Top Quark Production and Asymmetry at Tevatron

Yen-Chu ChenFermi National Accelerator Laboratory

On behalf of the CDF/D0 collaborations

June 11-21, 2012 @HQL2012

Outline

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• The Top quark• Tevatron summary• The CDF/D0 experiments• Top quark production• Production asymmetry• Conclusion

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The Top quark• Properties

Standard Model • Charge : + 2/3• Spin : 1/2• Mass : A free parameter • Life time: ~10-25 s

• Top quark pairs are produced via proton antiproton collisions at Tevatron.

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Tevatron Integrated luminosity

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The detectorsCDF D0

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Top quark physics

P Pt

tX

b

b

W

W

l q

l q

q'_

q'

__

Production of top quark

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t t decay channels used

l ± b

b_

q

q’

l +

l -

b

b_

Lepton + Jets, LJ Di-lepton, DIL

_

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Looking for secondary vertex

B tagging via Secondary vertex (SVX)

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Efficiency Mistagging

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Efficiency Mistagging

B tagging at D0

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69.7

Top di-lepton event candidate

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Top quark production• p p → t t

s ~ 7.22 pb at √S = 1.96 TeV: (NNLL, C. Schwinn, arXiv:1205.0988) 90% qq → t t; 10% gg → t t_

_

_ _

__

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…

DIL event selection• Two high pT leptons

– > 20 GeV (CDF), > 15 GeV(D0)– || < 2

• High missing ET due to the two neutrinos being missing in detection

• Suppressing of Z boson events • High total transverse energy• Two or more jets

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σpretag = 7.40 ± 0.58stat  ± 0.63syst ± 0.45lumi pb σbtag = 7.25 ± 0.66stat  ± 0.47syst ± 0.44lumi pb

Cross sectionusing dilepton events, 5.1 fb-1

Mtop = 172.5 GeV/c2

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Production cross section

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DIL alone: (mtop=172.5 GeV)s = 7.36 +0.90 -0.79 (stat + syst +lumi) pb

Combined with LJ:s =7.56 +0.63

–0.56 (stat+syst + lumi) pb

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Top production asymmetry

• Differential cross section

• Dependent on q2 and * Mtt and y• There is asymmetry with respect to beamline

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*22

2*22*2

2

2

cossin)1(cos1cos

sMq

qqd

d s

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Theoretical interest

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• Exotic gluons– massive chiral color – RS gluon– color sextets, anti-triplets

• Intermediate Vector B´– Z´, W´,…

• …

• Model building must contend with – total s in good agreement with SM– ds/dMtt in good agreement with SM

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Using the lepton + jets events

• high pT lepton (e/m)– Et/pt > 20 GeV (/c)– || < 1.0

• missing ET

• Four or more jets– || < 2.0

• at least one b-tagged jet– || < 1.0

• 2498 events• 505±123 non-tt background

– mostly W+jets

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l ± b

b_

q

q’

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CDF, full RUN II data, with good silicon, 8.7 fb-1

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Top reconstruction

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Asymmetry in the tt rest frame ytt

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)0()0()0()0(

)0()0()0()0(

ttt

ttt

ttt

ttt

ttFB

yNyNyNyN

yNyNyNyNA

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_

_

Asymmetryfor reconstructed tt

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Dependence on ytt

background subtracted

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Dependence on Mtt

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Afb in parton level• Using Unfolding Matrix to correct for the bias

caused by acceptance of the detector/trigger and the reconstruction.

• CDF, 8.7 fb-1

Afb = 0.162 0.047

• D0, 5.4 fb-1

Afb = 0.196 0.065 PRD 84, 112005 

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Conclusion• Both CDF and D0 have measured the total tt production cross

section. The results are consistent between the two experiments and with the Standard Model.

• Both CDF and D0 observe a large production asymmetry.

• CDF observe dependence on the ytt and Mtt. But the dependence on Mtt is not very strong at D0.

• More studies from both CDF and D0 will come in the near future.– Differential cross section in terms of cos, etc. – Result from DIL channel

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Thank you!!!

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Asymmetry from backgroundis small but not zero!

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tt rest frame lab frame

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Expecting Higgs at LHC/ATLAS

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Top reconstruction

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Inclusive asymmetry

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yhy

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Apply to not b tagged sampleS:B = 0.3

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tt frame lab frame

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Afb from di-lepton channel• In progress of blessing

• Shows also strong asymmetry

• Combining with the result from the lepton + jets channel and using full dataset we could reach 5 sigma, if this asymmetry is true.

• Also some puzzles similar to lepton + jets result!– Such as smaller asymmetry in the not b tagged sample– Is this just statistical or is it telling us something that we are not aware

of?

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Asymmetry categorized

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Separated by number of jets• data: the high mass asymmetry is significantly reduced for 5

jet events

• the NLO QCD asymmetry has a strong Njet dependence

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In the lab frame• Cross check using

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ppth yqy

?

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Production asymmetry

• “Evidence for a Mass Dependent Forward-Backward Asymmetry in Top Quark Pair Production”, – “Wine & Cheese” at Fermilab, Jan. 7, 2011.

• Paper submitted to PRD.• D0 also see similar effect.

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prior measurements• CDF, 1.9 fb-1 , inclusive, corrected to “parton-level”

– tt rest frame– NLO QCD

– lab (pp) frame– NLO QCD

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PRL 101, 202001 (2008)

A tt 0.24 0.14

08.017.0 ppA

A tt 0.060.01

01.004.0 ppA

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prior measurements• D0, inclusive, background subtracted “data-level”

– tt rest frame 0.9 fb-1

4.3 fb-1

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08.012.0 ttA

PRL 100, 142002 (2008)

40.008.0 ttA

ICHEP 2010

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Top charge asymmetry in QCD• Halzen, Hoyer, Kim; Brown, Sadhev, Mikaelian; Kuhn, Rodrigo; Ellis,

Dawson, Nason; Almeida, Sterman, Vogelsang; Bowen, Ellis, Rainwater

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C = - 1

C = +1

C = + 1

C = -1

+

+

tt

tt j

AFB ~ +10-12 % NLO

In t t rest frame:

AFB ~ -7 % NLO

AFB ~ 6 ± 1 % NLO

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Asymmetry in tt and pp frame

• The asymmetry in the lab. Frame is reduced by the uncontrolled boost along the beamline:

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ppFB

ttFB AA 5.1

_ _

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MC NLO study

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sim + recosim + reco +bkg

truth

• MCFM NLO calculation at “parton level”• MC@NLO + CDFSIM

• Pythia remains good approximation of SM

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Correction to parton level• Bin by bin in the histogram

– Pj : parton level distribution– Aj: acceptance of the analysis– Sij: smearing of the reconstruction– Ti: top signal

Ti = Sij x Aj x Pj

– Bi: background– Di: data distribution

Pj = Aj-1 x Sij

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Asymmetry is a function of ytt and Mtt

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