Top Pair Production Cross Section with Tight SecVtx Tags using 318 pb-1      
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 Salvatore Rappoccio
Daniel Sherman Joao Guimaraes da CostaAndy Foland Melissa Franklin 
Harvard University

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Systematics Q&A

Update to Gen5: MC Modeling Systematic (Pythia vs Herwig)
You need to update the Pythia vs Herwig systematic for Gen5.
In Gen5, Herwig has QED radiation turned ON by default, whereas in Gen4, it had QED radiation turned OFF by default. Because of this, we can compare default Herwig samples with default Pythia samples. The only care that must be taken is that the Herwig BR for W-> lnu is 0.111 (the theoretical value) whereas Pythia uses the value measured at LEP, 0.108. For this reason we reweight the Herwig values to get a fair comparison. The acceptance in Herwig MC before reweighting is 8.99%. After reweighting, the acceptance in Herwig is 8.75%. The Pythia acceptance is 8.66%, and so we assign a 1.0% systematic for the Pythia Vs Herwig systematic. This is a decrease from the Gen4 value of 1.4%.

Update to jetCorr04b
You need to update the JES systematic to jetCorr04b.
We have re-evaluated the JES systematic using jetCorr04b. We see that before the optimization, the systematic changed from 3.4% to 2.9%, and after the optimization, the systematic changed from 5.0% to 4.3%.