Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements

Study of Muon Drift Tube Detectors and Fast Readout Electronics for Very High Counting Rates

The Large Hadron Collider (LHC) at the European Center for Particle Physics, CERN, collides protons and lead ions with high luminosities at a center-of-mass energy of 13 TeV in the center of four detectors, of which one is the ATLAS experiment. After more than a decade of successful operation with the discovery of the Higgs boson, the instantaneous luminosity will be increased by almost an order of magnitude with the High Luminosity LHC (HL-LHC) in order to perform precision measurements of the Higgs boson and top quark and extend the searches for physics beyond the Standard Model of particle physics. A proposal for a further major increase of the collision energy and luminosities is the Future Circular Collider (FCC). In order to cope with the increased particle fluxes, upgrades of the LHC experiments and the development of new detector technologies are needed. In this thesis, studies of the potential of small-diameter Muon Drift Tube (sMDT) chambers for precise track reconstruction at high particle fluxes and concepts of new readout electronics are presented. Twelve new sMDT chambers for the ATLAS Muon Spectrometer have already been constructed, tested and installed in ATLAS in order to gain experience and to optimize them for future applications. New read-out electronics concepts with improved signal shaping, which allow for he full exploitation of the sMDT rate capability have been studied. Measurements with generated pulses and on sMDT chambers in a muon beam at CERN under unprecedentedly high γ irradiation show the advantages of the new shaping schemes. The findings are supported by detailed simulations of the high rate effects and allow for prediction of the performance of the sMDT chambers and new readout electronics at the background particle fluxes expected at the FCC

Measurement of the W±ZW^{\pm}Z boson pair-production cross section in pppp collisions at s=13\sqrt{s}=13 TeV with the ATLAS Detector

The production of $W^{\pm}Z$ events in proton--proton collisions at a centre-of-mass energy of 13 TeV is measured with the ATLAS detector at the LHC. The collected data correspond to an integrated luminosity of 3.2 fb$^{-1}$. The $W^{\pm}Z$ candidates are reconstructed using leptonic decays of the gauge bosons into electrons or muons. The measured inclusive cross section in the detector fiducial region for leptonic decay modes is $\sigma_{W^\pm Z \rightarrow \ell^{'} \nu \ell \ell}^{\textrm{fid.}} = 63.2 \pm 3.2$ (stat.) $\pm 2.6$ (sys.) $\pm 1.5$ (lumi.) fb. In comparison, the next-to-leading-order Standard Model prediction is $53.4^{+3.6}_{-2.8}$ fb. The extrapolation of the measurement from the fiducial to the total phase space yields $\sigma_{W^{\pm}Z}^{\textrm{tot.}} = 50.6 \pm 2.6$ (stat.) $\pm 2.0$ (sys.) $\pm 0.9$ (th.) $\pm 1.2$ (lumi.) pb, in agreement with a recent next-to-next-to-leading-order calculation of $48.2^{+1.1}_{-1.0}$ pb. The cross section as a function of jet multiplicity is also measured, together with the charge-dependent $W^+Z$ and $W^-Z$ cross sections and their ratio