A-level Physics (Advancing Physics)/Quarks

Quarks (pronounced like 'orcs' with a 'qu' on the front) are a subset of the fermions - they make up part of matter, most notably the nuclei of atoms. Quarks interact with all four of the fundamental forces: gravity, electromagnetism, and the weak and strong nuclear forces.

Generations

There are four quarks in each of the three generations of fermions. The first contains the up quark ($u$), down quark ($d$), antiup quark (${\bar {u}}$) and antidown quark (${\bar {d}}$). The second generation contains the charm quark ($c$), strange quark ($s$), anticharm quark (${\bar {c}}$) and antistrange quark (${\bar {s}}$). The third generation contains the top quark ($t$), bottom quark ($b$), antitop quark (${\bar {t}}$) and antibottom quark (${\bar {b}}$).

Charge

The up, charm and top quarks have a charge of +⅔e, and so their respective antiparticles have a charge of -⅔e. The down, strange and bottom quarks have a charge of -⅓e, and so their respective antiparticles have a charge of +⅓e.

Hadrons

When quarks are combined, they form larger particles, which are not fundamental. These larger particles are known as hadrons and are held together by the strong nuclear force. There are two types of hadrons: baryons and mesons.

Baryons

Baryons are hadrons which are made up of three quarks. The two most common baryons are the proton and the neutron. Protons are made up of two up quarks and one down quark, giving them a total charge of +1e. Neutrons are made up of one up quark and two down quarks, giving them net charge of 0.

Mesons

Mesons are hadrons which are made up of a quark and an antiquark. For example, pions are made up of two first generation quarks - the π⁰ is made up of either an up quark and an antiup quark, or a down quark and an antidown quark. The π⁺ is made up of an up quark and an antidown quark (total charge +1e), and The π^- is made up of a down quark and an antiup quark (total charge -1e).

Questions

1. The Δ⁺⁺ baryon is made up of up quarks. What is its total charge?

2. The Δ^- baryon has a total charge of -1e. Given that it is made up of only one type of first generation quark, what is this quark?

3. What is an antiproton made of? What is its charge?

4. A K+ meson is made of an up quark and an antistrange quark. What is its total charge?

5. Lambda (Λ) baryons are made up of an up quark, a down quark, and another quark (not an antiquark). The Λ⁰ is neutral, and contains a second generation quark. What is this quark?

Worked Solutions