<p> The high-energy universe is known to be violent. Ultra High Energy Cosmic Rays (UHECRs) have been observed with kinetic energies exceeding 10²⁰ eV. Their origin, despite decades of observations, remains elusive. A unique probe of the sources and production mechanisms of these high energy cosmic rays can be neutrinos, since they are inevitably produced when high-energy protons interact. The IceCube Neutrino Observatory, located at the geographical South Pole in Antarctica, continuously monitors a total volume of 1 km³ of clear Antarctic ice for neutrino interactions. For this purpose, a total of 5160 optical sensors (photomultiplier tubes) have been melted deep into the glacier at depths between 1450m and 2450m. In 2013 IceCube reported one of its biggest discoveries, the observation of highly energetic neutrinos that are consistent with a possible extra-galactic origin. </p><p> In this dissertation we use IceCube data (recorded from 2012 to 2015) to study the spectral properties of this astrophysical neutrino flux with focus on electron and tau neutrino flavors. We developed a new neutrino identification and muon background rejection method using state-of-the-art machine-learning techniques, more specifically multi-class gradient boosted decision trees. In addition to enlarging the number of detected neutrino events (>10x increase over previous works), we lowered the energy threshold to below 1 TeV and thereby greatly improved upon the control and treatment of systematic uncertainties. The sample contains ~400 astrophysical electron and tau neutrinos, which increases the significance of the original discovery to beyond 8 standard deviations. We find the astrophysical neutrino flux to be well described by a single power-law consistent with expectations from Fermi-type acceleration of high-energy particles at astrophysical sources and obtain leading constraints on its properties. We further studied the possibility of additional spectral complexity, which significantly increases measurement uncertainties. No evidence for such scenarios was found. Finally we searched for a contribution from atmospheric neutrinos related to heavy meson (charm) decay in Earth's atmosphere and derive a flux upper limit of 4.8 times the benchmark pQCD flux prediction at 90% confidence level, dominated by systematic uncertainties, especially related to photon transport in the glacial ice.</p><p>

<p> Searches for violations of discrete symmetries can be sensitive probes of physics beyond the Standard Model. Many models, such as supersymmetric theories, introduce new particles at higher masses that include new <i> CP</i>-violating phases which are thought to be of order unity. Such phases could generate measurable permanant electric dipole moments (EDMs) of particles. The ACME collaboration has measured the electron's EDM to be consistent with zero with an order of magnitude improvement in precision compared to the previous best precision (J. Baron et al., ACME collaboration, <i> Science</i> <b>343</b> (2014), 269-272) with a spin precession measurement performed in the <i>H</i> state of a beam of thorium monoxide (ThO). This limit constrains time-reversal violating physics for particles with masses well into the TeV scale. In this thesis I discuss the details of this measurement with an emphasis on the data analysis, search for systematic errors, and systematic error models that contributed to this result. I also discuss implemented and planned upgrades to the experimental apparatus intended to both improve the statistical sensitivity and reduce its susceptibility to systematic errors. At this time, the upgraded apparatus has been demonstrated to have a statistical sensitivity to the electron EDM that is more than a factor of 10x more precise than our previous measurement. </p><p>

<p> An interpretation of CMS searches for evidence of supersymmetry in the context of the minimal supersymmetric Standard Model (MSSM) is given. It is found that supersymmetric particles with color charge are excluded in the mass range below about 400 GeV, but neutral and weakly-charged sparticles remain non-excluded in all mass ranges. Discussion of the non-excluded regions of the model parameter space is given, including details on the strengths and weaknesses of existing searches, and recommendations for future analysis strategies. Advancements in the modeling of events arising from quantum chromodynamics and electroweak boson production, which are major backgrounds in searches for new physics at the LHC, are also presented. These methods have been implemented as components of CMS searches for supersymmetry in proton-proton collisions resulting in purely hadronic events (i.e., events with no identified leptons) at a center of momentum energy of 13 TeV. These searches, interpreted in the context of simplified models, exclude supersymmetric gluons (gluinos) up to masses of 1400 to 1600 GeV, depending on the model considered, and exclude scalar top quarks with masses up to about 800 GeV, assuming a massless lightest supersymmetric particle. A search for non-excluded supersymmetry models is also presented, which uses multivariate discriminants to isolate potential signal candidate events. The search achieves sensitivity to new physics models in background-dominated kinematic regions not typically considered by analyses, and rules out supersymmetry models that survived 7 and 8 TeV searches performed by CMS.</p>

Leptogenesis is a class of scenarios where the baryon asymmetry of the Universe is produced from a lepton asymmetry generated in the decays of a heavy sterile neutrino. We explain the motivation for<br />leptogenesis. We review the basic mechanism, and describe subclasses of models. We then focus on recent developments in the understanding of leptogenesis: finite temperature effects, spectator processes, and in particular the significance of flavour physics.

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Le travail présenté dans cette thèse a été réalisé grâce aux données enregistrées de 1991 à 1994 par le le détecteur DELPHI au LEP. Il porte sur la détermination du paramètre libre IVcbl du Modèle Standard de l'interaction électrofaible à partir de la désintégration semileptonique $\bar{B}^0_d -> D*^{+}l-\nu_l$. Une deuxième analyse sur la production d'états charmés orbitalement excités dans les désintégrations semileptoniques des mésons B est aussi présentée. La mesure de IVcb/ repose sur la détermination précise du spectre en moment transféré $q^2 = (m_b - m_D*)^2$ des événements reconstruits de manière exclusive. L'ajustement de ce spectre par l'expression théorique donnée par la théorie effective des quarks lourds (HQET) permet d'obtenir la valeur de IVcb| vec une incertitude théorique inférieure à 4 %. L'exposé de ce travail commence par un présentation du rôle de l'élément de matrice lVcbl et des outils théoriques nécessaires à sa mesure. Le dispositif expérimental utilisé est ensuite décrit. Les différentes étapes de l'analyse sont détaillées: sélection des événements, étude des bruits de fond, mesure du rapport d'embranchement Br($\bar{B}^0_d -> D*^{+}l-\nu_l$ qui est l'étape préliminaire à la mesure de |Vcb| . La mise au point des estimateurs du moment transf4ré est ensuite ex~oséeL. a détermination du spectre en moment transféré des événements -+ DS+l-fi sélectionnés et son ajustement conduisent finalement à la mesure de /VcbI . Les résultats obtenus sont: Br($\bar{B}^0 -> D*^{+}l-\nu_l) = (5,17$\pm 0,49(stat) \pm 0,71(syst))% $|V_{cb}| = (38,7\pm 3,4(stat) \pm 2,9(syst) \pm 1,2(th)) x 10^{-3}$ Ces mesures sont en bon accord avec les résultats obtenus par d'autres collaborations auprès du LEP ou des usines à mésons B (CESR, DESY). L'analyse concernant la recherche d'états charmés excités orbitalement dans les désintégrations semileptoniques des B a permis de mesurer le rapport d'enibranchement Br(b -> $D^0_{J}lX) x Br(D^0_J -> D*^{+}\pi-) = (6.0^{+1,7}_{-1,5}(stat) \pm 0,8(syst))) x 10^{-3}$ où $D^0_J$ correspond à la somme des deux états charmés étroits $D^0_1$ et $D*^0_2 . Cette mesure, quoique statistiquement peu précise indique la présence non négligeable d'excitations orbitales charmées dans les désintégrations sernilcptoniques des B.

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