understand that a hadron may be either a baryon (consisting of three quarks) or a meson (consisting of one quark and one antiquark)

Fundamental Particles – Hadrons (Cambridge 9702 Syllabus)

1. Scope of the syllabus (11.2)

• Six quark flavours – up (u), down (d), charm (c), strange (s), top (t) and bottom (b).

  
  These are the only quark flavours required for the exam.

• Electric charge of each flavour:

• Colour charge – red, green, blue (and the corresponding anticolours). Only colour‑neutral (singlet) combinations are observed.
• Six lepton flavours – e⁻, νₑ, μ⁻, νμ, τ⁻, ντ (do not feel the strong interaction).
• Composition of hadrons:

  • Baryons – three quarks (qqq)
  • Mesons – one quark and one antiquark (q\(\bar q\))

• Quark‑level description of β‑decay.
• Antiparticles for all hadrons (e.g. \(\bar p\), \(\pi^-\)).

2. Quarks

Quarks are elementary fermions (spin = ½) that carry three intrinsic properties:

• Flavour: u, d, c, s, t, b
• Electric charge** (see table above)
• Colour charge: red, green or blue. Colour is a property of the strong interaction; observable particles must be colour‑neutral.

2.1 Colour confinement

Only the following colour‑neutral combinations exist in nature:

• Baryons: three quarks of different colours (red + green + blue) → colour singlet.
• Mesons: a quark and an antiquark with matching colour–anticolour (e.g. red + anti‑red) → colour singlet.

3. Leptons

Leptons are elementary fermions that do not participate in the strong interaction.

4. Hadrons

Hadrons are composite particles that experience the strong nuclear force. They are divided into two families.

4.1 Baryons (qqq)

Three quarks of different colours combine to give a colour‑neutral fermion (half‑integer spin).

4.2 Mesons (q \(\bar q\))

A quark and its corresponding antiquark (colour–anticolour) form a colour‑neutral boson (integer spin).

5. Antiparticles of Hadrons

Every hadron has an antiparticle obtained by replacing each quark with its antiquark (and reversing the colour).

• Proton ↔ Antiproton: \(uud \leftrightarrow \bar u\,\bar u\,\bar d\)
• Neutron ↔ Antineutron: \(udd \leftrightarrow \bar u\,\bar d\,\bar d\)
• π⁺ ↔ π⁻: \(u\bar d \leftrightarrow \bar u d\)

6. Quark‑level description of β‑decay

In nuclear β⁻‑decay a down quark inside a neutron changes into an up quark by emitting a \(W^{-}\) boson, which subsequently decays into an electron and an electron‑antineutrino:

\[

d \;\rightarrow\; u + W^{-}, \qquad

W^{-}\;\rightarrow\; e^{-} + \bar{\nu}_{e}

\]

At the hadron level this appears as:

\[

n\;(udd) \;\rightarrow\; p\;(uud) + e^{-} + \bar{\nu}_{e}

\]

7. Summary of key differences