top quark production and asymmetry at tevatron
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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
June, 11-21, 2012
• 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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_ _
_
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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
-1 x (Di – Bi)June, 11-21, 2012 47@ HQL 2012
Asymmetry is a function of ytt and Mtt
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