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Quarks & Antiquarks

Physics: AQA A Level, topic 2.1.12

Quarks

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

Types of quark

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

Properties

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

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

Proton

  • A proton is made of two up quarks and one down quark.
  • Proton = uud
  • The charges add up to +1.
  • 23+23−13=1\frac{2}{3} + \frac{2}{3} - \frac{1}{3} = 1
  • The baryon numbers add up to +1.
  • 13+13+13=1\frac{1}{3} + \frac{1}{3} + \frac{1}{3} = 1
  • Strangeness and lepton numbers add to zero.
Antiproton

Antiproton

  • An antiproton is made of two anti-up quarks and one anti-down quark.
  • Antiproton = uud
  • The charges add up to -1.
  • −23−23+13=−1- \frac{2}{3} - \frac{2}{3} + \frac{1}{3} = -1
  • The baryon numbers add up to -1.
  • −13−13−13=−1-\frac{1}{3} - \frac{1}{3} - \frac{1}{3} = -1
  • Strangeness and lepton numbers add to zero.
Neutron

Neutron

  • A neutron is made of one up quark and two down quarks.
  • Neutron = udd
  • The charges add up to 0.
  • 23−13−13=0\frac{2}{3} - \frac{1}{3} - \frac{1}{3} = 0
  • The baryon numbers add up to +1.
  • 13+13+13=1\frac{1}{3} + \frac{1}{3} + \frac{1}{3} = 1
  • Strangeness and lepton numbers add to zero.
Antineutron

Antineutron

  • An antineutron is made of one anti-up quark and two anti-down quarks.
  • Antineutron = udd
  • The charges add up to 0.
  • −23+13+13=0-\frac{2}{3} +\frac{1}{3} + \frac{1}{3} = 0
  • The baryon numbers add up to -1.
  • −13−13−13=−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

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!

Test yourself on Quarks & Antiquarks

  • Which of the following is NOT a type of quark?

  • What are the quark combinations for a proton and an antiproton?

  • What are the quark make ups of a neutron and an antineutron?

  • What baryon is shown in this diagram?

  • Is it possible for particles to be their own antiparticle?

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