The Quark Model

The quark model explains the internal structure of hadrons (protons, neutrons, pions, and many other particles). It was proposed independently by Murray Gell-Mann and George Zweig in 1964 and is a cornerstone of the Standard Model of particle physics. This lesson covers quarks, antiquarks, hadrons, baryons, mesons, and the quantum numbers used to classify particles. This is assessed in AQA section 3.2.1.

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Quarks

Quarks are fundamental particles — they have no known internal structure. At A-Level, you need to know three flavours (types) of quark:

Quark	Symbol	Charge (e)	Baryon number (B)	Strangeness (S)
Up	u	+⅔	+⅓	0
Down	d	−⅓	+⅓	0
Strange	s	−⅓	+⅓	−1

Key Points About Quarks

• Quark charges are fractional: +⅔e or −⅓e. These are the only particles with fractional charges — all observable particles have integer charge.
• Quarks have baryon number +⅓ each (so three quarks make a baryon with baryon number +1).
• The strange quark has strangeness −1 (by convention — this is a historically assigned quantum number).
• Quarks are never observed in isolation — they are always confined within hadrons. This is called colour confinement (related to the strong force carried by gluons).

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Antiquarks

Every quark has a corresponding antiquark with the opposite charge, opposite baryon number, and opposite strangeness:

Antiquark	Symbol	Charge (e)	Baryon number (B)	Strangeness (S)
Anti-up	ū	−⅔	−⅓	0
Anti-down	d̄	+⅓	−⅓	0
Anti-strange	s̄	+⅓	−⅓	+1

Key Point: Every quantum number of an antiquark is the exact negative of the corresponding quark. So if the strange quark has strangeness −1, the anti-strange quark has strangeness +1.

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Hadrons

Hadrons are particles made of quarks. They feel the strong nuclear force. Hadrons are divided into two families:

Baryons (Three Quarks)

Baryons are made of three quarks (qqq). They have baryon number B = +1 (since each quark contributes +⅓).

Baryon	Quark composition	Charge (e)	Strangeness
Proton (p)	uud	+⅔ +⅔ −⅓ = +1	0
Neutron (n)	udd	+⅔ −⅓ −⅓ = 0	0
Sigma-plus (Σ⁺)	uus	+⅔ +⅔ −⅓ = +1	−1
Sigma-zero (Σ⁰)	uds	+⅔ −⅓ −⅓ = 0	−1
Sigma-minus (Σ⁻)	dds	−⅓ −⅓ −⅓ = −1	−1
Xi-minus (Ξ⁻)	dss	−⅓ −⅓ −⅓ = −1	−2
Omega-minus (Ω⁻)	sss	−⅓ −⅓ −⅓ = −1	−3

Antibaryons (Three Antiquarks)

Antibaryons are made of three antiquarks (q̄q̄q̄). They have baryon number B = −1.

• Antiproton (p̄): ūūd̄ — charge = −⅔ −⅔ +⅓ = −1
• Antineutron (n̄): ūd̄d̄ — charge = −⅔ +⅓ +⅓ = 0

Mesons (Quark-Antiquark Pair)

Mesons are made of one quark and one antiquark (qq̄). They have baryon number B = 0 (since +⅓ and −⅓ cancel).