AQA AS Level Physics Paper 2, June 2024: Question 16

✅ Correct Answer: C

The correct option is C ( ūs ).

To find the antiparticle of the K⁺ meson, you must first know the quark makeup of the K⁺ itself, then invert each quark to its corresponding antiquark (and vice versa).

💡 Key Knowledge

• Kaons are mesons consisting of a strange quark (or antiquark) and another light quark/antiquark.
• The quark composition of a K⁺ meson is us̄ (up quark and strange antiquark).
• Taking the antiparticle of a hadron means replacing every quark with its respective antiquark, and every antiquark with a quark.

🧠 Exam Technique

Don't guess! Break the problem down into two logical steps:

1. Recall or derive the quark structure of the positive kaon ( us̄ ).
2. Apply the antiparticle operation: the up quark ( u ) becomes an anti-up ( ū ), and the anti-strange quark ( s̄ ) becomes a strange quark ( s ). This yields ūs .

❌ Common Errors

• Forgetting charge conservation: K⁺ has a charge of +1. Its quark composition ( us̄ ) gives +2/3 + (+1/3) = +1. Therefore, its antiparticle must have a charge of -1.
• Mixed-up notation: Selecting option A ( us̄ ) which is the K⁺ particle itself, rather than its antiparticle. Always double-check whether the question asks for the particle or its antiparticle!