Dear ROOTers, I'm using TParticle class and I'm storing in an array of TParticles output of various MC event generators. Output of some MC generators (HERWIG, for example) contains off-shell particles with E^2 < P^2. Several TParticle constructors which are trying to calculate particle mass as fCalcMass = TMath::Sqrt(fE*fE -fPx*fPx -fPy*fPy -fPz*fPz); are crashing in this case. I'd think of putting in a protection against sqrt of negative number and, in order not to lose information, of making fCalcMass negative for off-shell particles with Q^2<0: a2 = fE*fE -fPx*fPx -fPy*fPy -fPz*fPz; if (a2 >= 0) { fCalcMass = TMath::Sqrt(a2); } else { fCalcMass = -TMath::Sqrt(-a2); } Other suggestions? -best, Pasha
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