Strange particles and strangenessAQA A-Level Physics: Revision notes
Section 1
Strange particles
Strange particles were found in cosmic ray and accelerator experiments. They are produced in collisions between hadrons through the strong interaction, which is very fast (about 10⁻²³ s), yet they decay through the weak interaction, and live about 10⁻¹⁰ s. This unexpected long life led physicists to call them strange.
Kaons (K⁺, K⁰, K⁻) are the main examples.
Section 2
Strangeness
Strangeness (symbol S) is a quantum number introduced to explain why strange particles are always created in pairs.
- K⁺ and K⁰ have S = +1
- K⁻ has S = −1
- protons, neutrons, pions and leptons have S = 0
In a pair such as K⁺ and K⁻ the total strangeness is zero, the same as the particles that collided.
Section 3
Strong interactions: strangeness is conserved
In the strong interaction strangeness is conserved, as are charge and baryon number.
Worked example. p + p → p + p + K⁺ + K⁻:
- charge: +2 → +1 + 1 + 1 − 1 = +2 ✓
- baryon number: +2 → +2 ✓
- strangeness: 0 → +1 − 1 = 0 ✓
With only K⁺ produced, strangeness would change from 0 to +1, so a strong interaction cannot make a single strange particle.
Saying that strangeness is conserved in every interaction. It is conserved in the strong interaction only.
Section 4
Weak interactions: strangeness can change
In the weak interaction strangeness can change by 0, +1 or −1.
Worked example. K⁰ → π⁺ + π⁻: S goes from +1 to 0, so ΔS = −1. K⁻ → π⁻ + π⁰: S goes from −1 to 0, so ΔS = +1. Both are allowed, so these decays occur through the weak interaction.
To decide the interaction: if strangeness is conserved, the strong interaction is possible; if it changes by one unit it must be the weak interaction.
Compute ΔS = S(after) − S(before). If ΔS = 0 the strong interaction is possible; if it is ±1 the interaction must be weak.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Strange particles and strangeness
- In a collision experiment a fast-moving proton collides with a stationary proton, and the reaction p + p → p + p + K⁺ + K⁻ is observed. The strangeness of K⁺ is +1.Explain why the K⁺ and K⁻ are produced together in this reaction.2 marks
- The neutral kaon K⁰ has strangeness +1 and decays into a π⁺ and a π⁻ with a mean lifetime of about 10⁻¹⁰ s. Reactions that produce kaons from collisions between hadrons typically take place in about 10⁻²³ s.Suggest why kaons are described as strange particles, using the information given.2 marks
- Physicists use conservation laws to decide which reactions can occur through the strong interaction. The strangeness of K⁺ and K⁰ is +1 and the strangeness of K⁻ is −1. Protons, neutrons and pions have strangeness 0.Determine whether the reaction p + p → p + n + K⁺ can occur through the strong interaction.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).