Quarks and antiquarksAQA A-Level Physics: Revision notes

Hadrons are made of quarks. Only the up (u), down (d) and strange (s) quarks and their antiquarks are needed.

• u: charge +2/3, baryon number +1/3, strangeness 0
• d: charge −1/3, baryon number +1/3, strangeness 0
• s: charge −1/3, baryon number +1/3, strangeness −1

Each antiquark has the opposite charge, baryon number and strangeness: $\bar{u}$ has charge −2/3 and baryon number −1/3; $\bar{d}$ has charge +1/3; $\bar{s}$ has charge +1/3 and strangeness +1.

A baryon is made of three quarks and an antibaryon of three antiquarks.

• proton: u u d (charge +2/3 + 2/3 − 1/3 = +1)
• neutron: u d d (charge +2/3 − 1/3 − 1/3 = 0)
• antiproton: $\bar{u}$ $\bar{u}$ $\bar{d}$ (charge −1)
• antineutron: $\bar{u}$ $\bar{d}$ $\bar{d}$ (charge 0)

Baryon number is 3 × (1/3) = +1 for a baryon and −1 for an antibaryon.

A meson is a quark and an antiquark, so its baryon number is 0.

• π⁺ = u $\bar{d}$, π⁻ = $\bar{u}$ d, π⁰ = u $\bar{u}$ or d $\bar{d}$
• K⁺ = u $\bar{s}$, K⁻ = $\bar{u}$ s, K⁰ = d $\bar{s}$

The pions contain no strange quarks, so S = 0. Kaons contain a strange quark or antiquark and are strange particles: K⁺ has S = +1 (from $\bar{s}$) and K⁻ has S = −1 (from s).

Neutron decay, n → p + e⁻ + $\bar{\nu}_e$, is a weak interaction in which a down quark changes into an up quark:

d → u + e⁻ + $\bar{\nu}_e$

The neutron (u d d) becomes a proton (u u d). Charge: −1/3 → +2/3 − 1 + 0 = −1/3 ✓. Baryon number: the three quarks keep B = +1, and the leptons have B = 0.