This thesis presents measurements of inclusive and differential cross-sections for top quark pair-production in fiducial and full phase-space regions, as well as an analysis of gas compositions in the ATLAS inner detector cooling systems. The inclusive top-anti-top quark production cross-section was measured using a sample of 0.085 1/fb of proton--proton collisions recorded by the ATLAS experiment at centre-of-mass energy of 13 TeV, using dilepton top-anti-top quark events with an opposite-sign ee or ÂμÂμ pair in the final state. The numbers of events with exactly one and exactly two jets originating from b-quarks were counted to determine the top-anti-top quark production cross-section . The efficiency to reconstruct and identify a jet as originating from a b-quark from a top quark decay was extracted simultaneously, minimising the associated systematics. The cross-section was measured to be 749 ± 57 (stat.) ± 79 (syst.) ± 74 (lumi.) pb, giving a total relative uncertainty of 16%. The differential top-anti-top quark production production cross-sections were measured using top-anti-top quark events with one electron and one muon of opposite sign electric charge, and at least two jets, one of which was required to be tagged as containing a b-hadron. Initially, the differential cross-sections were measured in a fiducial phase-space region, using an integrated luminosity of 3.2 1/fb of proton--proton data at a centre-of-mass energy of 13 TeV, recorded by the ATLAS experiment in 2015. Subsequently, the single and double differential cross-sections were measured in both a fiducial phase-space region and the full phase-space, using 36.1 1/fb of proton--proton data at 13 TeV, recorded by the ATLAS experiment in 2015 and 2016. The measured differential cross-sections are compared to predictions of next-to-leading order generators matched to parton showers. Custom ultrasonic instruments have been developed for simultaneous monitoring of binary gas mixture and flow in the ATLAS inner detector. These instruments are sensitive to leaks in the cooling systems of the ATLAS inner detector. Software for on- and off-line analysis of these instruments has been developed and proven with the analysis of the cool-down to operation temperature in January 2016.

This thesis contains phenomenological studies of top quark production in the forward region of phase space at the Large Hadron Collider (LHC). The production of highly forward top quarks has so far not been observed in nature. As the LHCb detector at the LHC is instrumented in the forward region, this opens the possibility of studying top quark properties in this yet unexplored region of phase space. This feasibility of performing cross section and charge asymmetry measurements of top quark pairs with available and future LHCb data is studied in detail. In both cases, potential analysis strategies are proposed, and sophisticated theoretical predictions are provided for both signal and background. By studying the expected experimental sensitivity of cross section measurements, the potential constraints on parton distribution functions is quantified. The sensitivity of the proposed charge asymmetry measurements with the full Run II LHCb data is also considered.

In this thesis, the search for the production of the Higgs boson in associationwith two top quarks is presented. The main focus of this work is on the analysis optimised for the decay of the Higgs boson to a b-quark pair. The analysis is performed using 36.1 fb−1 of ppcollision data at a centre of mass energy sqrt(s)=13 TeV collected by the ATLAS detector at the Large Hadron Collider during 2015 and 2016. The signal strength of ttH in relation to the Standard Model prediction for a Higgs boson with a mass of 125 GeV is measured to be mu(ttH) = 0.87 +0.64−0.61, with signal strengths greater than 2.0 excluded at the 95% confidence level. The combination of this analysis with searches targeting additional Higgs boson decay modes is subsequently presented. The measured signal strength in relation to the Standard Model prediction is mu(ttH) = 1.2 +/-0.3. This correspondsto an observed (expected) significance for ttH of 4.2sigma (3.8sigma), constituting evidence for the ttH production mode. Finally, a study into the ability to observe and model the colour connection of b-quarks in ttH(bb) and ttbar+jets events is presented. The jet pull angle observable is used to investigate the effect of colour connection on jet substructure. Such an observable is found to be sensitive to the underlying colour structure in events, showing differences between b-quarks which decay from a colour singlet in comparison to a colour octet. However, the effect is found to be small and a larger dataset is required to measure the effect in ttH events.

No description available.

No description available.

No description available.

No description available.

No description available.