Physics 9702 · Paper 2

Hadrons: Baryons & Mesons

Quarks join together in two ways. The particles they make are called hadrons.

Baryons and mesons

A is any particle made of quarks. There are two types.
A baryon is made of three quarks. A proton is uud and a neutron is udd.
A meson is made of one quark and one antiquark.

Protons and neutrons are each three quarks. Proton = uud: (+⅔ + ⅔ − ⅓)e = +1e. Neutron = udd: (+⅔ − ⅓ − ⅓)e = 0. The up quark is +⅔e, the down quark −⅓e — the fractional charges that build every nucleus.

The charges must add up to the charge of the whole particle, and that alone tells you the recipe. A proton ( $+1e$ ) is two ups and a down: $\tfrac{2}{3} + \tfrac{2}{3} - \tfrac{1}{3} = +1$ .
A neutron ( $0$ ) is one up and two downs: $\tfrac{2}{3} - \tfrac{1}{3} - \tfrac{1}{3} = 0$ .

Adding up the charge

Build a particle below and watch the charge add up. Try a proton (uud), a neutron (udd), and a meson (one quark and one antiquark).

Build a particle from quarks

No quarks yet.

Total charge: 0Pick some quarks to build a particle.

Antiparticles

Every particle has a matching with the same mass and the opposite charge. The antiparticle of the electron is the positron.
An antiproton is three antiquarks: anti-u, anti-u, anti-d. Its charge is $-\tfrac{2}{3} - \tfrac{2}{3} + \tfrac{1}{3} = -1e$ , the opposite of the proton. (9702/21/M/J/25 Q7(b)) (9702/23/M/J/25 Q8)
When a particle meets its antiparticle they annihilate: both disappear and their mass becomes energy, given out as gamma photons.

Your turn— tap to reveal the worked answer (9702/23/O/N/24 Q7)

A particle is made of one up quark and one anti-down quark. What type of particle is it, and what is its charge? (9702/23/O/N/24 Q7) (9702/23/M/J/24 Q6(c))

Answer: one quark and one antiquark is a meson. Its charge is $+\tfrac{2}{3} + \tfrac{1}{3} = +1e$ .

Common mistake

An antiquark flips the sign of the charge and keeps the size. An anti-up is

-\tfrac{2}{3}e

, not

-\tfrac{1}{3}e

. Change the sign, keep the fraction.

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