1. Status of NLOjet++ for dijet angular distributions
Lee Pondrom
University of Wisconsin
20 May 2010

2. Ingredients 1.1 fb-1 jet100 triggered data
1E10 nlojet++ events with CTEQ6
2E6 Pythia events with full CDFSim and CTEQ5
1E6 ‘standalone’ Pythia events with CTEQ6 and ISR, FSR turned off.

3. Pythia first
We have to use Pythia to correct the data to the hadron level.
We use a calculation of the subprocess cross sections to understand Pythia.
We learn that to reproduce the Pythia angular distributions, the 2->2 subprocesses with nonidentical final state partons must be u<->t symmetrized.

4. 2->2 symmetirzed jet_chi cross sections 600 GeV mass bin

5. Key to previous slide q1q2->q1q2 t channel gluon exchange
q1q2bar->q1q2bar t channel gluon*
q1q1->q1q1 t channel gluon
q1q1bar->q2q2bar s channel annihilation
q1q1bar->q1q1bar s and t channels*
q1q1bar->glueglue s channel annihilation
glueglue->q1q1bar/glueglue* s and t
q1glue->q1glue compton* *=large 

6. 2->2 subprocesses
The peaks at =1 come from the u<->t symmetrization
The t channel gluon exchange cross sections dominate, which is the motivation for the choice of scale Q2=pT2.
Now that we understand Pythia born, let us look at nlojet++ born

7. 2->2 Pythia compared to Nlojet born and jet_chi

8. 2->2 Pythia compared to Nlojet born and jet_chi

9. Normalization
Each set of four mass plots has one overall normalization.
All programs agree on the 1/mass4 dependence of the cross section.
Nlojet++ born agrees better with Pythia as the mass increases.

10. conclusion
We understand Pythia. It agrees well with the data, and strengthens the Pythia based quark substructure analysis.
To compare nlojet++ to the data, we need to correct the data to the hadron level using Pythia

11. Nlojet++ has no CDF trigger
After jet energy corrections the 100 GeV trigger moves to about 125 GeV
ET= M/(1+)=(Msin(*))/2 which has to be removed, in addition to other instrumental effects.

12. 125 GeV trigger threshold cut in the angular distribution

13. Correct the data to the hadron level using Pythia MC

14. Correct the data to the hadron level using Pythia MC

15. Corrected data agree well with hadron level Pythia Q2=pT2

16. Corrected data agree well with hadron level Pythia Q2=pT2

17. 2 for hadron level data compared to Q2=pT2 Pythia noqsub
20 bins one parameter fits
M (GeV) events (data) 2

18. Jet-jet angular distribution and quark substructure
Quark substructure effective contact color singlet Lagrangian of Eichten, et al is:
L = ±(g²/2Λ²(LLLL
Looks just like muon decay. Affects only the u and d quarks. Color singlet means that some diagrams have no interference term.
g²/4 = 1; strength of the interaction ~(ŝ/²)²
This measurement is not sensitive to the interference term. _ _ _

19.  Dependence of the angular distributions

20.  Dependence of the angular distributions

21. Plot the ratio R=(1<<7)/(7<<13) vs (mass)4 for each 

22. Fitted slopes vs (1/4) give sensitivity to quark substructure

23. Run nlojet++ 1010 events 0=ETavge

24. Vary 0 in NLOjet++

25. Fit nlojet++ to hadron level data

26. 2 for one parameter fits to first 12 bins of data with nlojet++
Mass GeV 0=Etav Etav Etav
No fit is particularly good, compared to Pythia

27. Compare lo and nlo 0=ETave K factor 1.1

28. Cuts in nlojet++ For 2 partons with highest ET ET>10 GeV ||<2
Cone size D=0.7 in (,) space
Rsep = 1.3. D and Rsep govern when the third parton is included with one of the other two to form a ‘jet’. Should have no effect on a born calculation.

29. Systematics
Calculate R(Nlojet++) for 0=ETave, 0.7 ETave, and 1.4ETave.
Calculate R(data) for level7JetE corrections, and 1 on JetE corrections
Average the results <R(data)> and <R(Nlojet)>

30. Table M(GeV) R(data) R(nlojet) ratio 600 .815.017 .818.009 1.0.02
  .02
  .02
  .06
  .1
Fitted slope s=0.160.07, intersept=1.0, 2=2.3

31. R(data)/R(Nlojet++) vs (mass)4

32. Conclusions
The original Pythia based analysis has been repeated, with the following changes.
Only Pythia with Q2=pT2 used.
Data corrected to hadron level with Pythia
Sensitivity to quark substructure uses Pythia integrated over smaller regions in  to accommodate Nlojet++.

33. Conclusions continued
Systematics are included in the comparison of data to nlojet++ by varying the jet energy corrections in data and the hard scale 0 in nlojet++.
 limit from the fitted slope: >2.1 TeV 95% confidence.
Expected limit for zero slope is >2.6 TeV 95% confidence.