STUDY OF ATLAS SENSITIVITY TO Wtb

STUDY OF ATLAS SENSITIVITY TO Wtb
ANOMALOUS COUPLINGS IN tt̄ EVENTS
J.A. Aguilar-Saavedra
Departamento de Física Teórica y del Cosmos and CAFPE, Universidad de Granada
J. Carvalho, N. CASTRO, A. Onofre and F. Veloso
LIP - Departamento de Física, Universidade de Coimbra
The ATLAS experiment sensitivity to Wtb anomalous couplings in top pair production with semileptonic decay (pp → t t̄ → W + bW − b̄, with one of the W bosons decaying leptonically and the other
hadronically) is presented. In particular, the sensitivity for the first fb−1 of data is discussed using
analyses with and without b-tagging. Different observables are examined, namely the W helicity ratios and angular asymmetries. Expected limits on the anomalous couplings are obtained
through the combination of the most sensitive observables.
most general Wtb vertex (top can be safely assumed on-shell):
how to measure the Wtb anomalous couplings?
g
L = − p b̄ γμ (VL PL + VR PR ) t Wμ−
2
g σ μν qν
(gL PL + gR PR ) t Wμ− + h.c.
− p b̄
MW
2
leptonic b
SM
VL=1 and gR=gL=VR=0
0.7
b
F0=Γ0/Γtot
t
ν
W
e (µ)
FL=ΓL/Γtot
b
Fi
W
0.4
➟
AFB
0.3
0.73
0.33
0.72
0.71
gR
F0 0.7
0.004
0.29
0.68
0.28
0
0.05
0.1
0.15
FR
0.2
gL
0.27
-0.2 -0.15 -0.1 -0.05
0
0.05
coupling
0.1
0.15
-0.006
-0.2 -0.15 -0.1 -0.05
0.2
PSfrag replacements
0.05
0.1
0.15
0.2
Probabilistic analysis (LR = LS / LB ):
0
analysis with b-tag (L=1 fb−1 )
presel. 3159 ± 149
back. final sel.
1072 ± 99
trigger
851 ± 94
presel. 13801 ± 107
signal final sel.
7984 ± 82
trigger
6586 ± 74
e+µ (b-tag)
800
700
500
ATLAS
L = 1 fb-1
e+µ (no b-tag)
600
e+µ (b-tag)
100
100
400
500
0
-3
l
200
-2
-1
—
0
1
2
3
LR = log (L /LB)
signal
10
S
0
-3
0
-1 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 1
cos θ*
l
corrected cos θ∗
(b-tag analysis):
ℓ
-2
-1
0
background
1
2
3
LR = log (L /LB)
10
S
final selection: LR > 0.1
correction factor
1000
0
-1 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 1
cos θ*
7
6
ATLAS
L = 1 fb-1
e+µ
600
5
0.5
0.75
1
{
ρR
AFB
A+
A−
Source
ρL
ρR
AFB
A+
A−
jet energy scale
luminosity
top mass
background
ISR+FSR
MC generator
pile-up
0.04
0.01
0.03
0.01
0.05
0.01
0.15
0.00
0.00
0.00
0.00
0.01
0.01
0.01
0.01
0.01
0.01
0.03
0.02
0.01
0.01
0.00
0.01
0.01
0.00
0.03
0.01
0.04
0.00
0.00
0.01
0.00
0.02
0.00
0.02
ATLAS
L = 1 fb-1
700
e+µ (b-tag)
0.25
systematic uncertainties (b-tag analysis)
200
200
0
0.45 ± 0.05 0.00 ± 0.01 -0.23 ± 0.03 0.54 ± 0.03 -0.83 ± 0.01
300
1500
-0.25
AFB = −0.223 (SM)
angular
A+ = 0.548 (SM)
asymmetries
A− = −0.840 (SM)
ρL = FL / F0 = 0.423 (SM)
∗
cos θℓ fit
ρR = FR / F0 = 5.1 × 10−4 (SM)
ρL
300
400
-0.5
observables ± stat error (b-tag analysis, L=1 fb−1 )
600
500
-0.75
➟
{
ATLAS
L = 1 fb-1
400
-1
➟
A-
cosθ*l
`b
reconstructed cos θ∗
:
ℓ
ATLAS
L = 1 fb-1
800
0.1
(W rest frame)
900
1000
A+
ν
FR
1 lepton (pT > 25(20) GeV for e(μ); |η| < 2.5)
≥ 4 jets (pT > 30 GeV; |η| < 2.5)
MET > 20 GeV
ΔR between reconstructed objects > 0.4
e+µ (no b-tag)
0
coupling
FL
ATLAS
L = 1 fb-1
gR
0.2
b
W
θ`∗ = π − θ`b
-0.004
Preselection:
2000
t
-0.002
F0
2500
gL
0
0.69
coupling
VR
0.002
VR
FL 0.3
gL
0.67
-0.2 -0.15 -0.1 -0.05
0.006
0.31
VR
θ
0.008
0.32
gR
jets from W (non b’s)
`
➟
anomalous couplings ⇒ deviations in W helicity fractions:
q
➟
q
FR=ΓR/Γtot
(rescaled)
0.5
t
➟
hadronic b
0.6
Total
(b-tag)
0.16 0.01 0.03 0.05 0.03
500
4
400
3
300
2
200
1
100
0
-1 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 1
cos θ*
l
ρ = 0.453 ± 0.048
L
ρ = -0.004 ± 0.007
R
0
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
cos θ*l
A+ , A− , ρL , ρR combined using TopFit
The expected 68.3% CL confidence regions on the anomalous couplings for
L=1 fb−1 are shown. In addition to
the allowed regions in the Figures,
additional solutions can be found at
gR ∼ 0.8. Such solutions are due to a
large cancellation between O(gR ) and
O(g2R ) terms and can be excluded by the
measurement at the LHC of the single
top cross-section.
For comparison, the constraints derived from the CDF measurement of
the W polarization fractions are also
shown.
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