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W±Z0
Anomalous Triple Gauge Couplings in
1.9 fb-1 of ppbar Collisions at √s=1.96 TeV
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- Authors:
- Data:
Run II, 1.9/fb
Contents
Introduction
The goal of this study is to form limits for anomalous
triple gauge couplings on the WWZ vertex. The analysis will
used previously published cross section results from PRL 98, 161801.
The W±Z0 final state used will be
3 e,&mu leptons and missing tranverse energy (MET) from the
unobserved neutrino in lepton decay of the W± boson.
At tree level, the W±Z0 final state has the
s-channel (left) and t-channel (right) contributions shown in
these diagrams:
The s-channel diagram provides sensitivity to the WWZ vertex coupling.
The W±Z0 final state is unique among
the triple gauge coupling (TGC) final states, providing access to this
coupling separately from the WW&gamma coupling. The analysis of triple
gauge couplings in WZ production is based on analysing the Z pT
distribution in the observed 25 events in 1.9fb-1 in the
cross section times branching fraction measurement of WZ->lllnu described
in CDF8539 and PRL 98, 161801. We consider the anomalous triple gauge
couplings lambda, delta g, delta kappa as defined in PRL D60,113006 and
Nucl. Phys. B282, 253 and implemented in MCFM. In the Standard Model
all three of these couplings are zero. Additionally the values delta
kappa = delta g = -1 turns off the WWZ vertex leaving only the t-channel
production and a cross section that violates unitarity. To avoid this
unitarity violation the WWZ vertex is modified by a form factor that
implements a cut-off by multiplying the coupling with 1/(1+shat/Lambda^2)^2.
We calculate the limits for two values of Lambda = 1.5TeV and 2.0TeV.
Summary of Results
Analysis Methods
The Z-pT distribution measured for the observed 25 events is fitted for
each of the paramaters: lambda, delta g, delta kappa. This is done
individually as well as two dimensional pairs. The Z-pT distribution is
used since it is sensitive to these couplings and it can be measured
experimentally.
In order to avoid CDF full simulation for every possible
coupling it was shown that the efficiency at a given Z-pT is the same for
any given coupling. This was done using leading order MCFM hadronization
via Pythia followed by full realistic Monte Carlo detector simulation
using cdfSim and standard reconstruction. The resulting efficiency curve
is then applied to MCFM next to leading order matrix element simulations
of a given coupling combination to arrive at an expected observed Z-pT
distribution for each combination of coupling values.
A -2log(Likelihood) is then formed for a binned distribution in data
to come from an expected Z pT distribution given any coupling
value. Shown below are the various -2log(likelihood) distributions
for the 3 couplings (lambda, delta g, delta kappa). The red curve
represents the systematically varied -2log(likelihood) while the
black curve is without systematics.
-2log(likelihood) distribution for lambda with /\=1.5TeV
-2log(likelihood) distribution for delta g with /\=1.5TeV
-2log(likelihood) distribution for delta kappa with /\=1.5TeV
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-2log(likelihood) distribution for lambda with /\=2.0TeV
-2log(likelihood) distribution for delta g with /\=2.0TeV
-2log(likelihood) distribution for delta kappa with /\=2.0TeV
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Table of Results for 1-dimensional Limits:
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/\=1.5
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lambda
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delta g
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delta kappa
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With Systematics:
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-0.14 < lambda < 0.16
delta g = delta kappa = 0
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-0.17 < delta g < 0.27
lambda = delta kappa = 0
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-0.86 < delta kappa < 1.36
delta g = lambda = 0
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/\=2.0
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lambda
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delta g
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delta kappa
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With Systematics:
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-0.13 < lambda < 0.14
delta g = delta kappa = 0
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-0.15 < delta g < 0.24
lambda = delta kappa = 0
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-0.82 < delta kappa < 1.27
delta g = lambda = 0
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Shown below are the systematically varied and nominal contours (inner curve is statistical only while outer curve is statistical+systematic). Both are at 95% confidence level.
delta g vs lambda 2-dimensional contour /\=2.0TeV central value=(0.07,0.01)
delta g vs delta kappa 2-dimensional contour /\=2.0TeV central value=(-0.17,0.05)
delta kappa vs lambda 2-dimensional contour /\=2.0TeV central value=(-0.16,0.02)
Rami Vanguri
