Past exam of the mathematics course of the University of Cambridge / 2025 / iii / Paper 301 / 3 / b / Solution

The pseudoscalar vertex replaces $-i\lambda$ by $g{\gamma }^5$, soUsing ${\gamma }^5/q{\gamma }^5=-/q$ with the conjugation sign givesThe interactions are experimentally distinguishable through the decay rate and its threshold behavior: scalar decay is proportional to $(1-4m^2/M^2{)}^{3/2}$, whereas pseudoscalar decay is proportional to $(1-4m^2/M^2{)}^{1/2}$. Spin and parity correlations provide further discrimination.