Welcome to the World of Quarks!

Up until now, you might have thought that protons and neutrons were the smallest things inside an atom's nucleus. It turns out there is something even smaller! In this chapter, we explore quarks—the fundamental building blocks that make up particles like protons and neutrons. Don't worry if this seems like a lot to take in; we will break it down piece by piece.

1. Meet the Quarks

In the AQA AS Level syllabus, you only need to know about three types (often called "flavours") of quarks: Up, Down, and Strange. These particles have fractional electric charges, which is unique because most particles we see have whole-number charges (like \(+1\) or \(-1\)).

Here are the properties you must memorize:

  • Up (\(u\)): Charge = \(+\frac{2}{3}e\), Baryon Number = \(+\frac{1}{3}\), Strangeness = \(0\)
  • Down (\(d\)): Charge = \(-\frac{1}{3}e\), Baryon Number = \(+\frac{1}{3}\), Strangeness = \(0\)
  • Strange (\(s\)): Charge = \(-\frac{1}{3}e\), Baryon Number = \(+\frac{1}{3}\), Strangeness = \(-1\)

Did you know? The name "quark" comes from a line in a book by James Joyce: "Three quarks for Muster Mark!" Physics can be poetic too!

2. The Mirror World: Antiquarks

Every quark has an antiquark. These are like mirror images. They have the same mass but opposite properties (opposite charge, opposite baryon number, and opposite strangeness).

The symbols for antiquarks have a bar over them:

  • Anti-up (\(\bar{u}\)): Charge = \(-\frac{2}{3}e\), Baryon Number = \(-\frac{1}{3}\), Strangeness = \(0\)
  • Anti-down (\(\bar{d}\)): Charge = \(+\frac{1}{3}e\), Baryon Number = \(-\frac{1}{3}\), Strangeness = \(0\)
  • Anti-strange (\(\bar{s}\)): Charge = \(+\frac{1}{3}e\), Baryon Number = \(-\frac{1}{3}\), Strangeness = \(+1\)

Quick Tip: If you forget the properties of an antiquark, just write down the quark's properties and flip the plus/minus signs!

3. Building Particles: Baryons and Mesons

Quarks never exist alone (a concept called "confinement"). They always group together to form hadrons. There are two main ways they do this:

A. Baryons (The "Triple Threat")

Baryons are made of three quarks (\(qqq\)). Because each quark has a baryon number of \(+\frac{1}{3}\), a baryon always has a total baryon number of \(+1\).

  • Proton: Combination is \(uud\).
    Check the math: \(+\frac{2}{3} + \frac{2}{3} - \frac{1}{3} = +1\) (Total charge \(+1\)).
  • Neutron: Combination is \(udd\).
    Check the math: \(+\frac{2}{3} - \frac{1}{3} - \frac{1}{3} = 0\) (Total charge \(0\)).

Antibaryons are made of three antiquarks (\(\bar{q}\bar{q}\bar{q}\)). For example, an antiproton is \(\bar{u}\bar{u}\bar{d}\).

B. Mesons (The "Perfect Pair")

Mesons are made of one quark and one antiquark (\(q\bar{q}\)). Their baryon number is always \(0\) (because \(+\frac{1}{3} - \frac{1}{3} = 0\)).

  • Pions (\(\pi\)): These are made of \(u\) and \(d\) quarks/antiquarks. They have zero strangeness. (Example: \(\pi^+\) is \(u\bar{d}\)).
  • Kaons (\(K\)): These must contain a strange quark (or antistrange). (Example: \(K^+\) is \(u\bar{s}\)).

Key Takeaway: If a particle has strangeness, it must be a Kaon or a baryon containing a strange quark. If it doesn't have strangeness, it's likely a Pion, Proton, or Neutron.

4. Changing Character: Beta Decay

In previous chapters, you learned about radioactive decay. Now we can see what happens inside the nucleons during these changes. This is called a change of quark character.

Beta-minus (\(\beta^-\)) Decay

In \(\beta^-\) decay, a neutron turns into a proton. Looking at the quarks:
\(udd \rightarrow uud\)
One down quark changes into an up quark. This happens via the weak interaction.

Beta-plus (\(\beta^+\)) Decay

In \(\beta^+\) decay, a proton turns into a neutron:
\(uud \rightarrow udd\)
One up quark changes into a down quark.

Common Mistake: Students often forget that quarks only change "flavour" during weak interactions. The strong interaction cannot change a \(d\) quark into a \(u\) quark!

5. Conservation Laws

When you look at a particle interaction equation, you must check that these properties are "conserved" (the total before equals the total after):

  1. Charge (\(Q\)): Always conserved.
  2. Baryon Number (\(B\)): Always conserved.
  3. Lepton Number (\(L\)): Always conserved (though quarks themselves have a lepton number of \(0\)).
  4. Strangeness (\(S\)): Conserved in strong interactions, but can change by \(0, +1,\) or \(-1\) in weak interactions.

Analogy: Think of these laws like a bank account. You can move "charge" or "baryon number" between different particles, but the total amount in the "account" must stay the same at the end of the transaction!

Quick Review Table

Use this to test yourself!

  • Proton: \(uud\), \(Q=+1\), \(B=1\), \(S=0\)
  • Neutron: \(udd\), \(Q=0\), \(B=1\), \(S=0\)
  • \(\pi^+\) Meson: \(u\bar{d}\), \(Q=+1\), \(B=0\), \(S=0\)
  • \(K^+\) Meson: \(u\bar{s}\), \(Q=+1\), \(B=0\), \(S=+1\)

Note: For any other particles not listed here, the exam paper will provide you with the data you need. You just need to apply the conservation rules!