1. 1 Pythia versus Alpgen for W+jets (High Mass Dibosons work) Sarah Allwood 07/12/06

2. 2 Introduction Our VBF signal is WWqq  WWq’q’: In this high mass case, p T W ≥ 300GeV whilst p T tag jet >20GeV. W + 3jets is an important background. Alpgen can make W + up to 6 jets. Pythia produces W + 1jet as the hard process; extra jets can come from parton showers (  tag jets could be faked by ISR). lνlν qq For high m WW, this is one jet with some substructure for cone ΔR =0.7 or k T R=0.5, and not quite 2 jets for cone ΔR =0.4.

3. 3 Samples Generation cuts for Alpgen:  W decays leptonically: p T lep >100GeV, |η lep | 100GeV, dR(l-j)>0.  p T jet >20GeV, |η jet | 0.7  njets = 3  …and then through Herwig / Pythia 6.323 Generation cuts for Pythia 6.323:  W decays leptonically  min p T of hard scatter (ckin 3) 250GeV …followed by ATLFAST (11.0.42) with massive cone algorithm. 5618 unweighted events for Alpgen: σ = 3.46±0.17 pb through Herwig σ = 3.29 pb through Pythia. 100000 events for Pythia: σ = 13.53 pb.

4. 4 Jet variables cone R=0.7 Required NJET>2, ptjet>20GeV, pt lepton>100GeV, ptmiss>100GeV, pt leptonic W>300GeV, pt hadronic W >300GeV. pythiaalpgen through pythia

5. 5 Leptonic W, cone R=0.7 Alpgen through pythia vs Pythia: Alpgen through Herwig vs Pythia:

6. 6 Tag jets, cone R=0.7 Found highest p T jets forward of the W candidates. Alpgen through Pythia Pythia Alpgen through Herwig Pythia

7. 7 Jet Variables cone R=0.4 pythia alpgen through herwig

8. 8 W’s and tag jets cone R=0.4 alpgen through herwig vs Pythia

9. 9 Summary Started a study of Alpgen versus Pythia for W+3jets as a background to high m WW VBF. Possible similarities:  After lepton p T >100GeV, W p T >300GeV and tag jet cuts Pythia W+jets and Alpgen W+3jets show similar numbers and tag jet distributions. Possible differences:  Alpgen shows a slightly harder p T distribution on additional jets.  dR(leptonicW, hadronicW) The cuts used at generation are quite different between Alpgen and Pythia – it’s possible I haven’t completely removed those differences in my ATLFAST cuts. Next:  Pythia 6.4 rather than 6.323.  more alpgen events, looser cuts, contributions from W+2jets, W+4jets?  generator level study  different jet algorithm  hadronic W cuts, central jet veto.