W-Pair Production in the Process $e^+e^- \to \ell \nu q\bar{q}'$ and Measurement of the $WW\gamma$ and $WWZ$ Couplings

Abstract

We performed a detailed analysis of the process $e^+e^-\to \ell \nu q\bar{q}'$ where we included all tree level Feynman diagrams that contribute to this final state. We studied the sensitivity of this process to anomalous trilinear gauge boson couplings of the $WW\gamma$ and $WWZ$ vertices using two popular parametrizations. We used a maximum likelihood analysis of a five dimensional differential cross-section based on the $W$ and $W$ decay product angular distributions. We concentrated on LEP-200 energies, taking $\sqrt{s}=175$ GeV, and energies appropriate to the proposed Next Linear Collider (NLC), a high energy $e^+e^-$ collider with center of mass energies $\sqrt{s}=500$ and 1~TeV. At 175 GeV, $g_1^Z$ can be measured to about $\pm 0.2$, $\kappa_Z$ to $\pm 0.2$ and $\kappa_\gamma$ to $\pm 0.3$, $\lambda_Z$ to $\pm 0.2$ and $\lambda_\gamma$ to $\pm 0.3$. at 95\% C.L. assuming 500~pb$^{-1}$ integrated luminosity. Although these will be improvements of existing measurements they are not sufficiently precise to test the standard model at the loop level and are unlikely to see deviations from SM expectations. At 500~GeV with 50~fb$^{-1}$ integrated luminosity, $g_1^Z$ can be measured to about $\pm 0.01$, $\kappa_Z$ and $\kappa_\gamma$ to $\pm 0.005$ and $\lambda_Z$ and $\lambda_\gamma$ to $\pm 0.003$ at 95\% C.L. while at 1 TeV with 200~fb$^{-1}$ integrated luminosity, $\kappa_V$ and $\lambda_V$ can be measured to about $\pm 0.005$ and $\pm 10^{-3}$ respectively. The 500~GeV measurements will be at the level of loop contributions to the couplings and may show hints of new physics while the 1~TeV should be sensitive to new physics at the loop level.