Deducing quark compositions from charge

Overview

Recall questions on Deducing quark compositions from charge.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Show that the quark composition uud gives the proton its correct charge.

   Model answer

   The charges add as +⅔ + ⅔ − ⅓ = +1 in units of e. That is exactly the observed charge of the proton.

2. 2

   State the quark composition of the neutron and show that it gives a neutral particle.

   Model answer

   The neutron is udd. Its charges add as +⅔ − ⅓ − ⅓ = 0, so the particle is uncharged.

3. 3

   Deduce the quark composition of the positive pion π⁺ from first-generation quarks.

   Model answer

   The π⁺ is a meson of charge +1e, so it needs one quark and one antiquark. The combination ud̄ gives +⅔ + ⅓ = +1, so the π⁺ is ud̄.

4. 4

   Deduce the quark composition of the negative pion π⁻.

   Model answer

   The π⁻ is the antiparticle of the π⁺, so it is ūd. Its charge is −⅔ − ⅓ = −1, as required.

5. 5

   Is the π⁺ a baryon or a meson, and how can you tell?

   Model answer

   It is a meson, because it is made of a single quark and a single antiquark rather than three quarks. That also gives it a baryon number of zero.

6. 6

   Deduce the quark composition of a first-generation baryon carrying a charge of +2e.

   Model answer

   A baryon needs three quarks, and the largest charge available is +⅔e from each up quark. The composition must therefore be uuu, giving +⅔ × 3 = +2e.

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