MEASUREMENTS AND SEARCHES FOR NEW PHYSICS IN DIBOSON PROCESSES WITH THE DØ DETECTOR by ALEXEY

This dissertation describes a comprehensive study of the Zγ → eeγ/μμγ/νν̄γ process. The data have been collected with the DØ detector at the Fermilab Tevatron pp̄ collider with the center-of-mass energy of 1.96 TeV, and correspond to up to 3.6 fb−1 of integrated luminosity. Firstly, in this study we perform a measurement of the Zγ production cross section. The measured cross sections in both ``γ (` = e or μ) and νν̄γ channels are the most precise cross sections measured at hadron colliders to date, and they agree within uncertainties with the theoretical calculations to the next-to-leading order. We claim the first observation of the Zγ → νν̄γ process at hadron colliders at 5.1 standard deviations statistical significance. We also conduct a search for anomalous Zγ processes. Such anomalous Zγ processes, where a Z boson couples to a photon, result in an increased cross section and an enhancement of the production of high-energetic photons. Thus, an observation of this kind might possibly indicate the presence of new physics. Having found no excess of data over the SM expectations, we set the most restrictive 95% C.L. limits on the anomalous trilinear Zγγ and ZZγ gauge couplings (ATGC) at hadron colliders to date: |hγ30| < 0.033, |h γ 40| < 0.0017, |h30| < 0.033, and |h40| < 0.0017, which mark almost three times improvement over the most recent results published to date. Three out of four current limits improve on the LEP limits, and are the tightest limits on ATGC in the world. Finally, we performed a search for narrow vector and scalar resonances that decay to Zγ. Many extensions of the SM predict such resonances, thus making this search a test of the gauge sector of the SM. We found no significant deviation of data from the SM predictions, and hence we set 95% C.L upper limits on the production cross section of the resonances. MEASUREMENTS AND SEARCHES FOR NEW PHYSICS IN DIBOSON PROCESSES WITH THE DØ DETECTOR by ALEXEY V. FERAPONTOV B.S., Moscow State University, 2002 M.S., Moscow State University, 2004 A DISSERTATION submitted in partial fulfillment of the requirements for the degree DOCTOR OF PHILOSOPHY Department of Physics College of Art and Sciences KANSAS STATE UNIVERSITY Manhattan, Kansas 2009