Study of Z Boson Pair Production and Search for Physics beyond Standard Model at LEP-II

The LEP collider at CERN, with its large data sample collected at energies that were never previously attained in e+e- collisions, is an ideal laboratory to test the predictions of the Standard Model (SM). One of the major areas to be explored at LEP is the self-interaction among the gauge bosons envisaged, due to the non-abelian nature of the SU(2)_LXU(1)_Y gauge group associated with the SM. The reaction e+e- -> ZZ, in particular, provides an ideal opportunity to test this theory by verifying that the anomalous couplings, V_ZZZ and V_ZZgamma, are absent at the tree level. Possible deviation from the SM expectation, in the form of a change in the measured Z-pair production cross section or a modification to the angular distribution of the Z bosons, would be a sign of new physics. Another strong motivation for studying this process is in connection with the production of the Higgs boson at LEP. The Higgs boson, the manifestation of spontaneous breaking of the underlying gauge symmetry, is responsible for generating the masses of the gauge bosons and the chiral fermions. The process e+e- -> ZZ forms the most dominant and irreducible background to the Higgs search at LEP. Events in which at least one Z boson decays into b quarks have a signature similar to that of the process e+e- -> H(-> bb-bar)Z(-> ff-bar). Thus by studying this final state in the Z-pair events, one can validate the experimental procedures used in the search for the Higgs particle. Z-pair production also offers an interesting test of theories beyond Standard Model, such as those with extra space dimensions. This thesis presents a study of the Z boson pair-production where one Z decays to qq-bar, leading to two hadronic jets, and the other Z to a pair of leptons. In a data sample of 682 pb^-1 integrated luminosity collected with the L3 detector (one of the four multi-purpose collider experiments at LEP) at center-of-mass energies ranging from 183 GeV up to 208 GeV, a total of 71 Z-pair events are detected. By performing a maximum likelihood fit to the invariant mass distribution of the Z bosons in the selected events, the pair-production cross sections are measured at several energy points, and are found to be in agreement with the SM predictions. Measurement of a b-quark enriched subsample of the ZZ events shows a similar trend and no significant evidence has been found to support the expectations from theories with extra space dimensions.