A search for new phenomena in final states with one or more energetic jets and large missing transverse momentum, ETmissT, is presented. An integrated luminosity of 20 fb-1 is collected from √s = 8 TeV pp collisions at the LHC with the ATLAS detector during 2012 operations. A selection criteria is imposed requiring events to have monojet signatures, with ETmissT > 350 GeV and no identified leptons. Standard Model backgrounds and systematics uncertainties are estimated using a maximum likelihood procedure. The number of events passing this selection criteria is in good agreement with Standard Model expectations. These events are then divided up into three orthogonal signal regions based on the outputs of two Random Forest classifiers trained to classify invisible decays of the Higgs boson produced through the vector boson fusion and Higgs-strahlung production modes. These results are then interpreted in terms of three different Higgs-portal models and translated into upper limits on WIMP-nucleon cross-sections for comparison with direct detection experiments. / Graduate

Although astrophysical evidence supports the existence of dark matter (DM), it remains one of the unanswered questions left by the Standard Model (SM) of Particle Physics. However, under hypotheses of new interactions, the production of dark matter can be detected as an excess of events with large missing transverse momentum (p_T^miss) over the SM background process. This thesis documents a search for new particles at the Compact Muon Solenoid (CMS) at the Large Hadron Collider (LHC), targeting events where large p_T^miss and energetic jets are produced in a proton-proton collision at 13 TeV. The data were collected from 2017 to 2018 during the second half of LHC Run-II. The analysis also targets events where a jet is produced from W or Z bosons identified by a deep-neural-network-based tagger. Multiple control regions targeting specific background processes are defined which estimate background yield in the signal region through a simultaneous fit across control regions of all search channels. The result for the Run-II data corresponding to an integrated luminosity of 137 fb^-1 is obtained by combining this analysis with the previously published 2016 data. No excess of events is observed compared to the SM background expectations. The result of this search is interpreted in several new physics models, including simplified dark matter models, large extra dimension model (ADD), Higgs portal models, and leptoquark models. Limits are set on model parameters providing the most stringent direct constraints on dark matter search from colliders.

The Large Hadron Collider is the most powerful particle accelerator built to date. It is a proton-proton and heavy ion collider which in 2015 and 2016 operated at an unprecedented center of mass energy of √s = 13 TeV. The Tile Calorimeter is the ATLAS hadronic calorimeter covering the central region of the detector. It is designed to measure hadrons, jets, tau particles and missing energy. In order to accurately be able to properly reconstruct these physical objects a careful description of the electronic noise is required. This thesis presents the work done in updating, monitoring and studying the noise calibration constants used in the processing and identication of hadronic jet in the 2011 data. Moreover the results of the searches for compressed supersymmetric squark-neutralino and large extra dimensions models are also presented in this thesis. The present work uses an experimental signature with a high energy hadronic jet and large missing transverse energy later often referred to as monojet signature. The search for supersymmetry is carried out using an integrated luminosity of 3.2 fb-1 recorded by the ATLAS experiment in 2015. The search for large extra dimensions presented in this work uses the full 2015 + 2016 dataset of 36.1 fb-1. No signicant excess compared to the Standard Model prediction has been observed on the production of squark pairs with the subsequent decay of the squark in a quark and a neutrino. Exclusion limits are set on squark production as a function of the neutralino mass. Squark masses up to 608 GeV are excluded for a mass difference between the squark and the neutralino of 5 GeV. In the second search for the presence of large extra spatial dimensions in the Arkani-Hamed, Dimopoulos and Dvali model scenario a good agreement between data and Standard Model prediction is observed and exclusion limits are set on the effective Planck scale MD of 7.7 and 4.8 TeV for two and six hypothesized large extra dimensions respectively signicantly improving earlier results.