Non-iterative mass constraint for decays

Non-iterative methods for the mass constraint of decays such as π 0 → γ γ are developed as an alternative to relatively complex and slow iterative methods. Constraints involving only the refit of the decay pair energies are presented in the form of a direct quadratic solution and an empirical solution. The empirical solution is then expanded to three-dimensions where the opening angle of the decay pair is also refitted. The performance of these methods is compared to that of traditional iterative chi-square methods.