 Correlated diphoton mass resolution

Logarithmic differentiation of the diphoton mass gives $dM/M=(d{E}_1/E_1+d{E}_2/E_2)/2$ at fixed angle. If $s_i$ are fractional energy standard deviations and $\rho$ their correlation coefficient, covariance propagation gives the displayed formula. A common multiplicative calibration error transfers directly to the reconstructed mass; independent errors combine differently. Angular errors, if present, add the corresponding derivative and covariance terms.