[Rivet] Final particles with negative squared mass

[Andy Buckley]

Paolo Francavilla wrote:
> Hi,
> Thanks for the help.
> I am using Rivet, but I found that in the projection for the final 
> state, from time to time I have some negative mass2(). What does it means?
> Usually the value is of the order of 10e-9.
> I am using Pythia6.421

Hi Paolo,

Presumably you mean the FourMomentum::mass2() method? Since mass(^2) is 
calculated as a Lorentz invariant by subtracting the (finite) space and 
time 4-momentum components from one another, massless vectors will in 
general not have mass^2 exactly equal to zero, by computational rounding 
errors. This error can be either positive or negative, and is typically 
of the size you've reported.

If you want to check that the mass2 is zero (or any other floating point 
equality check) then you need to use a "fuzzy" definition of equality to 
avoid rounding errors: the Rivet::fuzzyEquals(a, b) function is a 
reasonable definition, and the standard one for use in Rivet algorithms.

Best wishes,
Andy