Status of NLOjet++ for dijet
angular distributions
Lee Pondrom
9 May 2010
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.
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.
22 symmetrized jet_chi cross
sections
Key to previous slide
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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->glueglue/q1q1bar s and t
q1glue->q1glue compton
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
2->2 Pythia compared to nlojet
born and jet_chi
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.
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
Nlojet++ has no CDF trigger
• After jet energy corrections the 100 GeV
trigger moves to about 120 GeV
• ET= M/(1+)=(Msin(*))/2 which has to
be removed, in addition to other
instrumental effects.
120 GeV trigger threshold cut in the
angular distribution
correct the data to the hadron level
using Pythia MC
Corrected data agree well with
hadron level Pythia Q2=pT2
2 for hadron level data compared
to Q2=pT2 Pythia noqsub
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20 bins one parameter fits
Mass
2
600 GeV
32
700 GeV
38
800 GeV
17
900 GeV
17
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.
Repeat previous analysis using
hadron level monte carlo
Dependence of the angular distributions
 Depedence of the angular
distribution
Plot the ratio R=(1<<7)/(7<<13)
vs (mass)4 for each 
Fitted slopes vs (1/4) give
sensitivity to quark substructure
Run nlojet++ 1010 events
0=ETavge
Vary 0 in NLOjet++
Fit nlojet++ to hadron level data
2 for one parameter fits to first 12
bins of data with nlojet++
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Mass GeV
600
700
800
900
0=Etav
75
75
36
37
0.7Etav
110
48
48
35
1.4Etav
78
65
35
37
• No fit is particularly good, compared to
Pythia
Compare lo and nlo 0=ETave
K factor 1.1
Cuts in nlojet++
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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.
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)>
Table
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Mass bin
R(data) R(nlojet) ratio
600 GeV .815.017 .818.009 1.0.02
700 GeV .87.02 .81.01 1.07 .02
800 GeV .85.05 .82.02 1.04.06
900 GeV .92.09 .83.02
1.1.1
Fitted slope s=0.290.08, intersept=0.97,
2=2.4
R(data)/R(Nlojet++) vs (mass)4
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++.
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.