PHYS3036 · Condensed Matter and Particle Physics
The Particle Zoo & the Standard Model
The particle-physics module of University of Sydney PHYS3036 opens with the Standard Model's cast: three generations of leptons and quarks (spin-½ fermions), the gauge bosons (γ, g, W±, Z⁰) that carry the forces, and the spin-0 Higgs. Quarks are never free; they bind into hadrons — mesons (qq̄) and baryons (qqq) — whose charge, spin and baryon number follow from their quark content. The exam provides a particle-property data sheet, so the skill is using it to classify particles and compute quantum numbers.
What this chapter covers
- 01Leptons (spin-½, no strong force, can be free): e, μ, τ with charge −1; neutrinos νe, νμ, ντ with charge 0
- 02Quarks (spin-½, feel the strong force, never free, three colours): up-type u, c, t (charge +2/3); down-type d, s, b (charge −1/3); antiquarks carry opposite charge
- 03Gauge bosons (force carriers): γ (massless, EM), gluon (massless, strong), W± and Z⁰ (massive, weak); the graviton lies outside the Standard Model
- 04The Higgs boson H (spin 0, ~125 GeV): couples to massive particles, gives mass via the electroweak mechanism
- 05Hadrons: baryons = qqq (baryon number B = +1), antibaryons = q̄q̄q̄ (B = −1), mesons = qq̄ (B = 0)
- 06Charge of a hadron = sum of its quark charges; baryon number B = (1/3)(n_q − n_q̄)
- 07Excited states and exotics: same quark content, different mass (p vs Δ⁺); tetraquarks and pentaquarks
Classifying hadrons from their quark content
- +1(i) uds electric charge: Q = (+2/3) + (−1/3) + (−1/3) = 0. Three quarks (no antiquarks) ⇒ baryon number B = (1/3)(3 − 0) = +1 ⇒ it is a BARYON. This is the Λ (Lambda) baryon. [+1]
- +1Cross-check the Λ: a neutral, strangeness-carrying baryon of three different quarks — consistent with Q = 0, B = +1. [+1]
- +1(ii) us̄ electric charge: Q = (+2/3) + (+1/3) = +1 (the s̄ antiquark has charge +1/3). One quark + one antiquark ⇒ B = (1/3)(1 − 1) = 0 ⇒ it is a MESON. This is the K⁺ (kaon). [+1]
- +1Cross-check the K⁺: a positively charged, strange meson (qq̄, B = 0) — consistent with Q = +1, B = 0. [+1]
Key terms
- Lepton
- A spin-½ fundamental fermion that does not feel the strong force and can exist freely: the charged e, μ, τ and the neutral neutrinos νe, νμ, ντ.
- Quark
- A spin-½ fundamental fermion that feels the strong force, carries fractional charge (+2/3 or −1/3) and one of three colours, and is confined inside hadrons.
- Gauge boson
- A force-carrying particle: the massless photon (EM) and gluon (strong), and the massive W± and Z⁰ (weak); the spin-2 graviton is not part of the Standard Model.
- Higgs boson
- The spin-0, ~125 GeV particle whose field gives mass to the other massive particles through electroweak symmetry breaking.
- Hadron
- A composite particle built from quarks: a baryon (qqq, B = +1), an antibaryon (q̄q̄q̄, B = −1) or a meson (qq̄, B = 0).
- Baryon number (B)
- The conserved quantum number B = (1/3)(n_q − n_q̄); +1 for baryons, −1 for antibaryons, 0 for mesons.
The Particle Zoo & the Standard Model FAQ
Do I need to memorise particle masses for the PHYS3036 exam?
No. The particle-physics exam ships a data/formula sheet with a Selected Particle Properties table (quark content, mass, spin, strong isospin), the CKM matrix, constants and Clebsch–Gordan coefficients. The examined skill is using that sheet — reading off quark content, computing charge and baryon number, and reasoning about the particle — not reciting numbers. Confirm exactly what the sheet provides on Canvas.
How do I get a hadron's charge and baryon number?
Add the quark charges, remembering antiquarks have the opposite sign to their quark (an s̄ is +1/3, an ū is −2/3). For baryon number use B = (1/3)(n_q − n_q̄): three quarks give +1 (a baryon), three antiquarks give −1, and a quark–antiquark pair gives 0 (a meson). These two sums classify almost any hadron on the data sheet.
What's the difference between a fundamental particle and a hadron?
Fundamental particles — the leptons, quarks, gauge bosons and Higgs — have no known substructure. Hadrons are composite: baryons are three quarks and mesons are a quark–antiquark pair, bound by the strong force. Because quarks are confined, we only ever observe them inside hadrons, which is why the 'particle zoo' is mostly hadrons built from a handful of quarks.
How does this chapter show up in the exam?
As the foundation for the quark-model and interaction questions: classify particles, assign quantum numbers, and identify what a given quark content is. It pairs with the multiplet-diagram and conservation-law questions later in the module. Expect to lean on the data sheet throughout; confirm the paper's format and weight on Canvas and the unit outline.
Exam move
Get fluent with the bookkeeping rather than memorising the zoo. Practise reading the particle-property data sheet and, from any quark content, writing the charge (sum of quark charges, antiquarks flipped), the baryon number (qqq → +1, qq̄ → 0), and the strangeness. Keep the Standard-Model organisation clear in your head — three generations of leptons and quarks, four gauge bosons, one Higgs — and know which particles feel which force. Because the exam supplies the sheet, drill the process of extracting quantum numbers quickly and accurately; that is where the marks are. This foundation feeds the symmetry, quark-model and CP-violation chapters, so keep it warm across the semester. Ask Sia to quiz you on classifying random quark combinations and to check your quantum-number arithmetic.
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