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

In this thesis we present a search for the WZ and ZZ production in a final state ("W+2 jets") with a leptonically-decaying W and two energetic jets. We use the full dataset (R Ldt = 8:9 fb-1) recorded with the CDF detector at Fermilab. The challenge consists in extracting the small Z-hadronic peak from the large amount of background processes. Those processes also include the WW, whose hadronic peak cannot be distinguished from the Z peak, due to the poor calorimeter resolution. In the past such a signature was used to measure the diboson cross section, which is highly dominated by the WW cross section ([1]). A theoretical overview of the Standard Model (SM) of particle physics is presented in Chap. 1. In this chapter, we also explain the reasons for measuring the WZ=ZZ production cross section. In Chap. 2 and 3 we describe the accelerator complex and the detector (CDF). In Chap. 4 we describe the standard CDF algorithms for the identification of the final-state objects: electrons, muons, neutrinos, and jets. Since a particular attention in this analysis is given to the sub-sample with jets carrying b-flavor ("b-jets"), we spend Chap. 5 for describing the b-jet identification algorithm used in this analysis. The efficient identification of b-jets was the key for discovering the top quark ([2]) and for achieving evidence in the search of the SM light Higgs boson-like particle at the Tevatron ([3]). The CDF tremendous efforts devoted to develop the optimal b-tagger for the light Higgs searches is reported in App. A. The result of this effort produced the HOBIT b-tagger (App. A.0.5), of which I have been one of the developer 1. Because of detectors inefficiencies and physical effects the measured jet energy may deviate from the primary parton energy. For this reason, the jet energy must be corrected. In Chap. 6 we describe the standard CDF jet corrections. When checking the balancing of the corrected jets against the and Z bosons in the +jet and Z+jet samples, a signi cant discrepancy between data and simulations are found. Under the assumption that this discrepancy originates from di erences in the modeling of the jet response for quarks and gluons, we derive personalized corrections for quark and gluon jets (Sec. 6.2.4). Together with a graduate student of University of Chicago, I derived these corrections: I was in charge of comparing the jet balancing in data and simulation in the full CDF +jet dataset. Before describe the actual measurement, we devote Chap. 7 to describe the basic MonteCarlo (MC) techniques which are exploited in this thesis. Although I am not the author of any of the described MC generators, I think that understanding the details of the MC simulations should be a part of every HEP physicist's education. As preliminary step for measuring the WZ=ZZ cross section, it is imperative to model the W+2 jets dataset really well. Two years ago, the CDF collaboration published a discrepancy in the W+2 jets data which was not described by the theoretical predictions within the known statistical and systematic uncertainties ([117]). We investigated two main systematic e ects, which in control samples were found to spoil the agreement between data and predictions: modeling of the jet energy scale, and of the QCD multi-jets background. After all those e ects were corrected a good agreement between data and predictions was achieved. Details on the procedure to model the W+2 jets dataset are presented in Chap. 8. The effect on each single correction is reported in App. E. In order to accurately measure the WZ=ZZ cross-section, we optimize the dataset to be investigated: a number of studies to do so are described in Chap. 9. We measure the WZ=ZZ cross-section via a simultaneous t to the di-jet mass in the b-jet and light-quark jets enriched samples. We find a modest excess of events in di-jet mass, corresponding to a cross section WZ=ZZ = 4.7 3:0 2:5 pb, which is consistent the SM cross-section (5.1 0.2 pb). Such a result corresponds to a limit of WZ=ZZ < 12:2 pb at 95% C.L. The p-value is 1.4 . Details on the fitting procedure and results are given in Chap. 9. A summary of the results and conclusion remarks are reported in Chap. 10.