This dissertation describes a search for the Standard Model Higgs boson in Z+photon channel with the ATLAS detector at the LHC using 4.6 fb⁻¹ of proton-proton collisions at center of mass energy 7 TeV in 2011 and 20.7 fb⁻¹ of proton-proton collisions at center of mass energy 8 TeV in 2012. The distribution of the mass difference between M_Zphoton and M_Z is compared to the Standard Model (SM) background expectations. No significant deviation from the SM prediction is observed and the upper limits on the signal strength μ of a Higgs boson with a mass between 120 and 150 GeV are derived. The expected exclusion limits at 95% confidence level range between 7.3 and 22.3 times the predicted Standard Model cross section. The observed exclusion limits range between 5.3 and 38.7 times the Standard Model cross section. For a Higgs boson with mass of 125.5 GeV, the expected and observed limits are 13.2 and 17.0 times the Standard Model respectively.

A search for photons originating in the decay of a neutral long-lived particle produced in proton-proton collisions at sqrt(s) = 7 TeV is presented. The search was performed in the diphoton plus missing transverse energy final state, using the full data sample of 4.8 fb-1 of 7 TeV proton-proton collisions collected in 2011 with the ATLAS detector at the CERN Large Hadron Collider. The analysis exploits the capabilities of the ATLAS electromagnetic calorimeter to make precise measurements of the flight direction of photons, and utilizes the excellent time resolution of the calorimeter as an independent cross-check of the results. The search was conducted in the context of Gauge Mediated Supersymmetry Breaking models, where the lightest neutralino is the next-to-lightest supersymmetric particle and has a finite lifetime. In the family of models investigated, supersymmetric particles are produced in pairs due to R-parity conservation, eventually decaying to a pair of neutralinos, each subsequently decaying to a photon and a gravitino. The gravitinos escape the detector, giving rise to missing energy, while the photons can appear not to originate from the primary vertex of the event, and are measured with a delay with respect to the collision time. No excess was observed above the background expected from Standard Model processes. The results were used to set exclusion limits at 95% CL in the two-dimensional parameter space defined by the supersymmetry breaking scale and the lifetime of the lightest neutralino.

We present a search for electron antineutrino disappearance at the Chooz nuclear power plant in Chooz, France. Using the Double Chooz far detector and 101.5 days of detector run time, we measure sin^2(2θ(subscript 13)) = 0.086 ± 0.041 (stat.) ± 0.030 (syst.) from a rate and shape fit. A combined analysis of T2K and Double Chooz data finds that sin^2(2θ(subscript 13)) = 0 is excluded at the 3σ level.

A vacuum, a classically stable state, can decay to another vacuum by virtue of quantum tunneling. Although vacuum decay is an interesting topic itself in field theory, when it combines with gravity, it has wider applications and plays an essential role to understand the very early universe. The semi-classical solution of vacuum decay is well-described by the WKB approximation both in flat space and in curved space. Meanwhile, if we consider the configuration space of fields, we encounter many kinds of problems related with gravitational fields since the mode spectrum in curved space turns out to have an infinite number of negative modes in a de Sitter background. Despite of infinitely many negative modes, the regime in the weak gravity limit can be smoothly connected to flat space. To understand this, I discuss the nature of vacuum decay and various topics about negative modes in this thesis.

The K_L-K_S mass difference is a promising quantity to reveal new phenomena which lie outside the standard model. A state-of-art perturbation theory calculation has be en performed at next-to-next-to-leading order (NNLO) and a 40% error is quoted in the final result. We develop and demonstrate non-perturbative techniques needed to calculate the K_L-K_S mass difference, ΔM_K, in lattice QCD and carry out exploratory calculations. The calculations are performed on a 2+1 flavor, domain wall fermion, 16³ x 32 ensemble with a 421 Mev pion and a 24³ x 64 lattice ensemble with a 329 MeV pion. In the $16^3$ lattice calculation, we drop the double penguin diagrams and the disconnected diagrams. The short distance part of the mass difference in a 2+1 flavor calculation contains a quadratic divergence cut off by the lattice spacing. Here, this quadratic divergence is eliminated through the Glashow-Iliopoulos-Maiani (GIM) mechanism by introducing a quenched charm quark. We obtain a mass difference ΔM_K which ranges from 6.58(30) x 10⁻¹² MeV to 11.89(81) x 10⁻¹² MeV for kaon masses varying from 563 MeV to 839 MeV. On the 24³ lattice, we include all the diagrams and perform a full calculation. Our result is for a case of unphysical kinematics with pion, kaon and charmed quark masses of 330, 575 and 949 MeV respectively. We obtain ΔM_K=3.19(41)(96) x 10⁻¹² MeV, quite similar to the experimental value. Here the first error is statistical and the second is an estimate of the systematic discretization error. An interesting aspect of this calculation is the importance of the disconnected diagrams, a dramatic failure of the OZI rule.

A search for undiscovered particles decaying into top-antitop quark pairs produced in proton-proton collisions with the ATLAS experiment at the Large Hadron Collider utilizing 20.3 fb$^-1$ of data collected at $\sqrt{s}=8$ \tev\ center-of-mass energy during the 2012 data taking period is presented. The invariant mass spectrum of events containing multiple jets, exactly one lepton, and missing transverse energy and which are consistent with the decay of a top-antitop quark pair is studied and found to be consistent with that predicted by the Standard Model. Upper limits on the production cross section times branching ratio of several benchmark signal models are set at a 95\% confidence level.

In this dissertation we study the phenomenon of jet quenching in quark-gluon plasma using the AdS/CFT correspondence. We start with a weakly coupled, perturbative QCD approach to energy loss, and present a Monte Carlo code for computation of the DGLV radiative energy loss of quarks and gluons at an arbitrary order in opacity. We use the code to compute the radiated gluon distribution up to n=9 order in opacity, and compare it to the thin plasma (n=1) and the multiple soft scattering (n=\infty) approximations. We furthermore show that the gluon distribution at finite opacity depends in detail on the screening mass and the mean free path. In the next part, we turn to the studies of how heavy quarks, represented as "trailing strings" in AdS/CFT, lose energy in a strongly coupled plasma. We study how the heavy quark energy loss gets modified in a "bottom-up" non-conformal holographic model, constructed to reproduce some properties of QCD at finite temperature and constrained by fitting the lattice gauge theory results. The energy loss of heavy quarks is found to be strongly sensitive to the medium properties. We use this model to compute the nuclear modification factor R_AA of charm and bottom quarks in an expanding plasma with Glauber initial conditions, and comment on the range of validity of the model. The central part of this thesis is the energy loss of light quarks in a strongly coupled plasma. Using the standard model of "falling strings", we present an analytic derivation of the stopping distance of light quarks, previously available only through numerical simulations, and also apply it to the case of Gauss-Bonnet higher derivative gravity. We then present a general formula for computing the instantaneous energy loss in non-stationary string configurations. Application of this formula to the case of falling strings reveals interesting phenomenology, including a modified Bragg-like peak at late times and an approximately linear path dependence. Based on these results, we develop a phenomenological model of light quark energy loss and use it compute the nuclear modification factor R_AA of light quarks in an expanding plasma. Comparison with the LHC pion suppression data shows that, although R_AA has the right qualitative structure, the overall magnitude is too low, indicating that the predicted jet quenching is too strong. In the last part of the thesis we consider a novel idea of introducing finite momentum at endpoints of classical (bosonic and supersymmetric) strings, and the phenomenological consequences of this proposal on the energy loss of light quarks. We show that in a general curved background, finite momentum endpoints must propagate along null geodesics and that the distance they travel in an AdS5-Schwarzschild background is greater than in the previous treatments of falling strings. We also argue that this leads to a more realistic description of energetic quarks, allowing for an unambiguous way of distinguishing between the energy in the dual hard probe and the energy in the color fields surrounding it. This proposal also naturally allows for a clear and simple definition of the instantaneous energy loss. Using this definition and the "shooting string" initial conditions, we develope a new formula for light quark energy loss. Finally, we apply this formula to compute the nuclear modification factor R_AA of light hadrons at RHIC and LHC, which, after the inclusion of the Gauss-Bonnet quadratic curvature corrections to the AdS5 geometry, shows a reasonably good agreement with the recent data.

This thesis presents complementary measurements of the neutrino oscillation parameter sin²2θ₁₃ made by observing the electron antineutrino flux from two nuclear reactors in Chooz, France. Antineutrinos are identified through both products of the inverse beta decay interaction, bar νₑ + p → e⁺ + n, in a high-precision liquid scintillator detector located approximately 1 km from the reactors. The most sensitive signal channel involves neutron captures by a gadolinium dopant, while a search for neutron captures on hydrogen provides a high-statistics validation. In both channels, the value of sin²2θ₁₃ is revealed by an energy- and reactor power-dependent deficit of antineutrino-like events, compared to a reactor simulation. All analyses produce results consistent with one another and with findings of other experiments. These datasets also expose features of the antineutrino spectrum not predicted in reactor flux models. Data from the newly inaugurated near detector, anticipated in the final part of this thesis, holds unique potential to clarify these features, pursue signals from sterile neutrinos, and contribute to global knowledge of three-neutrino mixing.

Vidas médias de níveis mais baixos nas bandas de 1 e 3 quase-partículas em ANTPOT 133Ce foram medidas usando a técnica de distância de recuo e deslocamento Doppler RDDS. As amplitudes das transições E2 extraídas dos estados yrast de paridade negativa são bem descritas pelos modelos Rotor Triaxial + Quase-Partícula e geométrico. Os valores de B(M1) extraídos para os níveis na banda de 3 quase-partículas de paridade positiva estão consistentes com a atribuição da configuração Vh1l/2 (X) h 1l/2 (X) g7/2 para essa banda. Os níveis no 132,133Ce foram populados pela reação ANTPOT 120 Sn (ANTPOT 16 O,4n,3n) ANTPOT 132,133Ce na energia incidente de 70 MeV. O feixe foi fornecido pelo acelerador Tandem do Laboratório Pelletron. Os dados foram obtidos usando um detetor HP Ge (22% de eficiência, resolução de energia de 2.1keV) com supressão Compton BGO a 0° em relação ao feixe e um filtro de multiplicidades de 6 detetores 3\" x 3\" de NaI(Tl) num plano perpendicular ao plano da reação. O alvo foi produzido por evaporação ed ANTPOT 120 Sn metálico sobre um substrato fino de ouro voltado na direção de incidência do feixe, enquanto que o anteparo (stopper) foi produzido em chumbo. Os núcleos de recuo atingiram uma velocidade média de v= 0.008c. As medidas foram realizadas com um dispositivo plunger construído no Laboratório Pelletron. Os espectros foram obtidos para 13 diferentes distâncias alvo-stopper e foram normalizadas usando uma linha gama de 279 keV da excitação Coulombiana do substrato de Au. As distâncias de vôo alvo-stopper foram medidas usando a técnica de capacitância e também com um micrômetro de precisão. As vidas médias de estados de alto spin em ANTPOT 136 Pr, ímpar-ímpar, foram também medidas usando a reação ANTPOT. 123 Sb (ANTPOT 16 O,3n) na energia incidente de 65 MeV. Os espectros de raios gama foram adquirídos para 20 diferentes distâncias. O alvo foi confeccionado de ANTPOT 123 Sb de espessura de 1mg/cm², evaporado sobre um substrato de 2mg/cm² de Au. A análise mostra que a vida média do estado 8+ da banda yrast (h1l/2) (X) v(h 1l / 2) é de 1177 ± 64ps. Um limite superior para a vida média do estado 7- foi estabelecido em < 1ns. / Lifetimes of the low-lying levels in the one and three quasiparticle bands in 133Ce have been measured using the recoil-distance Doppler-shift technique RDDS. The E2 transition strengths extracted for the negative parity yrast states are well described by the Triaxial-Rotor-plus-Quasipartide and the geometrical model. The B(M1) values extracted for the levels in the positive parity 3 quasipartide band are consistent with the Vh11/2 X h 11/2 g7/2 configuration assignment to this band. Levels in 132,133Ce were populated by the 120Sn(16O,4n,3n)132,133Ce reaction at an incident energy of 70 MeV. The beam was provided by the Tandem accelerator of the Pelletron Laboratory. Data were obtained using a BGO Compton suppressed HPGe detector (22% efficiency, energy resolution 2.1 keV) at 0° to the beam and a multiplicity filter of six 3\" x 3\" NaI(TI) detectors in a plane perpendicular to the reaction plane. The target was produced by evaporation of enriched metallic 120Sn onto a thin Au backing which faced the incident beam, while the stopper was of lead. The recoil nuclei had an average velocity of v = 0.008c. The measurements were performed with a plunger device constructed in our laboratory. Spectra were obtained for 13 different target-stopper distances and were normalized using the 279 keV gamma line from the Coulomb excitation of the Au backing. The target-stopper flight distances were measured using the capacitance technique and also with a precision micrometer. Lifetimes of the high spin states in odd-odd 136Pr were also measured using the 123Sb(16O,3n) reaction at an incident energy of 65 MeV. Gamma spectra were collected at about 20 different distances. The target was of enriched 123Sb 1mg/cm2 thick, evaporated onto a Au 2mg/cm2 backing. Analysis shows that the lifetime of the 8+ state in the yrast band (h11 2Z) X V (h11/2) is 1177 ± 64ps. An upper limit for the 7- state was stablished in < 1ns.

O decaimento de ressonâncias gigantes (RG\'s) em núcleos tipicamente esféricos, ANTPOT.40 Ca, ANTPOT.90 Zr e ANTPOT.208 Pb, são interpretados em termos de suas larguras de escape nos processos direto e de pré-equilíbrio. Os cálculos são embasados na Teoria Estatística de Múltiplas Etapas de Núcleo Composto, de Feshbach, Kerman e Koonin (FKK). É feito um estudo comparativo, analítico e numérico, entre as aproximações FKK e de Oblozinsky, no contexto de densidade de níveis, no cálculo das larguras de damping e de escape. No caso das larguras de damping () não há diferença apreciável entre as referidas formulações no intervalo de energia estudado. Por outro lado, para as larguras de escape () podemos ter diferenças apreciáveis dependendo do núcleo, do tipo de núcleon emitido e da própria energia de excitação. A largura total das RG\'s, (= + ) , para algumas RG\' s nos núcleos mencionados acima, é comparada com dados experi-mentais e com outros cálculos teóricos, obtendo-se uma razoável concordância de resultados. No que se refere a núcleos deformados, apresentamos uma proposta teórica para o cálculo da largura de escape do decaimento direto de RG\' s. Consideramos um núcleo constituído de um coroço levemente deformado com simetria axial acoplado a um par partícula-buraco. O sistema formado decai, emitindo um núcleon, através do processo direto. / The decay of Giant Resonances (GR\'s) in typically spherical nuclei, 40Ca, 90Zr e 208Pb, are interpreted in terms of its escape widths in the direct and preequilibrium processes. The calculations are based in the Statistical MultiStep Compound theory of Feshbach, Kerman anel Koonin (FKK). It is made a comparative study, analytical and numeric, between the FKK and oI Oblozinsky approaches, in the level density context, for the calculations of the damping and escape widths. In the case of the damping widths () there is no appreciable difference among the mentioned formulations in the energy interval studied. On the other hand, for the escape widths () we can have significant differences depending on the nucleus, of the type of emitted nucleon and of the excitation energy. The total wiedth (= + ), for some GR´s of the nuclei mentioned above, it is compared with experimental data and with other theoretical calculations, being obtained a reasonable agreement among the results. For deformed nuclei, we presented a theorctical proposal for the calculation of the escape width in the direet decay of a given GR. We considered a nucleus constituted of a core softly deformed with axial symmetry coupled to a pair particle-hole. The formed system decay emitting a nucleon through direct process