Status of NLOjet++ for dijet angular distributions

Lee Pondrom

20 April 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.

22 symmetrized jet_chi cross sections

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->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=pT

2.

• 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=pT

2

2 for hadron level data compared to Q2=pT

2 Pythia noqsub

• 20 bins one parameter fits

• Mass 2

• 600 GeV 32

• 700 GeV 38

• 800 GeV 17

• 900 GeV 17

2 fits to quark substructure 600 GeV mass

2 fits to quark substructure 700 GeV mass

2 fits to quark substructure 800 GeV mass

2 fits to quark substructure 900 GeV mass

summary

• Mass bin noqsub 2 TeV 3 TeV 2

• 600 GeV 22.7 20.1 22.9

• 700 GeV 32.8 20.0 44.3

• 800 GeV 16.4 58.3 7.5

• 900 GeV 13.4 29.2 33.8

• No clear pattern

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++

• Mass GeV 0=Etav 0.7Etav 1.4Etav• 600 75 110 78• 700 75 48 65• 800 36 48 35• 900 37 35 37

• No fit is particularly good, compared to Pythia

ETave compared to mjj These two scales were fit to the data in Pythia

Compare lo and nlo 0=ETaveK factor 1.1

Compare lo and nlo 0=mjj

K factor 1.44

2->2 born level calculations, Pythia

• Pythia ‘stand alone’. 1E6 events• All initial and final radiation turned off:

mstp(61)=mstp(71)=mstp(81)=mstp(111)=0. No CDFSim. Q2=pT

2 mstp(32)=2.• QCD processes msub(381)->msub(386)

switched on.(qq->qq etc).• Ckin(1)=400, ckin(3)=90, lower limits on ŵ,

and on pT . Same cuts in the main CDF MC.

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.

2->2 and full Pythia compared

Conclusions from this exercise

• 2->2 Pythia and full Pythia at the parton level look the same. ISR and FSR have little effect.

• Hadron level Pythia looks like the data.

• Nlojet++ born does not agree with 2->2 Pythia.

Status

• The large K factor for mjj is reason to be suspicious of the accuracy of the nlo calculation for that scale.

• Etave has a more modest scale differece between lo and nlo calculations.

• Pythia fits the data better than nlojet++• I have not achieved as good a limit on

quark substructure using hadron level Pythia or nlojet++.