Analysis of a Gluonic Penguin Decay with the BaBar Detector

This thesis presents a branching fraction analysis of the neutral B meson decay channel B → ϕK0s where the K0s decays to π0π0. The decay is dominated by gluonic penguin transitions, which have been very important for the main program of BABAR: the search for physics beyond the Standard Model. The decay channel has been established and is included in the CP analysis, which is sensitive to new physics. The data set consists of 227 million BB̅ pairs recorded by the BABAR detector at the Stanford Linear Accelerator Center. Sophisticated analysis techniques have been applied primarily to suppress background from e+e- → quark/anti-quark reactions. The analysis of such rare decay channels with BABAR relies on the availability of a large set of computer simulated data. For that purpose a computer cluster has been built at the University of Tennessee as part of the distributed computing support work for BABAR. The design and performance of the cluster is a main subject of this thesis work

Status Report of the DPHEP Study Group: Towards a Global Effort for Sustainable Data Preservation in High Energy Physics

Data from high-energy physics (HEP) experiments are collected with significant financial and human effort and are mostly unique. An inter-experimental study group on HEP data preservation and long-term analysis was convened as a panel of the International Committee for Future Accelerators (ICFA). The group was formed by large collider-based experiments and investigated the technical and organisational aspects of HEP data preservation. An intermediate report was released in November 2009 addressing the general issues of data preservation in HEP. This paper includes and extends the intermediate report. It provides an analysis of the research case for data preservation and a detailed description of the various projects at experiment, laboratory and international levels. In addition, the paper provides a concrete proposal for an international organisation in charge of the data management and policies in high-energy physics

Cross-section measurements of top-quark pair production in association with a hard photon at 13 TeV with the ATLAS detector

25 years after the top quark's discovery, the Large Hadron Collider at CERN produces proton-proton collision data on unprecedented scales at unprecedented energies - and has heralded an era of top-quark precision measurements. The production of a top-quark pair in association with a photon ($t\bar{t}\gamma$) gives access to the electromagnetic top-photon coupling, one of the fundamental properties of the top quark. Various extensions of the Standard Model predict modifications of the coupling strength or structure, and deviations from the Standard Model prediction of the $t\bar{t}\gamma$ production cross-section would indicate new physics. With enough statistics available from the Large Hadron Collider, the electron-muon channel has gained particular interest due to its high signal purity and precise available theory predictions. This thesis presents results with the full Run 2 dataset collected with the ATLAS detector in proton-proton collisions at the Large Hadron Collider between 2015 and 2018 at 13 TeV centre-of-mass energy, corresponding to an integrated luminosity of 139 fb$^{-1}$. In order to compare the results to fixed-order calculations that include non-doubly-resonant diagrams, a combined measurement of $t\bar{t}\gamma + tW\gamma$ is performed. The focus is placed on a measurement of the fiducial inclusive cross-section in the electron-muon channel. Furthermore, the ATLAS data is unfolded to parton level and measurements of differential cross-sections as functions of several observables are presented. Both fiducial inclusive and differential results are compared to state-of-the-art fixed-order calculations at next-to-leading order in QCD. An additional focus of the thesis is placed on studies to use machine-learning techniques, in particular deep neural networks, for the identification of prompt photons.Comment: PhD dissertation (Univ. of G\"ottingen), 173 pages, 75 figures, 25 tables. Available on CERN CDS at https://cds.cern.ch/record/2725289

Search for the supersymmetric partner to the top quark in the all-hadronic final state with the ATLAS detector

The Large Hadron Collider and the experiments which rely on it constitute the largest research project for particle physics, both in terms of physical hardware and computing resources, to allow us to probe the frontiers of our understanding of nature, commonly referred to as the Standard Model (SM) of particle physics. A well motivated extension to the existing SM is that of supersymmetry (SUSY), posited to resolve some of the pressing limitations of the SM. SUSY predicts that for each given fermion spin state, there is an associated boson partner and vice-versa. This thesis will cover the decay where the scalar partner to the top quark ($\tilde{t}_1$) is pair produced and decays to a top quark and a stable supersymmetric particle called a neutralino ($\tilde{\chi}^0$). This search is undertaken using the ATLAS detector between LHC run periods 2015-2018 (LHC Run 2) corresponding to a centre-of-mass energy \sqrt{s}=13\TeV and a total integrated luminosity of 139.0\ifb, in particular considering events with no electrons or muons in the final state. Since the supersymmetric particles must be reconstructed from their final-state decay products, efforts were undertaken to constrain the SM backgrounds which produce the same final states, primarily the backgrounds arising from the \ttZ(\to\nu\nu) and \Zboson(\to\nu\nu)+\textrm{jets} processes. This thesis in addition also covers the workflows required from collision event (simulated using Monte Carlo or detected) to a final result. In the absence of any significant excesses, exclusion limits at the 95\% confidence level have been applied on the visible cross section for models with a branching ratio $BR(\tilde{t}_1\to t+\tilde{\chi}^0_1)=100\%$ up to 1.3\TeV in the case where \Delta(\mstop,\mneutralino)>m_{\textrm{top}}. Exclusion limits at the 95\% confidence level have been applied up to (\mstop,\mneutralino)=(600,450)\GeV for signal models where \Delta(\mstop,\mneutralino)\sim m_{\textrm{top}}, similarly up to (\mstop\sim 660\GeV) for models where \Delta(\mstop,\mneutralino) through the use of a new search strategy targeting four body decays ($\tilde{t}_1\to bqq'\tilde{\chi}^0$)