Short Answer
Two 511 keV gamma-ray photons (back-to-back) is the best answer.
Nuclear and particle questions tend to hinge on conservation laws, decay rules, interaction types, or scale. A clean answer usually comes from identifying the process before the details.
e⁻ + e⁺ → 2γ; each photon has energy mec² = 511 keV, emitted back-to-back to conserve momentum.
Why This Answer Is Correct
This is a Medium-level question in Nuclear & Particle Physics. The prompt is really testing whether you can connect the concept to its defining physical relationship instead of picking a nearby-but-wrong term.
Track conserved quantities carefully: energy, momentum, charge, lepton number, baryon number, and spin-like constraints when relevant.
Choices At A Glance
- A. A neutron
- B. Two 511 keV gamma-ray photons (back-to-back)
- C. One photon and a neutrino
- D. Alpha particle
When similar options appear on an exam, eliminate the ones that break the core law, use the wrong units, or confuse a definition with a consequence.
Study Next
Review the related formula library
review the theoretical physics hub
Topic Snapshot
Topic: Nuclear & Particle Physics
Difficulty: Medium
Best next move: Re-state the governing law in your own words, then solve one more example from the same topic before moving on.