This document investigates the W+4 jets background to the top quark forward-backward asymmetry at the Tevatron. It finds a large forward-backward asymmetry in the W+4 jets background simulation. The choice of neutrino rapidity in the event reconstruction can significantly impact the asymmetry. While a simple study, it provides hints that the W+4 jets process could contribute to the observed top quark asymmetry.

1. On
the
W+4
jets
background
to
the
top-­‐
quark
asymmetry
at
the
Tevatron
Yoshitaro
Takaesu
KNRC/KIAS,
Korea

2. 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

3. Event
SelecGon
&
ReconstrucGon

4. 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
T cut
p¯p t¯t
bW+¯bW l l b¯bjj
l l jjjjp¯p Wjjjj

5. Event
SelecGon
9/12/12
5
JPS
Fall
2012:
Yoshitaro
Takaesu
· one lepton (e or µ) (pT > 20 GeV, | | < 2.0)
· at least four jets (pT > 20 GeV, | | < 2.5)
· highest pjet
T > 40 GeV
· one or two b tagged jets
miss tag prob. = 8 %

6. ReconstrucGon
of
top
&
anGtop
9/12/12
JPS
Fall
2012:
Yoshitaro
Takaesu
6

7. Defini>on
of
Forward-­‐Backward
Asymmetry
of
top
9/12/12
JPS
Fall
2012:
Yoshitaro
Takaesu
7
y = yt y¯t
= qlep(yt,lep yt,had)
AF B =
N( y > 0) N( y < 0)
N( y > 0) + N( y < 0)

8. Results

9.
distribuGon
9/12/12
9
JPS
Fall
2012:
Yoshitaro
Takaesu
) [GeV]tM(t
300 400 500 600 700 800 900
Asymmetry
-0.05
0
0.05
0.1
0.15
tt
W+4j
+2jbW+b
[GeV]tt
M
300 400 500 600 700 800 900
)[fb/GeV]t/dM(tσd
-2
10
-1
10
1
10
tt
W+4j
+2jbW+b

10.
distribuGon
9/12/12
10
JPS
Fall
2012:
Yoshitaro
Takaesu

11. 9/12/12
JPS
Fall
2012:
Yoshitaro
Takaesu
11
) [GeV]tM(t
300 400 500 600 700 800 900
Asymmetry
-0.05
0
0.05
0.1
0.15
tt
W+4j
W+4j (PS)
+2jbW+b
)[fb/GeV]t/dM(td
-2
10
-1
10
1
10 tt
W+4j
W+4j (PS)
+2jbW+b
distribuGon
with
PS

12. y
-4 -3 -2 -1 0 1 2 3 4
0
100
200
300
400
500
< 450GeVtt
M
/dy[fb]d
50
100
150
200
250
300
tt
W+4 jets
W+4j +PS
W+3j +PS
W+2j +PS
> 450GeVtt
M
9/12/12
JPS
Fall
2012:
Yoshitaro
Takaesu
12
distribuGon
with
PS
PS
PS
y
-4 -3 -2 -1 0 1 2 3 4
0
100
200
300
400
500
600
W+4 jets
< 450GeVtt
M
/dy[fb]d
50
100
150
200
250
300
> 450GeVtt
M

13. 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.

14. 9/12/12
JPS
Fall
2012:
Yoshitaro
Takaesu
14
y
-4 -3 -2 -1 0 1 2 3 4
0
50
100
150
200
250
300
350 right or wrong
right
wrong
< 450GeVtt
M
/dy[fb]d
50
100
150
200
250
300
right or wrong
right
wrong
> 450GeVtt
M
W + 4jet
W + 4jet
Neutrino Rapidity: distributiony

15. 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

16. 9/12/12
16
JPS
Fall
2012:
Yoshitaro
Takaesu

17. Back
up
slides

18. 9/12/12
JPS
Fall
2012:
Yoshitaro
Takaesu
18
M(lvb) [GeV]
100 120 140 160 180 200 220 240 260 280 300
0
2
4
6
8
10
12
14
16
18
right
wrong
right
wrong
< 450GeVtt
M
100 120 140 160 180 200 220 240 260 280 300
/dM(lvb)[fb/GeV]σd
2
4
6
8
10
12
14
right
wrong
right
wrong
> 450GeVtt
M
t¯t
W4j
t¯t
W4j
Neutrino
Rapidity:
distributions
of W+4jets are almost
the same between
right or wrong neutrino
rapidity
Mt,lep
Mt,lep distribution

19. 9/12/12
JPS
Fall
2012:
Yoshitaro
Takaesu
19
(top) [GeV]
T
p
0 50 100 150 200 250 300 350 400
0
1
2
3
4
5
6
7
8
9
10
tt
W+4 jets
< 450GeVtt
M
(top)[fb/GeV]
T
/dpd
1
2
3
4
5
6
7
8
9
10
tt
W+4 jets
> 450GeVtt
M
top
distribuGon
pT

20.
distribuGon
9/12/12
20
JPS
Fall
2012:
Yoshitaro
Takaesu
MadGraph
+
PYTHIA

21. 9/12/12
JPS
Fall
2012:
Yoshitaro
Takaesu
21
y
-4 -3 -2 -1 0 1 2 3 4
0
100
200
300
400
500 tt
right
wrong
W+4j
right
wrong
< 450GeVtt
M
/dy[fb]d
50
100
150
200
250
300
350
tt
right
wrong
W+4j
right
wrong
> 450GeVtt
M

22. 9/12/12
JPS
Fall
2012:
Yoshitaro
Takaesu
22

23. 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

24.
distribuGon
9/12/12
24
JPS
Fall
2012:
Yoshitaro
Takaesu
y
-4 -3 -2 -1 0 1 2 3 4
0
100
200
300
400
500
tt
W+4 jets
< 450GeVtt
M
/dy[fb]d
50
100
150
200
250
300
tt
W+4 jets
> 450GeVtt
M

25. ReconstrucGon
of
top
&
anGtop
9/12/12
JPS
Fall
2012:
Yoshitaro
Takaesu
25

26. • 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?