3,497 research outputs found
On subgroups with narrow Schreier graphs
No full textWe study finitely generated pairs of groups H ≤ G such that the Schreier graph of H has at least two ends and is narrow. Examples of narrow Schreier graphs include those that are quasi-isometric to finitely ended trees or have linear growth. Under this hypothesis, we show that H is a virtual fiber subgroup if and only if G contains infinitely many double cosets of H. Along the way, we prove that if a group acts essentially on a finite dimensional CAT(0) cube complex with no facing triples then it virtually surjects onto the integers with kernel commensurable to a hyperplane stabiliser
Recherche de quarks vectoriels produits par l'échange de gluons lourds dans le cadre de modèles de Higgs composite avec le détecteur ATLAS
No full textLongtemps la pièce manquante du Modèle Standard (MS), le boson de Higgs a finalement été observé par l'expérience ATLAS en 2012. Malgré cette découverte, on a de bonnes raisons théoriques de penser que le MS n'est qu'une théorie effective à basse énergie, et un des objectifs principaux de l'expérience ATLAS reste de comprendre l'origine de la brisure de symétrie électrofaible. Parmi les extensions possibles au MS, les modèles de Higgs composite (CHM) apportent certaines réponses aux problèmes laissés ouverts par le MS, comme le problème de hiérarchie. Les CHM prévoient également l'existence de nouvelles particules dont les quarks vectoriels (VLQs). Les VLQs constituent une des extensions phénoménologiques les plus simples au contenu du MS et sont donc activement recherchés au LHC.
Les défis techniques et logistiques nécessaires pour une telle découverte font du détecteur ATLAS un des appareils les plus sophistiqués au monde. Le détecteur est également conçu pour sonder la physique au-delà du MS et tester les différents modèles théoriques.
Cette thèse présente les résultats obtenus dans la recherche de VLQs \BH de charge , en production simple, via l'échange d'un gluon lourd . L'analyse, basée sur une version simplifiée d'un CHM minimal, se concentre sur le processus pp \to G^* \to \BH\bar b/\bar B_\text{H}b \to H\bbbar \to \bbbar\bbbar. Les données analysées correspondent à 19.5~\fb de collisions effectuées à une énergie dans le centre de masse de ~TeV. Aucun excès d'événements n'est observé par rapport aux prédictions du MS. Des limites supérieures sur la section efficace et le rapport d'embranchement complet sont imposées. Pour un modèle de référence, une limite inférieure de 2.0~TeV sur la masse du gluon lourd est obtenue dans le cas où \mg = 2\mb.
Une analyse supplémentaire sur la recherche de nouvelles résonances se désintégrant en boson de jauge ou et un boson de Higgs pour les états finaux \nu\nu/\ell\nu/\ell\ell + \bbbar est résumée en annexe. Cette recherche a été réalisée dans le contexte de modèles discutés dans cette thèse.Being the missing piece of the Standard Model (SM) for decades, the Higgs boson was finally observed by the ATLAS experiment in 2012. Despite this discovery, it is justified to think that the SM is an effective theory at low energy scale, and one of the physics goal of the ATLAS experiment is to understand the origin of the electroweak symmetry breaking which the SM fails to explain. The technical and logistic challenges necessary for such a discovery makes the ATLAS detector one of the most sophisticated apparatus in the world. As a multipurpose detector, the ATLAS detector is built to investigate physics Beyond the SM and discover new particles. Composite Higgs models (CHM) are extensions to the SM that shed light on some problems left open, such as the mass hierarchy problem. Composite Higgs models also predict the existence of new particles such as vector-like quarks (VLQs). As one the simplest extension to the SM particle content VLQs are actively search for at the LHC.
This thesis presents the results obtained in the search for the single production of VLQs \BH of charge through the exchange of a heavy gluon . Based on a simplified minimal CHM, the search is performed for the process pp \to G^* \to \BH\bar b/\bar B_\text{H}b \to H\bbbar \to \bbbar\bbbar. The dataset corresponds to 19.5~\fb of collisions at a centre-of-mass energy of ~TeV. No significant excess of events with respect to SM predictions is observed. Therefore upper limits on the cross section times branching ratio are set. Using a specific benchmark model, a lower limit of 2.0~TeV on the mass is obtained when \mg = 2\mb
On subgroups with narrow Schreier graphs
No full textWe study finitely generated pairs of groups such that the Schreier graph of has at least two ends and is \emph{narrow}. Examples of narrow Schreier graphs include those that are quasi-isometric to finitely ended trees or have linear growth. Under this hypothesis, we show that is a virtual fiber subgroup if and only if contains infinitely many double cosets of . Along the way, we prove that if a group acts essentially on a finite dimensional CAT(0) cube complex with no facing triples then it virtually surjects onto the integers with kernel commensurable to a hyperplane stabiliser.17 pages, 2 figures. V2 extends the main result to a wider class of subgroups, V3 implements the referee\u27s comments. To appear in Bull. Lond. Math. So
Status and Prospects from the ATLAS Detector
No full textSince the startup of the LHC in December 2009, the ATLAS detector has been accumulating data from collisions at center of mass energies of 900 GeV and 7 TeV. Although the integrated luminosity is still low, it is increasing at an accelerated pace. The data have already made it possible to commission and calibrate the various subdetectors, understand their performance in detail and refine the trigger and software reconstruction algorithms. Initial measurements on charged particle multiplicities at sqrt{s} = 900 GeV and 7 TeV as a function of pseudorapidity and transverse momentum have allowed comparisons to results from other experiments at the lower center of mass energy and to various Monte Carlo models of minimum bias events. Standard Model electroweak processes are also being used as benchmarks for validating the analysis and simulation tools. With the higher luminosity expected in the coming year, stringent tests of higher order QCD processes could be achieved. Various models of new physics could be probed and significant constraints obtained. The status of the detector will be summarized, and a brief review of physics results and expectations from early analyses will be given
Prospects for the detection of Kaluza-Klein excitations of gauge bosons in the ATLAS detector at the LHC
No full textKaluza-Klein excitations of the gauge bosons are a notable feature of theories with “small” (~ 1 TeV) extra dimensions. The leptonic decays of the excitations of γ and Z bosons provide a striking signature which can be detected at the LHC. We investigate the reach for these signatures through a parametrized simulation of the ATLAS detector. With an integrated luminosity of 100 fb–1 a peak in the lepton-lepton invariant mass will be detected if the compactification scale (M
c
) is below 5.8 TeV. If no peak is observed, with an integrated luminosity of 300 fb–1 a limit of M
c
<12–13.5 TeV can be obtained from a detailed study of the shape of the lepton-lepton invariant mass distribution. If a peak is observed, the study of the angular distribution of the two leptons will allow to distinguish the KK excitations from alternative models yielding the same signature.
PACS: 11.10Kk – 11.25Mj – 13.85-
Erratum to: “EXOTIC – A heavy fermion and excited fermion Monte Carlo generator for e+e− physics” [Computer Physics Communication 126 (2000) 244–260]
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The Beyond the standard model working group: Summary report
No full textIn this working group we have investigated a number of aspects of searches for new physics beyond the Standard Model (SM) at the running or planned TeV-scale colliders. For the most part, we have considered hadron colliders, as they will define particle physics at the energy frontier for the next ten years at least. The variety of models for Beyond the Standard Model (BSM) physics has grown immensely. It is clear that only future experiments can provide the needed direction to clarify the correct theory. Thus, our focus has been on exploring the extent to which hadron colliders can discover and study BSM physics in various models. We have placed special emphasis on scenarios in which the new signal might be difficult to find or of a very unexpected nature. For example, in the context of supersymmetry (SUSY), we have considered: how to make fully precise predictions for the Higgs bosons as well as the superparticles of the Minimal Supersymmetric Standard Model (MSSM) (parts III and IV); MSSM scenarios in which most or all SUSY particles have rather large masses (parts V and VI); the ability to sort out the many parameters of the MSSM using a variety of signals and study channels (part VII); whether the no-lose theorem for MSSM Higgs discovery can be extended to the next-to-minimal Supersymmetric Standard Model (NMSSM) in which an additional singlet superfield is added to the minimal collection of superfields, potentially providing a natural explanation of the electroweak value of the parameter {micro} (part VIII); sorting out the effects of CP violation using Higgs plus squark associate production (part IX); the impact of lepton flavor violation of various kinds (part X); experimental possibilities for the gravitino and its sgoldstino partner (part XI); what the implications for SUSY would be if the NuTeV signal for di-muon events were interpreted as a sign of R-parity violation (part XII). Our other main focus was on the phenomenological implications of extra dimensions. There, we considered: constraints on Kaluza Klein (KK) excitations of the SM gauge bosons from existing data (part XIII) and the corresponding projected LHC reach (part XIV); techniques for discovering and studying the radion field which is generic in most extra-dimensional scenarios (part XV); the impact of mixing between the radion and the Higgs sector, a fully generic possibility in extra-dimensional models (part XVI); production rates and signatures of universal extra dimensions at hadron colliders (part XVII); black hole production at hadron colliders, which would lead to truly spectacular events (part XVIII). The above contributions represent a tremendous amount of work on the part of the individuals involved and represent the state of the art for many of the currently most important phenomenological research avenues. Of course, much more remains to be done. For example, one should continue to work on assessing the extent to which the discovery reach will be extended if one goes beyond the LHC to the super-high-luminosity LHC (SLHC) or to a very large hadron collider (VLHC) with {radical}s {approx} 40 TeV. Overall, we believe our work shows that the LHC and future hadronic colliders will play a pivotal role in the discovery and study of any kind of new physics beyond the Standard Model. They provide tremendous potential for incredibly exciting new discoveries
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