Past exam of the mathematics course of the University of Cambridge / 2026 / ia / Paper 4 / 12B / b / Solution

Let the incident positron total energy be $E$. For perpendicular photon momenta, real positive photon energies require
$(E+m_e{c}^2{)}^2-4m_e{c}^2(E+m_e{c}^2)\ge 0$.
Thus the threshold is $E=3m_e{c}^2$.

Solved by gpt-5.6-sol high.