ = \text{u}\,\bar{\text{d}}$ , kaon $\text{K}^+ = \text{u}\,\bar{\text{s}}$

So the test is simple: count the quarks. Three quarks means a baryon; a quark plus an antiquark means a meson.

What the antiquark does to the charge

In a meson one of the two particles is an antiquark, so its charge has the opposite sign to the corresponding quark. That is what lets mesons have the charges they do:

$\pi^+ = \text{u}\,\bar{\text{d}}$ : $+\tfrac{2}{3} + \tfrac{1}{3} = +1$ ;
$\pi^- = \bar{\text{u}}\,\text{d}$ : $-\tfrac{2}{3} - \tfrac{1}{3} = -1$ ;
$\pi^0 = \text{u}\,\bar{\text{u}}$ (or $\text{d}\,\bar{\text{d}}$ ): $+\tfrac{2}{3} - \tfrac{2}{3} = 0$ .

Notice the pattern: the $\pi^+$ and $\pi^-$ pair are antiparticles of each other, which is why their charges are equal and opposite.

Why baryons are the ones that last

A baryon contains three quarks, so it can hold a stable arrangement: the proton, with $\text{u}\,\text{u}\,\text{d}$ , does not decay at all.

A meson contains a quark and an antiquark. Such a pair can annihilate into other particles, so mesons are short-lived. That is why the particles you are made of are baryons, while mesons appear only briefly in collisions and cosmic rays.

Worked example: Example: classifying a particle

A particle is found with quark content $\bar{\text{s}}\,\text{u}$ . Is it a hadron, and if so which kind? What is its charge?

Have a go first?

Check 1 — counting quarks

CalculateQuestion 1 · Recall

Multiple choiceQuestion 2 · Recall

Answer the 2 questions above to continue.