[G12] CLAS fourVec method error

[Michael C. Kunkel]

Greetings,

For those that use the fourVec method to calculate quantities (all 
CLASEVENT invoking people), I want you to be aware of a problem.

The issue was found by simulating gamma p -> p pi0

Looking at the missing mass off of the proton this should always be the 
pi0 for events in PART0, the generated bank.

If you use the CLAS method of fourVec, this missing mass off of the 
proton was no longer the pi0 mass, however if I used the same Px, Py, Pz 
and E that I used in the fourVec method and put them into a ROOT method 
of TLorentzVector, this missing mass off of the proton is always the 
generated pi0. See attached pic, there are 9993 events, the red is the 
CLAS fourVec method and the blue is the ROOT method. The ROOT method has 
all 9993 events at pi0 mass.

The methods of obtaining the 4vectors quantities is safe from error, 
meaning the momenta and energy measurements are correct, but the method 
of calculating added fourVec is problematic.

I think we should discuss where to place this information so that it is 
not lost in the future.

BR
MK
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