Total cross sections in Sherpa: How do they compare to LO/NLO and, in particular, to Pythia

[Frank Krauss]

(email by Lenzi) I can confirm that when comparing virtuality ordered pythia shower and sherpa+up to 1 jet they look just the same (which is not the case for alpgen btw). What is still different however is the total cross section, that is lower for sherpa. I know that ME corrections in pythia cannot alter the cross section, so in pythia the ME corrected cross section is the same as the cross section for the qqbar-->e+e- process; but I'm a bit confused about the fact the in sherpa I get a different number. Can you remind me where this differece comes from?

(Follow up question) Is it correct that if I raise Q_cut up to, say s^{0.5 /2, I shoud get sigma_i ~ 0 for i > 0 and sigma_0 ~ sigma_pythia? About the formal accuracy, during the school in Debrecen Leif Lonnblad said that in CKKW for hadron collision "the total cross section becomes beyond LO (although not NLO)". I did not understand this statement very well. Is this the source of the difference (less that 10%) that I observe when comparing Pythia with Sherpa + up to 1 jet, Q_cut=20GeV?}