(Electron–positron annihilation: conceptual aspects) An electron at rest in free…
PHYSICS · Subjective · A Level · Physics · Particles and radiation · Particles and radiation
Problem
(Electron–positron annihilation: conceptual aspects)
An electron at rest in free space meets a positron that is also at rest. The two particles annihilate and only electromagnetic radiation is observed afterwards.
(i) State the mass–energy equivalence relation connecting mass m and energy E.
(ii) Using the law of conservation of linear momentum, explain carefully why the annihilation of an electron and a positron, both initially at rest, cannot result in the emission of a single photon.
(iii) Why is the simultaneous emission of two photons the minimum requirement? Justify your answer on the basis of the conservation laws and show that the least possible total energy carried away by the photons is equal to 2mec2, where me is the rest mass of the electron.