Doctor of Philosophy / Department of Physics / Glenn Horton-Smith / In March 2012, the Double Chooz reactor neutrino experiment published its most precise result so far: sin[superscript]2 2theta13 = 0.109 +/- 0.030(stat.) +/- 0.025(syst.). The statistical significance is 99.8% away from the no-oscillation hypothesis. The systematic uncertainties from background and detection efficiency are smaller than the first publication of the Double Chooz experiment. The neutron detection efficiency, one of the biggest contributions in detection systematic uncertainties, is a primary topic of this dissertation. The neutron detection efficiency is the product of three factors: the Gd-capture fraction, the efficiency of time difference between prompt and delayed signals, and the efficiency of energy containment. [superscript]252 Cf is used to determine the three factors in this study. The neutron detection efficiency from the [superscript]252 Cf result is confirmed by the electron antineutrino data and Monte Carlo simulations. The systematic uncertainty from the neutron detection efficiency is 0.91% used in the sin[superscript]2 2theta13 analysis. The seasonal variation in detector performance and the seasonal variations of the muon intensity are described in detail as well. The detector stability is confirmed by observation of two phenomena: 1) the [electron antineutrino] rate, which is seen to be uncorrelated with the liquid scintillator temperature, and 2) the daily muon rate, which has the expected correspondence with the effective atmospheric temperature. The correlation between the muon rate and effective atmospheric temperature is further analyzed in this thesis to determine the ratio of kaon to pion in the local atmosphere. An upper limit on instability of the neutron detection efficiency is established in the final chapter. The systematic error, 0.13%, from the relative instability is the deviation of the calibration runs. This thesis concludes with the potential systematic errors of neutron detection efficiency and estimation of how these potential systematic errors affect the result of sin[superscript]2 2theta13.

Using 1.0 fb^{-1} of data collected by the Collider Detector at Fermilab (CDF) from Run II of the Fermilab Tevatron, we measure the top-antitop production cross-section in events with two leptons, significant missing transverse energy, and at least jets, at least one of which is identified as a b-jet. As the Run II dataset grows, more stringent tests of Standard Model predictions for the top quark sector are becoming possible. The dilepton channel, where both top quarks decay t-> W b ->l nu b, is of particular interest due to its high purity. Use of an isolated track as the second lepton significantly increases the dilepton acceptance, at the price of some increase in background, particularly from W + jets events where one of the jets is identified as a lepton. To control the increase in background we add to the event selection the requirement that at least one of the jets be identified as a b-jet, reducing the background contribution from all sources. Assuming a branching ratio of BR(W->l nu) = 10.8% and a top mass of m_top = 175 GeV/c^{2} the measured cross-section is sigma = (10.5 +/- 1.8 stat. +/- 0.8 syst. +/- 0.6 lumi.) pb.

A measurement of the azimuthal decorrelation in di-jet events in proton-proton collisions at $\sqrt{s}= 7\,$TeV is presented. Using $19.6\,$nb$^{-1}$ of data collected in the ATLAS detector this measurement uses the angle ($\Delta\phi$) in the transverse plane between the two leading $p_T$ jets to measure the normalized differential cross section $\frac{1}{\sigma_{tot}}\frac{d\sigma}{d(\Delta\phi)}$. An unfolding correction is a applied to give a jet-level result. The unfolded spectrum is compared to the predictions of two Monte Carlo event generators: Pythia and Herwig++.

Using 1.0 fb^{-1} of data collected by the Collider Detector at Fermilab (CDF) from Run II of the Fermilab Tevatron, we measure the top-antitop production cross-section in events with two leptons, significant missing transverse energy, and at least jets, at least one of which is identified as a b-jet. As the Run II dataset grows, more stringent tests of Standard Model predictions for the top quark sector are becoming possible. The dilepton channel, where both top quarks decay t-> W b ->l nu b, is of particular interest due to its high purity. Use of an isolated track as the second lepton significantly increases the dilepton acceptance, at the price of some increase in background, particularly from W + jets events where one of the jets is identified as a lepton. To control the increase in background we add to the event selection the requirement that at least one of the jets be identified as a b-jet, reducing the background contribution from all sources. Assuming a branching ratio of BR(W->l nu) = 10.8% and a top mass of m_top = 175 GeV/c^{2} the measured cross-section is sigma = (10.5 +/- 1.8 stat. +/- 0.8 syst. +/- 0.6 lumi.) pb.

A measurement of the azimuthal decorrelation in di-jet events in proton-proton collisions at $\sqrt{s}= 7\,$TeV is presented. Using $19.6\,$nb$^{-1}$ of data collected in the ATLAS detector this measurement uses the angle ($\Delta\phi$) in the transverse plane between the two leading $p_T$ jets to measure the normalized differential cross section $\frac{1}{\sigma_{tot}}\frac{d\sigma}{d(\Delta\phi)}$. An unfolding correction is a applied to give a jet-level result. The unfolded spectrum is compared to the predictions of two Monte Carlo event generators: Pythia and Herwig++.

Doctor of Philosophy / Department of Physics / Timothy A. Bolton / This dissertation describes a search for the pair production of scalar top quarks, tilde[subscript] 1, using a luminosity of 995 pb[superscript-1] of data collected in p[bar over]p collisions with the D0 detector at the Fermilab Tevatron Collider at a center-of-mass energy sqrt[s] = 1.96 TeV. Both scalar top quarks are assumed to decay into a charm quark and a neutralino, [tilde chi superscript 0 subscript 1]where [(tilde chi superscript 0 subscript 1)]is the lightest supersymmetric particle. This leads to a final state with two acoplanar charm jets and missing transverse energy. The yield of such events in data is found to be consistent with the expectations from known standard model processes. Sets of tilde [subscript 1] and tilde chi [superscript 0 subscript 1] masses are excluded at the 95% confidence level that substantially extend the domain excluded by previous searches. With the theoretical uncertainty on the pair production cross section taken into account, the largest limit for m[subscript tilde subscript 1] is m[subscript tilde chi subscript 1> 150 GeV, for m [subscript tilde chi negative 0 subscript 1]= 65 GeV.

Doctor of Philosophy / Physics / Andrew G. Ivanov / In this thesis the measurement of production cross section of top-quark pairs in association with a photon in proton-proton collisions at a center of mass energy of 8 TeV is presented. The data was recorded at the CMS experiment at the LHC in 2012. This measurement aims to extend our knowledge of top quark properties and help to test consistency of the Standard Model (SM) of particle physics. Data-driven methods are used to estimate the photon identification efficiency and purity. The measured cross-section agrees with the standard model expectation.

Doctor of Philosophy / Department of Physics / Yurii Maravin / This dissertation describes a comprehensive study of the Zgamma [to] eegamma/mumugamma/nu[bar]nugamma process. The data have been collected with the D0 detector at the Fermilab Tevatron p[bar]p collider with the center-of-mass energy of 1.96 TeV, and correspond to up to 3.6 fb[superscript]-1 of integrated luminosity. Firstly, in this study we perform a measurement of the Zgamma production cross section. The measured cross sections in both llgamma (l = e or mu) and nu[bar]nugamma 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 Zgamma [to] nu[bar]nugamma process at hadron colliders at 5.1 standard deviations statistical significance. We also conduct a search for anomalous Zgamma processes. Such anomalous Zgamma 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 Zgammagamma and ZZgamma gauge couplings (ATGC) at hadron colliders to date: |h[subscript]30[superscript]gamma| < 0.033, |h[subscript]40[superscript]gamma| < 0.0017, |h[subscript]30[superscript]Z| < 0.033, and |h[subscript]40[superscript]Z| < 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 Zgamma. 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.

Doctor of Philosophy / Department of Physics / Glenn A. Horton-Smith / The next generation of nuclear fission reactor based neutrino experiments seeking to measure the Theta-13 mixing angle rely upon measurements made by detectors placed close to the reactor, and therefore less shielded from cosmic ray muons by the earth. 9Li production in liquid scintillator by these cosmic ray muons becomes a serious problem for these experiments that must be dealt with since the 9Li production rate is still a significant fraction of the neutrino interaction rate. This 9Li background reduces the experiment’s sensitivity to measure the Theta-13 mixing angle. This thesis discusses a small detector designed to measure the 9Li production rate in liquid scintillator at the earth’s surface by cosmic ray muons. The detector was designed, built, and finally, calibrated. The ability to find the signals necessary to actually measure the 9Li production rate is shown, establishing that this detector will be able to measure the production rate. A 90% significance level upper limit for the 9Li-like production rate, based on only 3.5 days worth of data, is reported as 213 9Li-like events per day per ton.

Doctor of Philosophy / Department of Physics / Yurii Maravin / In this Dissertation I present a detailed study of ppbar->WZ production using fully leptonic decays of W and Z bosons with electrons and muons in the final state. Data used for the study were collected by the D0 detector at the Fermilab proton-antiproton collider with a center-of-mass energy of 1.96 TeV and correspond to 4.1 fb[surepscript]-1 of integrated luminosity. The most precise measurement of the WZ production cross section is obtained and found to be in a good agreement with the standard model prediction. I also present a search for new phenomena in the WZ production by investigating the coupling between W and Z bosons and by searching for new charged particles that can decay into WZ boson pair. No evidence for new physics is found, and the most stringent limits are set on the anomalous WWZ coupling parameters and masses of charged resonances. This result also sets the stringest limit on one of the possible sources of electroweak symmetry breaking, a low-scale Technicolor with a typical heavy techni-pion hypothesis.