Cambridge International A-Level Physics · topic 11
Particle physics
Connect the ideas, make your working visible, and test the result. Use the hints when you need a starting point; open the full solution after your attempt.
Extra challenging questions
Work through the linked stages and explain what each result means.
An unseen particle in a decay event
A particle initially at rest decays into an electron, a positron and an unseen particle. The electron momentum components are (3,0) MeV/c and the positron components are (0,4) MeV/c; these supplied vectors are represented by points in momentum-component space. For this question the unseen particle is a photon. Use electron rest energy 0.511 MeV and E² = (pc)²+(mc²)². Supplied data (px / MeV c⁻¹; py / MeV c⁻¹): (0, 0), (3, 0), (0, 4). (a) Infer the photon's momentum components and energy, then determine the parent's rest energy. [3 marks] (b) Check charge and lepton-number conservation for the listed products, and compare leptons with a neutron's quark composition. [3 marks]
Measured momentum-vector endpoints
Supplied question data. Read the labelled axes and use the observations to test the physical model.
Vertical axis (y): py / MeV c⁻¹
Horizontal axis (x): px / MeV c⁻¹
Read the exact data as a table
| px / MeV c⁻¹ | py / MeV c⁻¹ |
|---|---|
| 0 | 0 |
| 3 | 0 |
| 0 | 4 |
These are the supplied values, not a worked answer. No fitted trend is drawn.
Show a hint
Begin with part (a): Infer the photon's momentum components and energy, then determine the parent's rest energy.
Reveal the worked solution
- The parent starts with zero momentum, so the unseen particle must cancel the measured vector sum: pγ = (−3,−4) MeV/c.
- Its momentum magnitude is √(3²+4²) = 5 MeV/c. A photon is massless and has Eγ = pc = 5 MeV.
- Electron energy is √(3²+0.511²)≈3.043 MeV and positron energy is √(4²+0.511²)≈4.033 MeV. Parent rest energy equals their sum plus 5, about 12.08 MeV.
- Electron charge is −e and lepton number +1; positron charge is +e and lepton number −1. The photon has neither charge nor lepton number, so both listed totals are zero.
- A neutron is a baryon containing one up quark (+2/3 e) and two down quarks (−1/3 e each), giving zero net charge and baryon number one.
- An electron is an elementary lepton, not a constituent quark. These conservation checks constrain a proposed process but do not alone establish that it occurs or determine all selection rules.
Answer (a) Photon momentum = (−3,−4) MeV/c; photon energy = 5 MeV; parent rest energy≈12.08 MeV. (b) Net charge and electron-family lepton number of the products are zero; neutron quarks are udd.
Compare the assumptions, method and units with your attempt. Explain any correction before trying another question.