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.

This book discusses searches for Dark Matter at the CERN’s LHC, the world’s most powerful accelerator. It introduces the relevant theoretical framework and includes an in-depth discussion of the Effective Field Theory approach to Dark Matter production and its validity, as well as an overview of the formalism of Simplified Dark Matter models. Despite overwhelming astrophysical evidence for Dark Matter and numerous experimental efforts to detect it, the nature of Dark Matter still remains a mystery and has become one of the hottest research topics in fundamental physics. Two searches for Dark Matter are presented, performed on data collected with the ATLAS experiment. They analyze missing-energy final states with a jet or with top quarks. The analyses are explained in detail, and the outcomes and their interpretations are discussed, also in view of the precedent analysis of theoretical approaches. Given its depth of coverage, the book represents an excellent reference guide for all physicists interested in understanding the theoretical and experimental considerations relevant to Dark Matter searches at the LHC.

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 brief book offers an interesting, fun, and widely accessible first-person tour of CERN, the European Center for Nuclear Research, the largest particle physics laboratory in the world. The facilities at CERN include the Large Hadron Collider (LHC), a 27-kilometer particle accelerator that straddles the border between Switzerland and France. The LHC was famously used to discover the Higgs boson, a long-sought fundamental particle. Physics historian Depambour (University of Paris) is enthusiastic about all aspects of CERN, especially its role as an agent for peace and international cooperation. The book focuses mainly on the physical layout of the CERN campus and its experimental facilities, but Depambour also includes an introduction to the standard model of particle physics and a history of the search for the Higgs boson. Supporting illustrations and interviews help convey the atmosphere and culture of CERN. The book can be read and enjoyed by virtually anyone interested in modern science, starting with students currently in high school. It will also be welcome as a useful orientation for undergraduates and graduate students whose research interests might eventually take them to CERN. Summing Up: Highly recommended. All readers.'CHOICEWhat lies within CERN's entrails? What is the path followed by the particles that are accelerated before they collide? What does the ATLAS detector look like? Does research at CERN find applications in everyday life?From the accelerator control room to the huge Computing Centre, via the auditorium where the discovery of the Higgs boson was announced in July 2012, I invite you to experience for one day an immersion in the world of research in particle physics! Discovering emblematic installations at CERN, walking through the places where people spend every working day, meeting with researchers in various fields, descending into the ATLAS cavern ... Our visit, whose path will mimic that of the particles during their journey, will be full of anecdotes and surprises.Follow me for a guided tour of CERN, the largest scientific collaboration in the world!

This thesis deals with two main procedures performed with the ATLAS detector at the Large Hadron Collider (LHC). The noise description in the hadronic calorimeter TileCal represents a very valuable technical job. The second part presents a fruitful physics analysis - the cross section measurement of the process p+p → Z0 → τ + τ. The Monte Carlo simulations of the TileCal are described in the first part of the thesis, including a detailed treatment of the electronic noise and multiple interactions (so-called pile-up). An accurate description of both is crucial for the reconstruction of e.g. jets or hadronic tau-jets. The second part reports a Standard Model measurement of the Z0 → τ + τ process with the emphasis on the final state with an electron and a hadronically decaying tau-lepton. The Z0 → τ + τ channel forms the dominant background in the search for Higgs bosons decaying into tau lepton pairs, and thus the good understanding achieved here can facilitate more sensitive Higgs detection.