Quarks, Leptons & Conservation Laws

Hadrons, baryons, mesons, leptons, quark model, strangeness, and the application of conservation laws to particle interactions.

Spec Points Covered
  • Classify particles as hadrons (baryons or mesons) or leptons.
  • State the quark composition of protons, neutrons, pions and kaons.
  • State the properties of up, down and strange quarks and their antiquarks.
  • Apply conservation of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C)., baryon numberA quantum number conserved in all interactions. Baryons (protons, neutrons) have baryon number +1, antibaryons have -1, and mesons/leptons have 0., lepton numberA quantum number conserved in all interactions. Leptons (electrons, neutrinos) have lepton number +1, antileptons have -1, and hadrons have 0. and strangeness to particle interactions.
  • Determine whether a given interaction is permitted or forbidden.
  • Explain quark transformations in beta decay using Feynman diagrams.
  • State which conservation laws apply in strong, electromagnetic and weak interactions.
Notes
01 Hadrons are particles made of quarks Hadrons 3.2.1.5 → 02 Leptons are fundamental particles not made of quarks Leptons 3.2.1.5 → 03 There are three quarks you need to know: up, down and strange 3.2.1.6 → 04 Beta decay involves a quark changing flavour 3.2.1.6 → 05 Conservation laws determine whether an interaction is allowed 3.2.1.7 → 06 Strangeness is only conserved in strong and electromagnetic interactions 3.2.1.7 →
Σ Key Equations Full Reference →
On Data Sheet
Not on Data Sheet
Charge of a quark combination
$$Q_{\text{total}} = \sum Q_{\text{quarks}}$$
  • Where:
    • $Q_total$ = total charge of the hadron
    • $Q_quarks$ = charge of each constituent quark
Up = +2/3, Down = −1/3, Strange = −1/3. Antiquarks have opposite charges.
Baryon number of a quark combination
$$B_{\text{total}} = \sum B_{\text{quarks}}$$
  • Where:
    • $B_total$ = total baryon number
    • $B_quarks$ = baryon number of each quark (+1/3) or antiquark (−1/3)
Baryons (3 quarks): B = 3 × (+1/3) = +1. Mesons (quark + antiquark): B = +1/3 + (−1/3) = 0.
Q Retrieval Practice All 12 Questions →
Q1. What is the difference between a hadron and a lepton?
  • Hadrons are made of quarks and experience the strong force.
  • Leptons are fundamental (not made of quarks) and do not experience the strong force.
Q2. What is the difference between a baryon and a meson?
  • A baryon is made of 3 quarks (baryon numberA quantum number conserved in all interactions. Baryons (protons, neutrons) have baryon number +1, antibaryons have -1, and mesons/leptons have 0. = +1).
  • A meson is made of a quark-antiquark pair (baryon numberA quantum number conserved in all interactions. Baryons (protons, neutrons) have baryon number +1, antibaryons have -1, and mesons/leptons have 0. = 0).
Q3. State the quark composition and chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). of (a) a proton and (b) a neutron.
(a) Proton = uud, chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). = +2/3 +2/3 −1/3 = +1. (b) Neutron = udd, charge = +2/3 −1/3 −1/3 = 0.
Q4. State the charge, baryon number and strangeness of the up, down and strange quarks.
  • Up: Q = +2/3, B = +1/3, S = 0.
  • Down: Q = −1/3, B = +1/3, S = 0.
  • Strange: Q = −1/3, B = +1/3, S = −1.
Q5. Which quark transformation occurs in β⁻ decay?
A down quark changes to an up quark (d → u), converting a neutron into a proton.