We report investigations of physical possibilities beyond the Standard Model, performed in the years between Runs I and II of the Large Hadron Collider (LHC). First, we explore the feasibility of using a hadron collider to unmask hidden sectors by means of a novel signal, the ``monocline". Dilepton production provides the cleanest channel to anticipate a monocline. A compelling sector to seek is dark matter with scalar messengers coupling it to standard fermions. We present current bounds from dilepton spectrum measurements at the LHC and make predictions for sensitivities at Run II of the LHC as well as at a future 100 TeV collider. Second, we corner the space of parameters of supersymmetric frameworks with an appreciable Yukawa coupling between the Higgs fields and a gauge singlet, the so-called Fat Higgs and $\lambda$-SUSY models, in the context of the discovery of the 125 GeV Higgs particle. These models are motivated by their alleviation of the electroweak fine-tuning that supersymmetry breaking entails, via raising the tree-level quartic coupling Higgs boson. Heavy Higgs scalars that couple strongly to the standard Higgs boson induce large radiative corrections to the Higgs quartic coupling, which is crucial to phenomenology; in particular, a very large ratio of the Higgs VEVs ($\tan \beta$), that was previously presumed unfavorable in these models, becomes viable and can be probed by future experiments. In such regions, the most stringent limits come from dark matter constraints on the lightest neutralino. Finally, we place limits on colored scalar production at the LHC in supersymmetric models where gauginos acquire both Dirac and Majorana masses, that we call ``mixed gauginos". While it was known that purely Dirac gluinos were less constrained by LHC searches than their purely Majorana counterparts, we find that the constraints further weaken or strengthen depending on which of the ``mixed" colored fermions acquires a Majorana mass. Also explored are the effects on squark production of turning on Majorana masses for electroweak gauginos. This dissertation consists of previously published and unpublished co-authored material.

Although about a century has passed since its discovery, and despite the scientific and technological progress of our society has gone through, the nature of the Dark Matter (DM) is a mystery not yet solved. It is a big challenge for the scientific community, its identification would mean the understanding of what seemingly makes up 84% of the matter content in the Universe. We say “seemingly” because so far all evidences for DM are purely gravitational. This implies that what we call dark matter could be either a manifestation of our incomplete understanding of gravity on large scales, or a new form of matter, in particular a new kind of elementary particle. Among the plethora of possible DM candidates, this work will consider the Weakly Interactive Massive Particle (WIMP). Up to know the observed value for the relic abundance is the only solid parameter which we can count on, and the WIMP is the candidate that, through a fashionable mechanism of production, gives a result for the relic abundance in agreement with the cosmological observations. In order to demonstrate that this is indeed the valid explanation to the DM problem, a non-gravitational signal and also a model to interpret a possible Dark Matter message are needed. This may be expressed in various ways, and our approach is based on so-called simplified model. We have built a new t-channel simplified model which promotes a real scalar particle as DM and a vector-like fermion as mediator, dubbed the Vector-like Portal. In our framework, there are very few free parameters, the DM mass, the mass of the mediator and at least one Yukawa coupling. We have discovered an intriguing feature in the annihilation cross-section, a d-wave suppression in the limit of light final state fermions (compared to the DM mass). This seemingly innocuous observation will turn out to be crucial both for thermal freeze-out and for indirect searches for DM, for which higher order effects become relevant. In particular, we have performed a comprehensive analysis of the impact of next-to-leading order corrections to the annihilation cross-section, including coannihilation, as well as on elastic scattering processes. We have exploited the complementarity of direct, indirect and collider searches to set constraints on the parameter space of some simple vector-like portal scenarios, including coupling to Standard Model leptons, light quarks and the top quark. In addition, we have studied the phenomenological consequences of electroweak corrections and the detectability ofour candidate. / Bien que presque un siècle ait passé depuis sa découverte, et malgré les progrès scientifiques et technologiques de notre société, la nature de la matière noire (MN) est un mystère irrésolu. C’est un grand challenge pour la communauté scientique, car sa découverte signifierait la compréhension de ce qui semble constituer 80 % du contenu en matière de l’univers. Nous disons “semble” car jusqu’à présent toutes les évidences de MN sont purement gravitationnelles. Cela implique que ce que nous appelons matière noire pourrait être soit une manifestation de notre compréhension incomplète de la gravité à grandes échelles, soit une nouvelle forme de matière, en particulier un nouveau type de particule élémentaire. Parmi la pléthore de candidats de MN possibles, ce travail considèrera la Weakly Interactive Massive Particle (WIMP). A ce jour, la valeur observée de l’abondance relique est le seul paramètre solide sur lequel nous pouvons compter, et la WIMP est le candidat qui, par un élégant mécanisme de production, donne un résultat pour l’abondance relique en accord avec les observations cosmologiques. Pour démontrer que cela est en effet l’explication valide du problème de la MN, un signal non gravitationnel ainsi qu’un modèle pour interpréter un possible message de MN sont requis. Cela peut s’exprimer de plusieurs façons, et notre approche est basée sur les modèles dits simplifiés. Nous avons construit un nouveau modèle simplifié avec le canal t qui promeut une particule scalaire réel comme MN et un fermion de type vector-like comme médiateur, et communément appelé le portail vector-like. Dans notre cadre, il y a très peu de paramètres libres, la masse de la MN, la masse du médiateur et au moins un couplage de Yukawa. Nous avons découvert une caractéristique intrigante dans la section efficace d’annihilation, une suppression de type d-wave dans la limite de fermions légers dans l’état final (en comparaison avec la masse de la MN). Il se trouve que cette observation, qui semble anodine au premier abord, est cruciale pour le freeze-out et pour les recherches indirectes de MN, pour lesquelles les effets aux ordres supérieurs deviennent relevants. En particulier, nous avons réalisé une analyse compréhensive de l’impact des corrections après l’ordre dominant de la section efficace d’annihilation, ainsi que des processus de diffusion élastique. Nous avons exploité la complémentarité des recherches directes, indirectes et aux collisionneurs afin de contraindre l’espace des paramètres de quelques scénarios simples de portails vector-like, dont le couplage au leptons du Modèle Standard et au quark top. De plus, nous avons étudié les conséquences phénoménologiques des corrections électrofaibles, et la détectabilité de notre candidat. / Option Physique du Doctorat en Sciences / info:eu-repo/semantics/nonPublished

The cosmological constant + cold dark matter (ΛCDM) theory is the 'standard model' of cosmology. Encoded in it are extremely accurate descriptions of the large scale structure of the Universe, despite a very limited number of degrees of freedom. The model struggles, however, to explain some measurements on galactic and smaller scales. The shape of the dark matter distribution toward the centres of galaxies is predicted to be steeply increasing in density ('cuspy') by the theory, yet observations of the rotation curves of some galaxies suggest that it instead reaches a central density plateau (a 'core'). This discrepancy is termed the 'cusp-core problem'. I propose a new way of quantifying this problem as a diversity in the central mass content of galaxies. This characterization does not distinguish between dark and ordinary ('baryonic') matter, but the apparent problem is so severe that the signature of the cusp-core discrepancy is still obvious. By formulating the problem in this way, several uncertain modelling steps are effectively removed from the discussion, allowing for a more narrowly focussed examination of remaining steps in the analysis. My subsequent comparison of recent results from galaxy formation simulations and observed galaxies in the space of the baryonic Tully-Fisher relation (BTFR) reveals some galaxies with an apparent anomalously low dark matter content not only in the centre, but out to the largest measurable radii. These objects are very difficult to explain within the ΛCDM framework; the most plausible interpretation which emerges is that the effect of systematic uncertainties in modelling the kinematics in these galaxies – particularly in the estimate of their inclinations – has been substantially underestimated. This motivates a re-examination of rotation curve measurement methods. I use a collection of simulated galaxies to demonstrate that, when these are synthetically 'observed' and modelled analogously to real galaxies, non-circular motions present in the gas discs give the appearance of cores, even though all of the simulated galaxies have central cusps. The errors are large enough to reproduce the full width of the observed scatter in rotation curve shapes. Provided the simulations produce sufficiently faithful models of real galaxies, these modelling errors could constitute a solution to the cusp-core problem within the ΛCDM paradigm. Regardless, the kinematic models must be better understood before drawing any strong cosmological conclusions. / Graduate

Galaxies, galaxy groups and galaxy clusters are embedded in large dark matter halos. Most galaxies in the local universe are found in the galaxy group environment. Locating the centres of galaxy groups is crucial in order to study their properties such as their halo masses. It is often assumed that the most massive galaxy (or brightest galaxy) resides at the centre of the gravitational potential. With the aim of evaluating the validity of this paradigm in galaxy groups, we used two different methods to probe the centres of galaxy group halos: the weak gravitational lensing and dynamical methods. We use these two methods to determine the best definition of galaxy group centres. Our sample is composed of 49 optically (spectroscopically) selected groups and 36 high quality X-ray-selected groups. In total our sample is composed of 78 distinct groups in the redshift range 0.1 < z < 0.9 from the GEEC sample. Our weak lensing analysis suggests that the weighted centre is a better definition than the most massive galaxy position. We address the question of whether or not the result is significantly different for X-ray and optically selected systems. For optically selected systems, the weighted centre is a significantly better assumption of the group centre than the most massive galaxies position. For the X-ray selected groups, the weighted centre and the most massive galaxy appear to trace the centre equally well, although the best definition is the location of the peak in X-ray emission. We evaluate, for the first time, the impact of dynamically complex groups on weak lensing analysis. Once we removed dynamically complex systems from our sample, the lensing signals for all centre definitions are in better agreement suggesting that groups with large offsets between the centre definitions are unevolved systems. For the dynamical method, velocity dispersion profiles suffer from large uncertainties and, therefore, we are unable to place any constraint on the centre definition from this technique. / Thesis / Master of Science (MSc)

The Standard Model of particle physics is a phenomenologically successful description of the strong, weak, and electromagnetic interactions at all currently accessible energy scales with few exceptions \cite{Agashe:2014kda}. The notable deficiencies of the Standard Model are its inability to explain the matter anti-matter asymmetry, the existence of neutrino oscillations \cite{Fukuda:1998mi,Ahmad:2002jz}, the anomalous magnetic moment of the muon \cite{Bennett:2006fi,Hagiwara:2011af}, and its failure to provide a suitable candidate for the gravitationally observed dark matter \citep{Dolgov:1995np}. We explore an extension of the Standard Model that introduces a new gauge symmetry $L_\mu-L_\tau$ along with three right-handed neutrinos, and a symmetry breaking scalar field. The inclusion of right-handed neutrinos are motivated by the aforementioned neutrino oscillation data while the scalar field is motivated by cosmological bounds on a new $Z'$. We attempt to fit our model to the observed neutrino mass textures in the see-saw limit. Despite having a Lagrangian density with three Yukawa couplings, and four right-handed mass parameters we found the left handed neutrino mass matrix was controlled by only four independent quantities. We were attempting to fit to a set of five measured parameters $\{ \Delta m_{12}^2,\Delta m_{13}^2,\theta_{12},\theta_{23},\theta_{13} \}$. This was found to be impossible with our proposed model. Higher dimensional operators were introduced to allow the model to generate neutrino textures that agree with experiment. Our first minimal model was able to reproduce the correct neutrino textures with the exception of one of either $\theta_{13}$ or $\theta_{12}$ the disagreements was at the level of $25\%$. We found that our model was able to fit to the central value of neutrino data after the introduction of various combinations of dimension-five operators. The parametric dependence of these solutions were found to be incompatible with the $Z'$ as a progenitor of dark matter scenario proposed by Shuve and Yavin \cite{Shuve:2014doa}. The $Z'$ progenitor scenario and the see-saw mechanism seem to be distinct entities in the sense that for the former to be viable the dark matter candidate cannot play a significant role in the generation of neutrino textures. / Thesis / Master of Science (MSc)

Astounding evidence for invisible "dark" matter has been found from galaxy clusters, cosmic and stellar gas motion, gravitational lensing studies, cosmic microwave background analysis, and large scale galaxy surveys. Although all studies indicate that there is a dominant presence of non-luminous matter in the universe (about 22 percent of the total energy density with 5 times more dark matter than baryonic matter), its identity and its "direct" detection (through non-gravitational effects) has not yet been achieved. Dark matter in the form of massive, weakly interacting particles (WIMPs) could be detected through their collisions with target nuclei. This requires detectors to be sensitive to very low-energy (less than 100 keV) nuclear recoils with very low expected rates (a few interactions per year per ton of target). Reducing the background in a direct dark matter detector is the biggest challenge. A detector capable of seeing such low-energy nuclear recoils is difficult to build because of the necessary size and the radio- and chemical- purity. Therefore it is imperative to first construct small-scale prototypes to develop the necessary technology and systems, before attempting to deploy large-scale detectors in underground laboratories. Our collaboration, the DarkSide Collaboration, utilizes argon in two-phase time projection chambers (TPCs). We have designed, built, and commissioned DarkSide-10, a 10 kg prototype detector, and are designing and building DarkSide-50, a 50 kg dark matter detector. The present work is an account of my contribution to these efforts. The two-phase argon TPC technology allows powerful discrimination between dark matter nuclear recoils and background events. Presented here are simulations, designs, and analyses involving the electroluminescence in the gas phase from extracted ionization charge for both DarkSide-10 and DarkSide-50. This work involves the design of the HHV systems, including field cages, that are responsible for producing the electric fields that drift, accelerate, and extract ionization electrons. Detecting the ionization electrons is an essential element of the background discrimination and gives event location using position reconstruction. Based on using COMSOL multiphysics software, the TPC electric fields were simulated. For DarkSide-10 the maximum radial displacement a drifting electron would undergo was found to be 0.2 mm and 1 mm for DarkSide-50. Using the electroluminescence signal from an optical Monte Carlo, position reconstruction in these two-phase argon TPCs was studied. Using principal component analysis paired with a multidimensional fit, position reconstruction resolution for DarkSide-10 was found to be less than 0.5 cm and less than 2.5 cm for DarkSide-50 for events occurring near the walls. DarkSide-10 is fully built and has gone through several campaigns of operation and upgrading both at Princeton University and in an underground laboratory (Gran Sasso National Laboratory in Assergi, Italy). Key DarkSide two-phase argon TPC technologies, such as a successful HHV system, have been demonstrated. Specific studies from DarkSide-10 data including analysis of the field homogeneity and the field dependence on the electroluminescence signal are reported here. / Physics

This thesis is devoted to the study of the nature of Dark Matter (DM). To this end we investigate both its generation mechanisms and detection possibilities. We mainly focus on interesting simple models and follow closely experimental constraints, in order to reveal the features of DM as model-independently as possible.<p><p><p><p><p>Throughout the whole thesis, we consider the framework of standard cosmology, which is first introduced in Chapter.1 (and supplemented in Appendices). Background knowledges of DM physics from the cosmological and experimental aspects are given in Chapter.2 and Chapter.3, respectively. <p><p><p>Following the scenario that a hidden sector, including DM particles, might decouple from the Standard Model (SM) sector at a very early time of the Universe, we study how through a portal interaction such a hidden sector can be created by the SM sector to yield the proper relic density of dark matter. In Chapter.4 we discuss the case of a massless portal using the gauge kinetic mixing model. It turns out that there are four basic ways to achieve the observed DM relic density for both massless and massive mediator cases: freeze-in from SM sector, reannihilation determined by the balance of the two sectors, freeze-out with hidden or portal interactions. Various models with massive portals, especially $Z'$ and scalar portals, are then explored in Chapter.5. Provided that DM annihilation within the hidden sector is kinetically allowed, similar conclusions would appear. If there is no hidden annihilation for DM, only the first and last ways are allowed. Nevertheless, chemical thermalization in hidden sector can still influence the results significantly.<p><p>In Chapter.6, we explore the hypothesis that dark matter particles partially annihilate to photons via a new heavy charged particle which is also "gauged" under SU(3)_C. By applying this hypothesis to the claimed<p>Fermi-LAT 130 GeV gamma line, it is showed that the correct dark matter relic density could be naturally obtained from the gluon channel and how for such a class of models the cosmic antiproton, diffuse gamma-ray, direct detection and LHC constraints<p>are fully correlated. <p><p><p>In Chapter.8, concluding remarks and perspectives for future DM research are presented. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

No description available.

The existence of a significant non-baryonic component to the Universe is widely accepted, with worldwide efforts underway trying to detect this so-called dark matter. The ZEPLIN-III detector utilises liquid xenon (Xe) as a target medium in the search for the expected rare interactions of Weakly Interacting Massive Particles, or WIMPs, with ordinary baryonic matter. The neutralino, arising in supersymmetric extensions to the standard model of particle physics, provides a particularly well-motivated candidate. The ZEPLIN-III experiment, operating in two-phase (liquid/gas) mode, measures both the scintillation and ionisation signatures produced during an interaction. The first science run (FSR) of ZEPLIN-III was performed during three months in 2008. The run culminated in a published result which excluded a WIMP-nucleon interaction cross-section above 8:1 x 10-8 pb for a 60 GeVc-2 WIMP at the 90% confidence level. ZEPLIN-III then entered an upgrade period where the photomultiplier tube (PMT) array, previously the dominant source of background, was replaced with new, ultra-low background, PMTs. The radio-contamination of components used to make these PMTs has been thoroughly studied and their impact on the background rates in ZEPLIN-III characterised. Additionally, a new 1.5 tonne plastic scintillator veto detector was constructed, increasing the ability to reject WIMPlike signals caused by neutron induced nuclear recoil events and improving the γ-ray discrimination capability of ZEPLIN-III. The second science run (SSR) of ZEPLIN-III began in June 2010 and continued for 6 months, with a projected upper limit for the interaction cross-section of 1:52 x 10-8 pb for a 55 GeVc-2 WIMP at the 90% confidence level.

No description available.