Many Beyond Standard Model theories predict particles that couple strongly to top/anti-top quark pairs. This makes tt(bar) a promising signature for new physics searches. The analysis presented in this dissertation uses 20.3 fb⁻¹ of √s = 8 TeV pp collision data from the ATLAS experiment to search for evidence of new physics in the single-lepton plus jets tt(bar) decay channel. This is done by requiring one electron or muon, large missing transverse momentum and additional hadronic jets. The tt(bar) system is reconstructed using techniques optimized for either resolved or boosted event topologies. The results of the analysis are used to set upper limits on the production cross-section times branching ratio for heavy resonances decaying to tt(bar). These limits are interpreted in the context of several benchmark models including the leptophobic topcolor Z', the Randall-Sundrum Kaluza-Klein gluon (gKK), the Randall-Sundrum Kaluza-Klein graviton (GKK) and a spin-0 scalar particle.

Graduate

A search for new physics in events with an energetic jet and large missing transverse momentum is performed with the ATLAS detector, using 2011 dataset corresponding to 4.7 fb−1 integrated luminosity. A model–independent approach is adopted, making predictions in various kinematic regions sensitive to potentially new physics scenarios. Data–driven background determination methods are developed to obtain robust predictions of the Standard Model expectations of the number of events in each probed kinematic region. No deviation from the Standard Model expectation is observed, and the results are hence interpreted in the context of the ADD scenario of Large Extra Dimensions, and pair production of WIMP dark matter candidates. This results in the world’s tightest constraints on the size of the D-dimensional Planck scale as the fundamental parameter of the ADD theory. The constraints obtained on Dark Matter suppression scale are stronger than those obtained from dedicated direct and indirect dark matter experiments for a large range of WIMP masses. Furthermore, data-driven estimates of various Standard Model processes contributing to the mono-jet final state allows a precise direct measurement of the invisible decay width of the Z boson. This results in a measurement at 5% precision level, comparable to the results of the L3 experiment, and better than all other LEP direct measurements.

A search for new physics in events with an energetic jet and large missing transverse momentum is performed with the ATLAS detector, using 2011 dataset corresponding to 4.7 fb−1 integrated luminosity. A model–independent approach is adopted, making predictions in various kinematic regions sensitive to potentially new physics scenarios. Data–driven background determination methods are developed to obtain robust predictions of the Standard Model expectations of the number of events in each probed kinematic region. No deviation from the Standard Model expectation is observed, and the results are hence interpreted in the context of the ADD scenario of Large Extra Dimensions, and pair production of WIMP dark matter candidates. This results in the world’s tightest constraints on the size of the D-dimensional Planck scale as the fundamental parameter of the ADD theory. The constraints obtained on Dark Matter suppression scale are stronger than those obtained from dedicated direct and indirect dark matter experiments for a large range of WIMP masses. Furthermore, data-driven estimates of various Standard Model processes contributing to the mono-jet final state allows a precise direct measurement of the invisible decay width of the Z boson. This results in a measurement at 5% precision level, comparable to the results of the L3 experiment, and better than all other LEP direct measurements.

Diss. Naturwiss. Bern (kein Austausch). / Bibliogr.

Zugl.: Bonn, Univ., Diss., 2004. / Computerdatei im Fernzugriff.

Wuppertal, Univ., Diss., 2005. / Computerdatei im Fernzugriff.

Mannheim, Univ., Diss., 2005. / Erscheinungsjahr an der Haupttitelstelle: 2004. Computerdatei im Fernzugriff.

Dortmund, Univ., Diss., 2001. / Computerdatei im Fernzugriff.

Dortmund, Univ., Diss., 2001. / Computerdatei im Fernzugriff.