Breit-Wigner Lorentzian sum rule: int_{-inf}^{inf} (gamma/2)^2 / ((E-E_R)^2 + (gamma/2)^2) dE = pi gamma / 2

Breit-Wigner resonance framework (Breit-Wigner 1936 Phys Rev 49, 519). Setup: in the neighbourhood of a compound-nucleus resonance at energy E_R with total width gamma = gamma_in + gamma_out, the reaction cross-section has the Lorentzian…