Measurement of the relative fraction ofthe gg fusion in tt production process at
1.96 TeV pp collisions using CDF
Naoki Kimura
University of Tsukuba
for the CDF Collaborations
Rencontres de Moriond EW 2009
Young Scientists ForumMar 12 2009

Motivation
The tt production processes in 1.96 TeV pp colisions

qq annihilation

qParton distribution function

t

qt

85%
gg fusion

ggt
Top quark mass

g15 %tggt

gt

Tevatron energytt

We are going to measure this ratio in experimentally to verify theperturbative QCD calculation and search for new physics.

Analysis methodology• qqÆtt and ggÆtt have different spin configuration
qq annihilation

qSpin

At threshold ttbar production

gqparallel spin state

gg fusionSpinganti-parallel spin stateJ = 1 Jz = ±1

J = 0 Jz = 0• top quark is the only quark that we can use spin correlation. Top quark decay
before hadronization.
•We measure the gg fusion fraction using the distribution of the azimuthal angle
between l+ and l- (∆φ) in top dilepton channel (tt -> WbWb - >lvlvbb).

ltt+In azimuthal plane

ltllltltt
Same possibility for parallel and anti-parallelt/t spin is in anti-parallel

no correlation in flight
direction between l+ and l-same flight direction ofl+ and l- is favoredEvent Selection••Signal selection:– 2 leptons with ET>20 GeV– At least 2 tight jets with ET>15 GeV, |Eta|<2.5– MET>25 GeV
Background rejection:– L-cut (MET>50 GeV if the minimum angle between MET and any lepton or
jet is less than 200)
– Z-veto– HT>200 GeV– Oppositely charged leptons

νtt b - jetw+tw-νl-l+tb - jet∆φ distribution of leptons
qq annihilation

Fqq

We make the ∆φ distribution of leptons using Monte
Carlo simulation. And we could find different
distribution between qq annihilation and gg fusion ttbar
production processes.
After all check, we perform unbinned likelihood fit todata as a function of gg fraction using following function.
F(∆ϕ)all = (fgg F(∆ϕ)gg + (1-fgg) F(∆ϕ)qq ) x fsig + F(∆ϕ)bg x (1-fsig)
L=ΠFallgg fusion

Fgg

backgroundFBG

Results and Summary• We presented a measurement of gg fraction in ttbar
production using dilepton channel spin correlation.
+ 0 . 36

Fgg = 0 . 53 − 0 . 38(+ 0 . 35

Standard Model Prediction

Fgg = 0 . 15 ± 0 . 05

• The results is consistent with Standard
Model prediction.
• Tevatron keep running and we
improving our analyses, we are aiming at
the about 10% uncertainty finally.
• four times data from Tevarton• combine other channel and method• multi validate analysis+ 0 . 07

( Stat .) − 0 . 08 ( Syst .)− 0 . 37
Best fir results)

Backup

Analysis methodology (cont.)
In actual pp collisions, t/t are produced with pT > 0
Enhance opposite flight directions of l+ and lIn gg fusion

In qq annihilation
Generator level (HERWIG)
Without t and tbar
Pt effect

same
angle between l+ and l-opposite∆ϕ [rad]tt product in
actual collisions

same
angle between l+ and l-

The qq fraction can be determined with ∆ϕ distribution

opposite∆ϕ [rad]0 jet control region

Background distribution check using 0jet
and 1jet control region
Before HT and Opposite Sign lepton cut
0jet control region
1jet control region

KS prob = 7 %KS prob = 37 %Summary of uncertainty

Data Fitting+ 0 .36

Fgg = 0 .53 − 0.38

ResultsFeldman-Cousins Bands(+ 0 .35+ 0 .07(Stat.)(Syst.)− 0 .37− 0 .08)