W+jets/Wbb+jets background in ttbar measurement (muon+jets)

Brad Abbott, Guangshun Huang, Pat Skubic

OU ATLAS Group

We are measuring ttbar cross section using the mixing sample and the MC08 samples, following the CSC event selection. We have a couple of questions on W+jets/Wbb+jets backgrounds, and hope some experts could clarify the situation. Specifically, can we have larger statistics for these background MC, and is it practical to generate W(bb) with more jets? Thanks.

Questions!

2/10/2009 G.S.Huang W+jets/Wbb+jets background 2

Question on ratio PS/MEFrom Wouter’s talk:

Are they also PS only? If yes,what is the ratio PS/ME?

Ratio PS/ME = 4% above PT=20 & DR=0.7 separate Wbb+jets samples were generated.Seems the current MC08 samples have PS+ME for Wbb+jets, butonly PS for W+jets (is this true?). For absolute normalization, do we lose anything (e.g. ME) for W+jets?

2/10/2009 G.S.Huang W+jets/Wbb+jets background 3

W+jets/Wbb+jets: jet pT spectraShown only leading jets,

• More jets, higher jet pT;

• Is this expected?

2/10/2009 G.S.Huang W+jets/Wbb+jets background 4

w/o b-tagging

3-jet mass: background shapeNormalizedto 146 1/pb

with b-tagging

Wbb+jets

W+jets

2/10/2009 G.S.Huang W+jets/Wbb+jets background 5

W+jets/Wbb+jets efficiencies• Efficiency keeps going higher with more jets for

existing MC samples;• Interesting to know when it begins falling!

This is without b-tagging.

2/10/2009 G.S.Huang W+jets/Wbb+jets background 6

W+jets/Wbb+jets event rates• W+5jets roughly same amount with W+4jets;• Wbb+3jets more than Wbb+2jets;• Do we worry about W+6jets and Wbb+4jets?

Has it been justified that we don’t need them?

This is without b-tagging.