Search for $WZ/ZZ$ Production in the Lepton(s) + MET + Jets Channel with the CDF Experiment at the Tevatron Collider

Abstract

In this thesis we present a search for $WZ$ and $ZZ$ production in a final state with a leptonically-decaying $W$ and two energetic jets ($W$+2jets). We use the full dataset ($\int \mathcal{L} dt=8.9$ $fb^{-1}$) recorded with the collider detector at Fermilab (CDF). The challenge consists in extracting the small $Z$-hadronic peak from the large amount of background processes, including the irreducible $WW$ background. We develop personalized corrections for simulated quark and gluon jets, and an appropriate tuning of the multi-jets-background model. Once these corrections are incorporated, the CDF $W$+2jets data is well described by the standard-model expectations. A 95$\%$ confidence level upper limit is set on the cross section of a $W$ boson produced in association with a new particle decaying into two jets and with a mass of 144 GeV/$c^2$. We also measure the $WZ/ZZ$ cross-section via a simultaneous fit to the di-jet mass in the $b$-jet and light-quark jets enriched samples by using an optimized multivariate discriminant. We find an excess of events in di-jet mass, corresponding to a cross section $\sigma_{WZ/ZZ}=$ 4.7 $\pm^{3.0}_{2.5}$ pb, which is consistent with the SM cross-section (5.1$\pm$0.2 pb). Such a result corresponds to a limit of $\sigma_{WZ/ZZ} < 12.2$ pb at 95$\%$ C.L.