口頭発表 @ 物理学会秋季大会 2012.09.13
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On the W+4 jets background to the top-‐quark asymmetry at the Tevatron
Yoshitaro Takaesu KNRC/KIAS, Korea
IntroducGon • 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 mode.
9/12/12 2 JPS Fall 2012: Yoshitaro Takaesu
Event SelecGon &
ReconstrucGon
Event generaGon
9/12/12 4 JPS Fall 2012: Yoshitaro Takaesu
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 SelecGon
9/12/12 5 JPS Fall 2012: Yoshitaro Takaesu
· 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 %
ReconstrucGon of top & anGtop
9/12/12 JPS Fall 2012: Yoshitaro Takaesu 6
Defini>on of Forward-‐Backward Asymmetry of top
9/12/12 JPS Fall 2012: Yoshitaro Takaesu 7
�y = yt � yt̄
= qlep(yt,lep � yt,had)
AFB =N(�y > 0)�N(�y < 0)N(�y > 0) + N(�y < 0)
Results
distribuGon
9/12/12 9 JPS Fall 2012: Yoshitaro Takaesu
) [GeV]tM(t300 400 500 600 700 800 900
Asym
met
ry
-0.05
0
0.05
0.1
0.15tt
W+4j+2jbW+b
[GeV]ttM300 400 500 600 700 800 900
) [fb
/GeV
]t
/ d M
(tσd
-210
-110
1
10tt
W+4j+2jbW+b
distribuGon
9/12/12 10 JPS Fall 2012: Yoshitaro Takaesu
9/12/12 JPS Fall 2012: Yoshitaro Takaesu 11
) [GeV]tM(t300 400 500 600 700 800 900
Asym
metry
-0.05
0
0.05
0.1
0.15ttW+4j
W+4j (PS)+2jbW+b
) [fb
/GeV
]t
/ d M
(tmd
-210
-110
1
10 ttW+4jW+4j (PS)
+2jbW+b
distribuGon with PS
y6-4 -3 -2 -1 0 1 2 3 4
0
100
200
300
400
500 < 450GeVttM
/ d
y [f
b]
md
50
100
150
200
250
300tt
W+4 jetsW+4j +PSW+3j +PSW+2j +PS
> 450GeVttM
9/12/12 JPS Fall 2012: Yoshitaro Takaesu 12
distribuGon with PS
PS
PS
y6-4 -3 -2 -1 0 1 2 3 4
0
100
200
300
400
500
600W+4 jets < 450GeVttM
/ d
y [
fb]
md
50
100
150
200
250
300 > 450GeVttM
9/12/12 13 JPS Fall 2012: Yoshitaro Takaesu
Neutrino Rapidity
Neutrino rapidity has two possibility in this analysis, and this is fixed from the top mass constraint.
Taking right (generated) or wrong rapidity affects on the AFB significantly.
9/12/12 JPS Fall 2012: Yoshitaro Takaesu 14
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 �y
Summary • We inves>gated W+4 jets background on distribu>ons.
• Large is seen in W+4 jets background.
• Neutrino rapidity may affect on the FB asymmetry in W+4jets.
• Although this is a naïve study and far from real experiments, there would be some hints in W+4jets.
9/12/12 15 JPS Fall 2012: Yoshitaro Takaesu
9/12/12 16 JPS Fall 2012: Yoshitaro Takaesu
Back up slides
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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 2
468
101214
right wrong right wrong
> 450GeVttM
tt̄
W4j
tt̄
W4j
Neutrino Rapidity:
distributions of W+4jets are almost the same between right or wrong neutrino rapidity
Mt,lep
Mt,lep distribution
9/12/12 JPS Fall 2012: Yoshitaro Takaesu 19
(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 distribuGon pT
distribuGon
9/12/12 20 JPS Fall 2012: Yoshitaro Takaesu
MadGraph + PYTHIA
9/12/12 JPS Fall 2012: Yoshitaro Takaesu 21
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
9/12/12 JPS Fall 2012: Yoshitaro Takaesu 22
Event SelecGon • # 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 %
9/12/12 23 JPS Fall 2012: Yoshitaro Takaesu
distribuGon
9/12/12 24 JPS Fall 2012: Yoshitaro Takaesu
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
ReconstrucGon of top & anGtop
9/12/12 JPS Fall 2012: Yoshitaro Takaesu 25
• W+4jets can have large AFB at the Parton Shower level.
• However, AFB in W+jets is not observed in 0 b-‐tag sample (background-‐enriched).
9/12/12 JPS Fall 2012: Yoshitaro Takaesu 26
Neutrino rapidity may give a hint for this contradiction.
Why?
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