CDF Logo W+W- Anomalous Triple Gauge Couplings in 3.6 fb-1 of ppbar Collisions at √s=1.96 TeV CDF Logo



The goal of this study is to form limits for anomalous triple gauge couplings on the WWZ vertex. This analysis is an extension of the WW cross section measurement using 3.6 fb^-1 of integrated luminosity. The W+W- final state used will be 2 e,&mu leptons and missing tranverse energy (MET) from the two unobserved neutrinos in lepton decay of the W± boson. At tree level, the W+W- 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+W- 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 leading lepton pT distribution in the observed 654 events in 3.6fb-1 in the cross section times branching fraction measurement of WW->llnunu described in CDF9723 found here. We consider the anomalous triple gauge couplings &lambdaZ, &Delta g1Z, &Delta &kappaZ, &lambda&gamma, &Delta g1&gamma, &Delta &kappa&gamma as defined in PRL D60,113006 and Nucl. Phys. B282, 253 and implemented in MCFM. These 6 parameters are then unified to make 3 independent parameters using the HISZ scheme (Phys Rev D 48, 2182 (1993) )which the limits are set on: &Delta g1Z, &Delta &kappa&gamma and &lambdaZ. 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+s/&Lambda^2)^2. We calculate the limits for two values of &Lambda = 1.5TeV and &Lambda = 2.0TeV.

Summary of Results

Leading Lepton pT Distribution (eps)

Limits, Expected Limits and Probabilities for Obtaining Observed Limits (tex)

Analysis Methods

The leading lepton pT distribution measured for the observed 654 events is fitted for each of the paramaters: &lambdaZ, &Delta g1Z and &Delta &kappa&gamma. The leading lepton 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 leading lepton pT is similar for any given coupling. This was investigated using leading order MCFM interfaced to 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 leading lepton 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 leading lepton pT distribution given any coupling value. Shown below are the various -2logL distributions for the 3 couplings (&lambdaZ, &Delta g1Z, &Delta &kappa&gamma).

-2logL distribution for &lambdaZ with &Lambda=1.5TeV eps

-2logL distribution for &Delta g1Z with &Lambda=1.5TeV eps

-2logL distribution for &Delta &kappa&gamma with &Lambda=1.5TeV eps

-2logL distribution for &lambdaZ with &Lambda=2.0TeV eps

-2logL distribution for &Delta g1Z with &Lambda=2.0TeV eps

-2logL distribution for &Delta &kappa&gamma with &Lambda=2.0TeV eps

Rami Vanguri