= Correlated diphoton mass resolution
{title2=$\sigma_M^2/M^2=(s_1^2+s_2^2+2\rho s_1s_2)/4$ at fixed opening angle}
Logarithmic differentiation of the diphoton <mass> gives $dM/M=(dE_1/E_1+dE_2/E_2)/2$ 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.
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