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A measurement of the differential Z boson production cross section in proton-proton collisions is presented. It furnishes a precision test of the Standard Model, and constrains the parton distribution functions of the proton. Moreover, it is a building block for future measurements of the mass of the W± boson. A study of the efficiency of lepton identification algorithms is performed which drives the precision of the measurement at lower values of transverse momentum. In tandem, a search for new physics in events with a Z boson produced in association with large missing transverse momentum is presented. The results of this search are interpreted in the context of several dark matter models: generic spin-0 or spin-1 mediators, invisible decays of a Higgs-like boson, unparticles, and large extra spatial dimensions. A multivariate analysis was developed to enhance the sensitivity of the invisible Higgs interpretation. The theoretical uncertainty on the irreducible background from electroweak diboson processes is constrained by emulating the missing energy using pure control samples in the fully leptonic final states. The data were collected with the Compact Muon Solenoid detector at the Large Hadron Collider and correspond to an integrated luminosity of 35.9 fb-1. No significant deviations from the Standard Model are found.
This thesis describes the search for Dark Matter at the LHC in the mono-jet plus missing transverse momentum final state, using the full dataset recorded in 2012 by the ATLAS Experiment. It is the first time that the number of jets is not explicitly restricted to one or two, thus increasing the sensitivity to new signals. Instead, a balance between the most energetic jet and the missing transverse momentum is required, thus selecting mono-jet-like final states. Collider searches for Dark Matter have typically used signal models employing effective field theories (EFTs), even when comparing to results from direct and indirect detection experiments, where the difference in energy scale renders many such comparisons invalid. The thesis features the first robust and comprehensive treatment of the validity of EFTs in collider searches, and provides a means by which the different classifications of Dark Matter experiments can be compared on a sound and fair basis.
This thesis reports on the search for dark matter in data taken with the ATLAS detector at CERN’s Large Hadron Collider (LHC). The identification of dark matter and the determination of its properties are among the highest priorities in elementary particle physics and cosmology. The most likely candidate, a weakly interacting massive particle, could be produced in the high energy proton-proton collisions at the LHC. The analysis presented here is unique in looking for dark matter produced together with a Higgs boson that decays into its dominant decay mode, a pair of b quarks. If dark matter were seen in this mode, we would learn directly about the production mechanism because of the presence of the Higgs boson. This thesis develops the search technique and presents the most stringent production limit to date.
In the field of particle and astrophysics, one of the major unresolved problems is to understand the nature and properties of dark matter, which constitutes almost 80% of the matter content of the universe. This book gives a pedagogical introduction to the field of dark matter in general, and in particular to the model building perspective. Starting from the evidence and need for dark matter, it goes into the deeper understanding of how to accommodate a dark matter candidate in a particle physics model. This book focuses on teaching the basic tools for model building of dark matter, starting from the easiest to gradually the difficult one. Although there are plenty of dark matter models available in the literature, this book concentrates on the important ones. This book aims to motivate the reader to propose a new dark matter model complying with all observational constraints.
A host of astrophysical measurements suggest that most of the matter in the Universe is an invisible, nonluminous substance that physicists call “dark matter.” Understanding the nature of dark matter is one of the greatest challenges of modern physics and is of paramount importance to our theories of cosmology and particle physics. This text explores one of the leading hypotheses to explain dark matter: that it consists of ultralight bosons forming an oscillating field that feebly interacts with light and matter. Many new experiments have emerged over the last decade to test this hypothesis, involving state-of-the-art microwave cavities, precision nuclear magnetic resonance (NMR) measurements, dark matter “radios,” and synchronized global networks of atomic clocks, magnetometers, and interferometers. The editors have gathered leading experts from around the world to present the theories motivating these searches, evidence about dark matter from astrophysics, and the diverse experimental techniques employed in searches for ultralight bosonic dark matter. The text provides a comprehensive and accessible introduction to this blossoming field of research for advanced undergraduates, beginning graduate students, or anyone new to the field, with tutorials and solved problems in every chapter. The multifaceted nature of the research – combining ideas and methods from atomic, molecular, and optical physics, nuclear physics, condensed matter physics, electrical engineering, particle physics, astrophysics, and cosmology – makes this introductory approach attractive for beginning researchers as well as members of the broader scientific community. This is an open access book.
This thesis describes in detail a search for weakly interacting massive particles as possible dark matter candidates, making use of so-called mono-jet events. It includes a detailed description of the run-1 system, important operational challenges, and the upgrade for run-2. The nature of dark matter, which accounts for roughly 25% of the energy-matter content of the universe, is one of the biggest open questions in fundamental science. The analysis is based on the full set of proton-proton collisions collected by the ATLAS experiment at the Large Hadron Collider at √s = 8 TeV. Special attention is given to the experimental challenges and analysis techniques, as well as the overall scientific context beyond particle physics. The results complement those of non-collider experiments and yield some of the strongest exclusion bounds on parameters of dark matter models by the end of the Large Hadron Collider run-1. Details of the upgrade of the ATLAS Central Trigger for run-2 are also included.
A search for evidence of particle dark matter (DM) and unparticle production at the LHC has been performed using events containing two charged leptons, consistent with the decay of a Z boson, and large missing transverse momentum. This study is based on data collected with the CMS detector corresponding to an integrated luminosity of 19.7 inverse femtobarns of pp collisions at the LHC at a center-of-mass energy of 8 TeV. No significant excess of events is observed above the number expected from the standard model contributions. The results are interpreted in terms of 90% confidence level limits on the DM-nucleon scattering cross section, as a function of the DM particle mass, for both spin-dependent and spin-independent scenarios. Limits are set on the effective cutoff scale Lambda, and on the annihilation rate for DM particles, assuming that their branching fraction to quarks is 100%. Additionally, the most stringent 95% confidence level limits to date on the unparticle model parameters are obtained.
This dissertation describes a search for the invisible decays of dark matter particles produced in association with a Z boson, where the latter decays to a charged lepton pair. The dataset for this search includes 13.3 1/fb of collisions recorded in 2015 and 2016 at a centre-of-mass energy of 13 TeV in the ATLAS detector at the Large Hadron Collider in Geneva, Switzerland. The invisible particles manifest themselves as missing transverse momentum, or MET, in the detector, while the charged leptons of interest are electron (e+e-) or muon (mu+mu-) pairs. The models simulated for this study are vector mediated simplified models with Dirac fermionic dark matter particles with couplings g_q = 0.25, g_X = 1 and g_l = 0 . The main background to this analysis, ZZ->llvv, is irreducible, as it shares the same signature as the signal. It is estimated with Monte Carlo simulations including contributions from both qq->ZZ and gg->ZZ production modes. Where possible, other backgrounds are estimated using data-driven techniques and reduced through various selection criteria. The final search is performed by looking for a deviation from the Standard Model background expectation in the MET distribution using two signal regions, e+e- and mu+mu-. This is done using statistical tools to make a likelihood fit and set a 95% confidence level limit as no deviations are found. Limits are placed on the presented model of dark matter for mediator masses up to 400 GeV and for a range of dark matter masses from 1 to ~200 GeV.
Results of a search for new phenomena in events with large missing transverse momentum and a Higgs boson decaying to two photons are reported. Data from proton-proton collisions at a center-of-mass energy of 8 TeV and corresponding to an integrated luminosity of 20.3 fb-1 have been collected with the ATLAS detector at the LHC. The observed data are well described by the expected standard model backgrounds. Upper limits on the cross section of events with large missing transverse momentum and a Higgs boson candidate are also placed. Exclusion limits are presented for models of physics beyond the standard model featuring dark-matter candidates.
A search for dark matter is performed using events with large missing transverse momentum and a Higgs boson decaying either to a pair of bottom quarks or to a pair of photons. The data from proton-proton collisions at a center-of-mass energy of 13 TeV, collected with the CMS detector at the LHC, correspond to an integrated luminosity of 2.3 inverse-femtobarns. Results are interpreted in the context of a Z'-two-Higgs-doublet model, where a high-mass resonance Z' decays into a pseudoscalar boson A and a CP-even scalar Higgs boson, and the A decays to a pair of dark matter particles. No significant excesses are observed over the background prediction. Combining results from the two decay channels yields exclusion limits in the signal cross section in the m[Z']-m[A] phase space. The observed data exclude, for Z' coupling strength g[Z'] = 0.8 and m[A] = 300 GeV for example, the Z' mass range of 600 to 1860 GeV. This is the first result on a search for dark matter produced in association with a Higgs boson that includes constraints on h to gamma-gamma obtained at sqrt(s) = 13 TeV.