This review summarizes the program in the physics of the top quark being pursued at Fermilab's Tevatron proton-antiproton collider at a center of mass energy of 1.96 TeV. More than a decade after the discovery of the top quark at the two collider detectors CDF and D0, the Tevatron has been the only accelerator to produce top quarks and to study them directly. The Tevatron's increased luminosity and center of mass energy offer the possibility to scrutinize the properties of this heaviest fundamental particle through new measurements that were not feasible before, such as the first evidence for electroweak production of top quarks and the resulting direct constraints on the involved couplings. Better measurements of top quark properties provide more stringent tests of predictions from the standard model of elementary particle physics. In particular, the improvement in measurements of the mass of the top quark, with the latest uncertainty of 0.7% marking the most precisely measured quark mass to date, further constrains the prediction of the mass of the still to be discovered Higgs boson.

Review of Top Quark Properties Measurements at the Tevatron

In recent years caverns deep underground at Sanford Underground Research Facility (SURF) near Lead, SD, have been proposed to house a large-scale liquid argon detector. This was once intended to be part of the ``Deep Underground Science and Engineering Laboratory" (DUSEL) project to build extensive laboratory space for multidisciplinary scientific research, well-shielded from cosmic rays. The ``Long Baseline Neutrino Experiment" (LBNE) later proposed such a detector at SURF and in 2015 LBNE evolved into the ``Deep Underground Neutrino Experiment" (DUNE). All of these proposals have involved a detector suitable for the study of neutrino oscillation parameters, primarily using a nu_mu / anti_nu_mu beam generated at Fermilab in Batavia, IL. The large target mass also enables a nucleon decay search, as well as the study of other phenomena. Cosmic-ray muons are capable of penetrating deep below the surface of the Earth and causing events that mimic rare signal events. Monte Carlo simulations of the cosmic muon-induced background for proton decay via the channel p \rightarrow K+ anti_nu have been performed, where the signal is the observation of a single K+ track. Cuts have been developed to reject the background K+/K- observed, which has been estimated for a 10 kton (fiducial mass) LBNE/DUNE detector at a depth of 4850 ft and a 40 kton (fiducial mass) detector at 800 ft suitable for a DUSEL cavern. For the former the annual rate of background events for p \rightarrow K+ anti_nu after cuts is <2.3 * 10^{-3} kton^{-1} yr^{-1} (at 90% C.L.). For the latter this is <0.026 kton^{-1} yr^{-1}. The cosmic ray-induced background for nu_e appearance from the nu_mu beam has also been estimated for a near-surface level detector (10 kton fiducial mass) once proposed for LBNE. The signal in this instance is an electromagnetic (EM) shower. After cuts the background has been estimated as 2.88 +/- 0.25 yr^{-1} due to EM showers that start with gamma \rightarrow e+ e-. The upper limits of the annual rates of EM showers starting with K^0_L \rightarrow pi+/pi- e-/e+ anti_nu_e/nu_e in muon, proton and neutron-induced events are < 0.21 yr^{-1}, < 0.01 yr^{-1} and < 0.002 yr^{-1} (at 90% C.L.) after cuts. The upper limits of the annual rates of EM showers starting with pi0 \rightarrow e+ e- gamma in muon, proton and neutron-induced events are < 0.18 yr^{-1}, < 0.01 yr^{-1} and < 0.03 yr^{-1} (at 90% C.L.) after cuts.

Cosmic ray background estimations in large liquid argon detectors for proton decay via p to K+ anti_nu and nu_e appearance from nu_mu beams

In this thesis we consider extending the standard model of particle physics (SM) to include a fourth generation of elementary particles (SM4). The fourth generation would have to be sufficiently heavy to have escaped detection; specifically, its neutrino is required to be kinematically inaccessible to the $Z$ boson in order to agree with the very precise LEP measurements of the $Z$ width. This extension is appealing since the current theory (the SM) exhibits tension with some phenomena observed in nature. Such phenomena are, for example, the replication and number of fermion families, the ratio of matter to antimatter in the observable universe, charge-parity violation and the mixings between the fermions. Up to very recently the issue of the origin of mass, that is, the mechanism of electroweak symmetry breaking in the SM, was lacking experimental verification. However, during the writing of this thesis there have been some very exciting advances in this domain. In July 2012 the CMS and ATLAS collaborations at the LHC have reported the observation of a resonance at $\sim$ 125 GeV, an observation confirmed by experiments at another high-energy particle collider, the Tevatron. This resonance seems to correspond to the Higgs particle, the quantum of the scalar field responsible for the breaking of the electroweak symmetry. Besides verifying the answer to the theoretically fundamental question about the origin of mass, the experimental discovery also serves as a constraint for any theory of particle physics. The goodness of models describing particle physics are generally tested by performing global fits to the data, with the data set usually taken to be the most precisely measured quantities available -- the electroweak precision observables. When the recent Higgs signal strengths are included in the data set, it is seen that the SM4 is not a correct theory of nature. Specifically, the Higgs signals predicted by the SM4 are not in agreement with the data, and the model has in September 2012 been quite decisively excluded at the statistical significance of 5.3$\sigma$. Following the developments in the field we next consider the phenomenological effects of adding another scalar doublet to the previously considered SM4. In the SM and SM4 there is just one scalar (Higgs) doublet: the models have a {\em minimal} scalar sector. The fermion sector, however, is not minimal: there are at least three replicas of a fermion family and so it is possible that the scalar sector is not minimal either. There are in fact arguments in favor of several Higgs doublets, for example supersymmetry and the baryon asymmetry of the universe. Two doublets give rise to five physical particles and so the phenomenology of such models is much richer than in the minimal scenario. Four family-models have received a great deal of interest in the last decade: some 500 articles are reported to have been published concerning their phenomenology during this time. This thesis is a review of the recent developments is this field.

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Particle physics models with four generations

Radiative decays of B mesons, a good example of Flavor Changing Neutral Currents, are interesting for the study of physics beyond the Standard Model, as new high-energy phenomena can enter in the loop. The LHCb experiment, designed to study b- and c-hadron physics at the LHC, is an ideal candidate for the study of this type of decays. However, in order to effectively study radiative B decays, good trigger performance is needed. Two trigger strategies have been studied, one exclusive and one inclusive: on one side, the already existing exclusive lines have been modified, and, on the other side, a new set of inclusive topological lines, specific for radiative B decays, has been introduced. Their performance in 2011 has been assessed both on simulation and on data, and the strategy for 2012 has been outlined. In addition, the full 2011 dataset collected by LHCb has been used to extract the ratio of branching fractions BR(B ? K??)/BR(Bs ? ??), which has been measured to be 1.31 ± 0.08(stat) ± 0.04(syst) ± 0.10(fs/fd). From this result, and using the well-known value for B ? K??, the branching fraction of Bs ? ?? has been found to be (3.3 ± 0.3 )x10?5, largely improving previous results. This work puts the basis for future developments within the radiative decays group in LHCb, as it settles the trigger strategy on one hand and provides a detailed study of the backgrounds involved in the two studied decays, on the other. This will be very useful for future analyses, such as the CP asymmetry measurements in B ? K?? or the photon polarization in Bs ? ??.

First measurements of radiative B decays in LHCb

The Standard Model of particle physics assumes that the world consists of leptons (electrons, muons, taus and neutrinos) and quarks that are elementary particles. We measure that leptons and quarks have a mass hierarchy that could be explained by an underlying substructure of even smaller particles. Then leptons and quarks would be the lowest stable energetic states and higher energy excited states would exist and could be probed experimentally. In this thesis a search for these excited states is described using the data of the ATLAS detector at the Large Hadron Collider (LHC). The analysis is focused on excess production of events containing at least 3 Standard Model leptons that could indicate the presence of excited states. The results of the search for all 6 lepton flavors (e*, μ*, τ*, ν(e)*, ν(μ)*, ν(τ)*) using the 2011 and 2012 ATLAS data sets are presented and perspectives for the next LHC run which started 2015 are shown. Since no hints for excited lepton states were found, limits were set on the mass of the excited leptons and the compositeness scale, Λ. For excited taus and excited neutrinos these limits are the first limits set by any LHC experiment.

B. Pontecorvo

Papers

Neutrino experiments and the question of leptonic-charge conservation.