WZ Anomalous Triple Gauge Couplings

WZ Anomalous Triple Gauge Couplings in 7.1 fb^-1 of ppbar Collisions at √s=1.96 TeV

Authors:
Blessed: Thursday, July 14, 2011
Data: Run II, 7.1/fb

Introduction

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 WZ cross section measurement using 7.1 fb^-1 of integrated luminosity presented here. The WZ final state used will be 3 e,&mu leptons and missing tranverse energy (MET) from the unobserved neutrino in lepton decay of the W. At tree level, the WZ 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 WZ 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 p_T distribution in the observed data events in 7.1fb^-1 in the cross section times branching fraction measurement of WZ->lllnu described here. We consider the anomalous triple gauge couplings &lambda^Z, &Delta g₁^Z, &Delta &kappa^Z 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^Z = &Delta g₁^Z = -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 2.0TeV.

Summary of Results

Z p_T Distribution (eps)

Z p_T Distribution with WWZ off (eps)

Analysis Methods

The Z p_T distribution measured for the observed events in data is fitted for each of the paramaters: &lambda^Z, &Delta g₁^Z, &Delta &kappa^Z. This is done individually with the other two couplings set to 0. The Z p_T distribution is used since it is sensitive to these couplings and it can be measured experimentally.

(eps)

In order to avoid CDF full simulation for every possible coupling it was shown that the efficiency at a given Z p_T is the same 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 Z p_T distribution for each combination of coupling values.

(eps)

A -2log(Likelihood) is then formed for a binned distribution in data to come from an expected Z p_T distribution given any coupling value. Shown below are the various -2log(likelihood) distributions for the 3 couplings (&lambda^Z, &Delta g₁^Z, &Delta &kappa^Z).