Electromagnetic calorimetry requirements from flavour physics for application at FCC-ee

This technical note reports an academic exercise to assess the electromagnetic (EM) calorimeter energy resolution performance that would be needed in order to separate candidates of the radiative quark transitions $b \to s \gamma$ and $b\to d \gamma$. The specific measurement considered in this work, the relative branching fractions of the decays $\overline{B}^0 \to K^{*0}(892)\gamma$ and $B^0_s \to K^{*0}(892) \gamma$ can only be performed with a stochastic term of the EM well-below $10\,%$.