Polarized cross-section issue when including QED
Dear,
we are computing ttbar production cross-section for many polarized cases: ++, +-, -+ and --. We are following the guideline in https:/
When using only QCD,
pp > t t~ QED=0
we find that the polarized cross-sections correctly sum the total cross-section within the uncertainty.
However, when simulating using
pp > t t~ QED=2
we find that there is mismatch between computing polarized cross-sections separately and adding them, that computing altogether. This disagreement is far beyond the uncertainty.
We find, for instance, that (using QED=2 for LHAPDF 262000 and a dynamical renormalization scale of transverse mass / 4)
sigma(++) = 229.5 pb
sigma(- -) = 230.1 pb
sigma (++ plus - - ) = 430.6 pb
sigma (+ - ) = 115.9 pb
sigma (- + ) = 116.6 pb
sigma ( + - plus - + ) = 205.1 pb
sigma (pp > tt~ total) = 605.5 pb
All uncertainties below 1pb.
Any ideas of why this could be? Do you see a workaround ?
Thank you so much! Ezequiel.
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