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What are quarks, and how do their combinations build the protons, neutrons and mesons we observe?

The properties of up, down and strange quarks and their antiquarks, the quark composition of baryons and mesons, the application of conservation laws to quark changes, and the quark model of beta decay.

A focused answer to AQA A-Level Physics 3.2.1.7, covering the charge, baryon number and strangeness of up, down and strange quarks, the quark composition of the proton, neutron, pions and kaons, and how beta decay is explained at the quark level.

Generated by Claude Opus 4.810 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
  2. Quark properties
  3. Quark composition of hadrons
  4. Beta decay at the quark level
  5. Try this

What this dot point is asking

AQA specification point 3.2.1.7 wants you to recall the charge, baryon number and strangeness of the up, down and strange quarks and their antiquarks, work out the quark composition of baryons and mesons, and use the quark model to explain beta-minus and beta-plus decay.

Quark properties

The fractional charges are a defining feature of quarks; no free particle has ever been observed with a fractional charge, because quarks are always confined inside hadrons.

Quark composition of hadrons

You can deduce any composition by ensuring the quark charges, baryon number and strangeness add up to the known values of the hadron. An antiparticle of a hadron is made of the corresponding antiquarks, so the antiproton is uβ€Ύuβ€Ύdβ€Ύ\overline{\text{u}}\overline{\text{u}}\overline{\text{d}}. The kaons illustrate strangeness: K+\text{K}^+ (usβ€Ύ\text{u}\overline{\text{s}}) has strangeness +1+1 because the antistrange quark carries S=+1S = +1, while Kβˆ’\text{K}^- (uβ€Ύs\overline{\text{u}}\text{s}) has strangeness βˆ’1-1. Checking strangeness this way is exactly how exam questions expect you to assign the quantum numbers of an unfamiliar meson.

Beta decay at the quark level

In beta-minus decay a down quark changes into an up quark, so a neutron (udd) becomes a proton (uud):

In beta-plus decay an up quark changes into a down quark, so a proton becomes a neutron, emitting a positron and an electron neutrino, mediated by a W+\text{W}^+ boson. These quark changes are only possible through the weak interaction, which is the only force that can change one type of quark into another.

Try this

Q1. State the quark composition of the proton and of the neutron. [2 marks]

  • Cue. Proton is uud; neutron is udd.

Q2. Describe, in terms of quarks, what happens in beta-plus decay. [2 marks]

  • Cue. An up quark changes into a down quark, so a proton becomes a neutron, emitting a positron and a neutrino.

Q3. State the charge of an up quark. [1 mark]

  • Cue. +23+\tfrac{2}{3}.

Exam-style practice questions

Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

AQA 20194 marksShow that the quark composition uud gives the correct charge and baryon number for a proton, using the quark properties (up: charge +23+\tfrac{2}{3}, down: charge βˆ’13-\tfrac{1}{3}, each baryon number +13+\tfrac{1}{3}).
Show worked answer β†’

Charge: (+23)+(+23)+(βˆ’13)=+43βˆ’13=+1\left(+\tfrac{2}{3}\right) + \left(+\tfrac{2}{3}\right) + \left(-\tfrac{1}{3}\right) = +\tfrac{4}{3} - \tfrac{1}{3} = +1, matching the proton's charge.

Baryon number: 13+13+13=+1\tfrac{1}{3} + \tfrac{1}{3} + \tfrac{1}{3} = +1, matching the proton's baryon number.

So uud correctly reproduces both the charge (+1+1) and baryon number (+1+1) of the proton.

Markers reward summing the three quark charges to +1+1 and the three baryon numbers to +1+1.

AQA 20214 marksDescribe, in terms of quarks, what happens in beta-minus decay, and write the quark-level equation, naming the exchange particle involved.
Show worked answer β†’

In beta-minus decay a down quark within a neutron (udd) changes into an up quark, turning the neutron into a proton (uud). The change is mediated by a Wβˆ’\text{W}^- boson.

The quark-level equation is dβ†’u+eβˆ’+Ξ½β€Ύe\text{d} \rightarrow \text{u} + \text{e}^- + \overline{\nu}_e, the underlying process of nβ†’p+eβˆ’+Ξ½β€Ύe\text{n} \rightarrow \text{p} + \text{e}^- + \overline{\nu}_e.

Markers reward the down-to-up quark change, the neutron becoming a proton, the correct quark equation, and naming the Wβˆ’\text{W}^- boson.

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