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Quarks and antiquarks

Three quarks, up, down and strange, build every hadron on the syllabus. Baryons are quark trios, mesons are a quark bound to an antiquark, and the neutron's decay is one down quark quietly changing flavour.

Year 12AQA 3.2.1.6

Builds on Classification of particles.

IN THIS TOPIC

  • Quote the charge, baryon number and strangeness of the u, d and s quarks and their antiquarks.
  • Build the proton, neutron, antiproton, antineutron, pions and kaons from quarks.
  • Describe the decay of the neutron in terms of its quarks.

WHAT YOU PROBABLY THINK

Protons and neutrons are fundamental.

Three flavours, one table

Every hadron AQA tests is built from three quark flavours. The up quark: charge +⅔ e, baryon number +⅓, strangeness 0. The down: −⅓ e, +⅓, 0. The strange: −⅓ e, +⅓, and strangeness −1, the flavour the quantum number is named for. Each has an antiquark, written with a bar (u, d, s), with every sign flipped. The full table is printed in the data booklet, so the skill is not memorising it but building with it.

Baryons: three quarks

The proton is uud and the neutron is udd: the quark charges add to plus one and zero exactlyprotonu+2/3u+2/3d−1/3total charge +1neutronu+2/3d−1/3d−1/3total charge 0same recipe book, one swapped ingredient
FIG. 1The proton is uud, charge ⅔ + ⅔ − ⅓ = +1. The neutron is udd, charge 0. One swapped quark apart.

A baryon is three quarks, and its baryon number is three thirds: +1, as it must be. The proton is uud and the neutron is udd, their charges falling straight out of the fractions. The antibaryons are the mirror recipes: the antiproton is uud and the antineutron udd, with charge −1 and 0 and baryon number −1.

Mesons: a quark and an antiquark

The specified mesons: each is one quark and one antiquark, and the charges and strangeness follow from the pairπ⁺udcharge +1S = 0π⁻ducharge −1S = 0K⁺uscharge +1S = +1K⁰dscharge 0S = +1the amber circle is always the antiquark
FIG. 2The specified mesons as pairs: π⁺ is u with anti-d, π⁻ is d with anti-u, K⁺ is u with anti-s, K⁰ is d with anti-s.

A meson is a quark bound to an antiquark: baryon number ⅓ − ⅓ = 0, exactly as the classification demanded. The pion family carries no strange quarks, so pions have S = 0. The kaons each contain one strange or anti-strange quark, which is where their strangeness of ±1 lives: a K containing s has S = +1, and one containing s has S = −1. Only u, d and s and their antiquarks will be tested; the rest of the quark family stays off the paper.

The neutron's decay, at last explained

Neutron decay at the quark level: one down quark becomes an up quark, and the W minus delivers the electron and antineutrinouddneutron: udduudproton: uudW⁻e⁻+νthe change of quark character that beta-minus decay is
FIG. 3Neutron decay at quark level: udd becomes uud as one down quark changes to an up, the W minus delivering the electron and antineutrino.

Beta-minus decay, met two lessons ago as “a neutron becomes a proton”, is really one line of quark chemistry: one down quark becomes an up quark, udd turning into uud, with a W− carrying away the charge difference and becoming the electron and the electron antineutrino. The other two quarks are spectators. AQA states plainly that the decay of the neutron should be known, and this is the level at which to know it.

THE EXAM BIT

  • Recipes cold: proton uud, neutron udd, antiproton uud, antineutron udd; mesons are quark-antiquark, pions strangeness 0, kaons ±1.
  • Verify any composition by adding thirds: the charges must sum to a whole number of e, and a trio of quarks must give B = +1.
  • The quark properties table is printed in the booklet. Spend the exam second reading it, not reciting it.
  • Neutron decay at quark level: d → u, with a W−, then e− and an electron antineutrino. Writing “n → p” alone at this point in the paper undersells the mark.
  • Antiquarks flip every sign: charge, baryon number and strangeness. Flipping only the charge is the classic table-reading slip.

CHECK YOURSELF

Using quark charges, show that the proton's charge is +1 and the neutron's is 0, then write the neutron's decay at quark level.

Show a hint

Add thirds, then change exactly one flavour.

Show the answer

Proton, uud: ⅔ + ⅔ − ⅓ = +1. Neutron, udd: ⅔ − ⅓ − ⅓ = 0. Both are whole charges, as hadrons must be.

Decay: one d becomes a u, so udd → uud + e− + an electron antineutrino, the W− mediating.

Charge audit at the vertex, in thirds: −1 = +2 − 3. The books balance, and the neutron's decay is one quark's change of flavour.

Baryons are three quarks; mesons are a pair.

The neutron's decay is one d becoming u.

No animated video for this topic yet; these notes stand alone. InkPhysics on YouTube.