2.1.12

# Quarks & Antiquarks

Test yourself

## Quarks

Quarks are fundamental particles. Hadrons (including protons and neutrons) are made of quarks. Different combinations of quarks make different hadrons.

### Types of quark

• There are six different types of quark but you only need to know about the following three:
• Up, u.
• Down, d.
• Strange, s.

### Properties

• The table summarises the key properties of up, down and strange quarks.

### Antiquarks

• Every quark has a corresponding antiquark.
• They have the opposite properties to their quarks.

## Quark Combinations - Baryons

Different combinations of quarks form different hadrons. Here we look at two important baryons - protons and neutrons - and their antiparticles.

### Proton

• A proton is made of two up quarks and one down quark.
• Proton = uud
• The charges add up to +1.
• $\frac{2}{3} + \frac{2}{3} - \frac{1}{3} = 1$
• The baryon numbers add up to +1.
• $\frac{1}{3} + \frac{1}{3} + \frac{1}{3} = 1$
• Strangeness and lepton numbers add to zero.

### Antiproton

• An antiproton is made of two anti-up quarks and one anti-down quark.
• Antiproton = uud
• The charges add up to -1.
• $- \frac{2}{3} - \frac{2}{3} + \frac{1}{3} = -1$
• The baryon numbers add up to -1.
• $-\frac{1}{3} - \frac{1}{3} - \frac{1}{3} = -1$
• Strangeness and lepton numbers add to zero.

### Neutron

• A neutron is made of one up quark and two down quarks.
• Neutron = udd
• The charges add up to 0.
• $\frac{2}{3} - \frac{1}{3} - \frac{1}{3} = 0$
• The baryon numbers add up to +1.
• $\frac{1}{3} + \frac{1}{3} + \frac{1}{3} = 1$
• Strangeness and lepton numbers add to zero.

### Antineutron

• An antineutron is made of one anti-up quark and two anti-down quarks.
• Antineutron = udd
• The charges add up to 0.
• $-\frac{2}{3} +\frac{1}{3} + \frac{1}{3} = 0$
• The baryon numbers add up to -1.
• $-\frac{1}{3} - \frac{1}{3} - \frac{1}{3} = -1$
• Strangeness and lepton numbers add to zero.

## Quark Combinations - Mesons

Different combinations of quarks form different hadrons. Mesons are a type of hadron containing two quarks.

### Pions and kaons

• Various mesons are created by u, d, s and their associated antiquarks.
• Pions have u and d quarks (and antiquarks) only.
• Kaons have one s or s quark.
• Notice that some pions and kaons are antiparticles of each other.
• The π0 meson is its own antiparticle!