Searches for the top quark in p{bar p} collisions at (square root)s = 1.8 TeV are described. The analyses are based on data with an integrated luminosity of 4.4 pb−1 recorded with the Collider Detector at Fermilab in the 1988--1989 run. An upper limit on the t{bar t} cross section is obtained. The top quark with mass below 89 GeV/c2 is excluded at the 95% CL. Prospects for searches for the top quark in the future are presented. We also briefly present results on searches for supersymmetric particles. 14 refs., 10 figs.
This PhD thesis documents two of the highest-profile searches for supersymmetry performed at the ATLAS experiment using up to 80/fb of proton-proton collision data at a center-of-mass energy of 13 TeV delivered by the Large Hadron Collider (LHC) during its Run 2 (2015-2018). The signals of interest feature a high multiplicity of jets originating from the hadronisation of b-quarks and large missing transverse momentum, which constitutes one of the most promising final state signatures for discovery of new phenomena at the LHC. The first search is focused on the strong production of a pair of gluinos, with each gluino decaying into a neutralino and a top-antitop-quark pair or a bottom-antibottom-quark pair. The second search targets the pair production of higgsinos, with each higgsino decaying into a gravitino and a Higgs boson, which in turn is required to decay into a bottom-antibottom-quark pair. Both searches employ state-of-the-art experimental techniques and analysis strategies at the LHC, resulting in some of the most restrictive bounds available to date on the masses of the gluino,neutralino, and higgsino in the context of the models explored.
This book reports a search for theoretically natural supersymmetry (SUSY) at the Large Hadron Collider (LHC). The data collected with the ATLAS detector in 2012 corresponding to 20 /fb of an integrated luminosity have been analyzed for stop pair production in proton–proton collisions at a center-of-mass energy of 8 TeV at the Large Hadron Collider (LHC) in the scenario of the higgsino-like neutralino. The author focuses on stop decaying into a bottom quark and chargino. In the scenario of the higgsino-like neutralino, the mass difference between charginos and neutralinos (Δm) is expected to be small, and observable final-state particles are likely to have low-momentum (soft). The author develops a dedicated analysis with a soft lepton as a probe of particles from chargino decay, which suppresses the large amount of backgrounds. As a result of the analysis, no significant SUSY signal is observed. The 95% confidence-level exclusion limits are set to masses of stop and neutralino assuming Δm = 20 GeV. The region with ΔM (the mass difference between stop and neutralino) 70 GeV is excluded for the first time at stop mass of less than 210 GeV. The author also excludes the signals with ΔM 120 GeV up to 600 GeV of stop mass with neutralino mass of less than 280 GeV. The author clearly shows very few remaining parameter spaces for light stop (e.g., topology of stop decay is extremely similar to the SM top quark) by combining his results and previous ATLAS analyses. His results provide a strong constraint to searches for new physics in the future.
These course-tested lectures provide a technical introduction to Supersymmetric Grand Unified Theories (SUSY GUTs), as well as a personal view on the topic by one of the pioneers in the field. While the Standard Model of Particle Physics is incredibly successful in describing the known universe it is, nevertheless, an incomplete theory with many free parameters and open issues. An elegant solution to all of these quandaries is the proposed theory of SUSY GUTs. In a GUT, quarks and leptons are related in a simple way by the unifying symmetry and their electric charges are quantized, further the relative strength of the strong, weak and electromagnetic forces are predicted. SUSY GUTs additionally provide a framework for understanding particle masses and offer candidates for dark matter. Finally, with the extension of SUSY GUTs to string theory, a quantum-mechanically consistent unification of the four known forces (including gravity) is obtained. The book is organized in three sections: the first section contains a brief introduction to the Standard Model, supersymmetry and the Minimal Supersymmetric Standard Model. Then SUSY GUTs in four space-time dimensions are introduced and reviewed. In addition, the cosmological issues concerning SUSY GUTs are discussed. Then the requirements for embedding a 4D SUSY GUT into higher-dimensional theories including gravity (i.e. String Theory) are investigated. Accordingly, section two of the course is devoted to discussing the so-called Orbifold GUTs and how in turn they solve some of the technical problems of 4D SUSY GUTs. Orbifold GUTs introduce a new set of open issues, which are then resolved in the third section in which it is shown how to embed Orbifold GUTs into the E(8) x E(8) Heterotic String in 10 space-time dimensions.
Events with signatures involving large missing transverse energy (E{sub T}) are among the quintessential search modes for R-parity conserving supersymmetry. CDF has conducted two recent analyses for supersymmetry which use E{sub T} and jets. The E{sub T} and monojet signature is employed to determine process independent limits for the production of new physics beyond the Standard Model and then applied to models of spontaneous breaking of supersymmetry to determine limits on the supersymmetry breaking parameter and the gravitino mass. Direct searches for scalar top and scalar bottom quarks within the framework of supersymmetric models are performed using a signature of E{sub T} and two heavy flavor jets. Since the data is found to be consistent with Standard Model expectations, limits are determined in the mass planes m({tilde {chi}}1°)-m({tilde t}1) and m({tilde {chi}}1)°-m({tilde b}1).
Supersymmetry (SUSY) is an extension to the Standard Model (SM) which introduces supersymmetric partners of the known fermions and bosons. Top squark (stop) searches are a natural extension of inclusive SUSY searches at the Large Hadron Collider (LHC). If SUSY solves the naturalness problem, the stop should be light enough to cancel the top loop contribution to the Higgs mass parameter. The 3rd generation squarks may be the rst SUSY particles to be discovered at the LHC. The stop can decay into a variety of nal states, depending, amongst other factors, on the hierarchy of the mass eigenstates formed from the linear superposition of the SUSY partners of the Higgs boson and electroweak gauge bosons. In this study the relevant mass eigenstates are the lightest chargino (X ±/1) and the neutralino (X 0/1). A search is presented for a heavy SUSY top partner decaying to a lepton, neutrino and the lightest supersymmetric particle (X 0/1) , via a b-quark and a chargino (X ±/1) in events with two leptons in the nal state. The analysis targets searches for a SUSY top partner by means of a Multivariate Analysis Technique, used to discriminate between the stop signal and the background with a learning algorithm based on Monte Carlo generated signal and background samples. The analysis uses data corresponding to 20.3 fb−1 of integrated luminosity at root s = 8 TeV, collected by the ATLAS experiment at the LHC in 2012.
Supersymmetry is at an exciting stage of development. It extends the Standard Model of particle physics into a more powerful theory that both explains more and allows more questions to be addressed. Most importantly, it opens a window for studying and testing fundamental theories at the Planck scale. Experimentally we are finally entering the intensity and energy and sensitivity regions where superpartners and supersymmetric dark matter candidates are likely to be detected, and then studied. There has been progress in understanding the remarkable physics implications of supersymmetry, including the derivation of the Higgs mechanism, the unification of the Standard Model forces, cosmological connections such as a candidate for the cold dark matter of the universe and consequences for understanding the cosmological history of the universe, and more.This volume begins with an excellent pedagogical introduction to the physics and methods and formalism of supersymmetry which is accessible to anyone with a basic knowledge of the Standard Model of particle physics. Next is an overview of open questions, followed by chapters on topics such as how to detect superpartners and tools for studying them, the current limits on superpartner masses as we enter the LHC era, the lightest superpartner as a dark matter candidate in thermal and non-thermal cosmological histories, and associated Z' physics. Most chapters have been extended and updated from the earlier edition and some are new.This superb book will allow interested physicists to understand the coming experimental and theoretical progress in supersymmetry and the implications of discoveries of superpartners, and will also help students and workers to quickly learn new aspects of supersymmetry they want to pursue.
Astrophysical observations implying the existence of Dark Matter and Dark Energy, which are not described by the Standard Model (SM) of particle physics, have led to extensions of the SM predicting new particles that could be directly produced at the Large Hadron Collider (LHC) at CERN. Based on 2015 and 2016 ATLAS proton-proton collision data, this thesis presents searches for the supersymmetric partner of the top quark, for Dark Matter, and for DarkEnergy, in signatures with jets and missing transverse energy. Muon detection is key to some of the most important LHC physics results, including the discovery of the Higgs boson and the measurement of its properties. The efficiency with which muons can be detected with the ATLAS detector is measured using Z boson decays. The performance of high-precision Monitored Drift Tube muon chambers under background rates similar to the ones expected for the High Luminosity-LHC is studied.
The PASCOS (International Symposium on Particles, Strings and Cosmology) series brings together the leading experts and most active young researchers in the closely related fields of elementary particle physics, string theory and cosmology/astrophysics. These areas of research have become increasingly intertwined in recent years, each having direct impact on the others. In particular, there has been a dramatic expansion of ideas from particle theory and string theory that have vast impact on cosmology, especially our picture of the early universe and its evolution. Correspondingly, the proliferation of data regarding the early universe, and its increasing precision, has begun to strongly constrain the theoretical models. Meanwhile, observations of neutrino oscillations and cosmic ray excesses, and limits on new physics from colliders and other particle experiments, as well as the resulting restrictions on theoretical and phenomenological modeling, are becoming ever stronger. During PASCOS99, it became clear that the long-awaited era of convergence of these fields is truly at hand.The proceedings of PASCOS 99 reflect the accelerating overlap and convergence of the fields of elementary particles physics, string theory and cosmology/astrophysics. Plenary reviews by leading figures in these fields provide perspectives on these interrelationships and up-to-the-minute summaries of recent progress in the various areas. Parallel talk summaries focus on many of the topics within each field of greatest current interest and activity. Both the plenary and parallel writeups are designed to be descriptive in nature and avoid being overly technical. As a result, the volume can serve as a useful reference for students and professionals in all three fields. Careful referencing allows further pursuit of a given topic. Overall, the proceedings are unique in that they not only bring together in a single volume comprehensive overview of the great progress being made in all three of these very exciting fields, but also provide a snapshot of how particles, strings and cosmology are increasingly impacting one another.
Derived from a course given at the University of Maryland for advanced graduate students, this book deals with some of the latest developments in our attempts to construct a unified theory of the fundamental interactions of nature. Among the topics covered are spontaneous symmetry breaking, grand unified theories, supersymmetry, and supergravity. The book starts with a quick review of elementary particle theory and continues with a discussion of composite quarks, leptons, Higgs bosons, and CP violation; it concludes with consideration of supersymmetric unification schemes, in which bosons and leptons are considered in some sense equivalent. The second edition is updated and corrected and contains new chapters on recent developments. From reviews of the first edition: "This book captures the exciting developments of grand unification and supersymmetry of fundamental interactions in quantum field theory... gives a self-contained field-theoretic treatment of the complete subject... almost every possible development is included here." #Mathematical Reviews#1
