talk @ qcd12 2012.07.03
TRANSCRIPT
On the W+4 jets background to the top-‐quark asymmetry at the Tevatron
Yoshitaro Takaesu KEK, Japan
IntroducCon • Large top forward-‐backward asymmetry ( ) has been observed at the Tevatron.
• Contribu>ons of new physics, NNLL QCD, EW correc>ons, NLO of , etc. have been discussed, but we do not know the sa>sfactory answer yet.
• We focus on W+4 jets background for this anomalous in semi-‐leptonic channel.
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Event SelecCon &
ReconstrucCon
Event generaCon
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matrix-‐element level (MadGraph 5)
• PDF: CTEQ 6L1 • Factoriza>on scale = ( ) (20 GeV) (W+4 jets) pjet
Tcut
pp̄� tt̄� bW+b̄W� � l�l bb̄jj
� l�l jjjjpp̄�Wjjjj
Event SelecCon
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· one lepton (e orµ) (pT > 20 GeV, |�| < 2.0)
· at least four jets (pT > 20 GeV, |�| < 2.5)
· highest pjetT > 40 GeV
· one or two b�tagged jets
miss tag prob. = 8 %
ReconstrucCon of top & anCtop
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Defini>on of Forward-‐Backward Asymmetry of top
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�y = yt � yt̄
= qlep(yt,lep � yt,had)
AFB =N(�y > 0)�N(�y < 0)N(�y > 0) + N(�y < 0)
Results
distribuCon
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distribuCon
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y6-4 -3 -2 -1 0 1 2 3 4
0
100
200
300
400
500 ttW+4 jets
< 450GeVttM
/ dy
[fb]
md
50
100
150
200
250
300tt
W+4 jets > 450GeVttM
• W+4jets can have large AFB at the matrix element level.
• However, AFB in W+jets is not observed in background-‐enriched 0 b-‐tag sample.
Why?
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Neutrino rapidity may give a hint for this contradiction.
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Neutrino Rapidity
Neutrino rapidity has two possibility and this is fixed from the top mass constraint.
Taking right (generated) or wrong rapidity affects on the AFB significantly.
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M(lvb) [GeV]100 120 140 160 180 200 220 240 260 280 30002468
1012141618
right wrong right wrong
< 450GeVttM100 120 140 160 180 200 220 240 260 280 300
/ dM
(lvb)
[fb/
GeV]
σd 2468
101214
right wrong right wrong
> 450GeVttM
tt̄
W4j
tt̄
W4j
Neutrino Rapidity: distribu>on Mt,lep
top mass distributions in W+4jets are almost the same between right or wrong neutrino rapidity
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If b-tag introduces a bias to take right neutrino rapidity in Data, W+4jets background could have large AFB in b-tagged events, while 0-tagged events would show smaller AFB.
y6-4 -3 -2 -1 0 1 2 3 4
050
100150200250300350 right or wrong
rightwrong
< 450GeVttM
/ dy
[fb]
md
50
100
150
200
250
300right or wrongrightwrong
> 450GeVttM
W + 4jet
W + 4jet
Neutrino Rapidity: distribution �yOne possibility
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(top) [GeV]T
p0 50 100 150 200 250 300 350 400
0123456789
10
ttW+4 jets
< 450GeVttM
(top)
[fb/
GeV]
T /
dpmd
123456789
10
ttW+4 jets
> 450GeVttM
top distribuCon pT
distribuCon
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MadGraph + PYTHIA
Summary • We inves>gated W+4 jets background on , and distribu>ons.
• Large is seen in W+4 jets background.
• Anomaly in by D0 may suggests BG contamina>ons in signal than expected.
• Although this is a naïve matrix-‐element-‐level study and far from real experiments, it would be helpful to re-‐evaluate W+4 jets BG at Tevatron.
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Back up slides
distribuCon
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y6-4 -3 -2 -1 0 1 2 3 4
0
100
200
300
400
500 ttrightwrongW+4jrightwrong
< 450GeVttM
/ dy
[fb]
md
50100150200250300350
ttrightwrongW+4jrightwrong
> 450GeVttM
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Event SelecCon • # lepton ( e or mu ) = 1 ( pT > 20 GeV, |eta|<2.0 )
• Missing ET > 20 GeV
• # jets >= 4 ( pT > 20 GeV, |eta|<2.5 )
• delta Rjl > 0.4, delta Rjj > 0.4, delta Rll > 0.4
• # b-‐tagged jet = 1 or 2 ( pT > 20 GeV, |eta|<2.5) • b-‐tag efficiency = 70% • miss-‐b-‐tag prob. = 8 %
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ReconstrucCon of top & anCtop
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