Past exam of the mathematics course of the University of Cambridge / 2023 / iii / Paper 354 / 1 / b / i

At spacelike separation $x_{12}^2>0$, conformal symmetry fixes the scalar-primary two-point function toApplying the d'Alembertian at $x_2$ givesaway from contact terms. Thus, up to a real multiplicative constant, $F_0\propto (x_{12}^2{)}^{-\Delta -1}$. The scalar conformal unitarity bound makes the coefficient nonnegative and makes it vanish when the bound is saturated, as expected for the null free-field equation of motion.