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Opponent: Associate professor Pietro Govoni, Milano-Bicocca University, Italien
Supervisor: Associate professor Jonas Strandberg KTH
This thesis presents my contributions to measurements of the Higgs boson production cross-sections at the Large Hadron Collider, and a measurement of the luminosity. The data were recorded by the ATLAS experiment during the Run 2 of the LHC (2015-2018) at a center-of-mass energy of 13 TeV. Only the 2018 data is used for the luminosity measurement. The measurement of Higgs boson cross-sections considers two production modes: gluon-gluon fusion and vector-boson fusion. In both production modes, the analysis focuses on the decay channel in which the Higgs boson decays into a pair of W bosons, which both decay into a lepton and the corresponding neutrinos: H→WW*→ lνlν. The data selection requires events containing exactly one electron (or positron), one muon (or anti-muon), and significant missing transverse energy. The events are then divided into several categories using the simplified template cross section (STXS) framework. The cross-sections are measured across seven categories for gluon-gluon fusion production and eight categories for vector boson fusion production.
Furthermore, measurements are performed with an alternative event categorisation that offers sensitivity to the properties of Higgs boson couplings under charge-conjugation and parity inversion (CP). For this, events featuring multiple jets are categorised by the signed difference of the azimuthal angles of the two jets with the largest transverse momentum.
The overall cross-section times branching ratio is measured to be 12.4+1.3(-1.2) pb for Higgs boson production via gluon-gluon fusion and 0.79+0.18(-0.16) pb for the vector boson fusion channel. These results align well with the Standard Model predictions of 10.4 ± 0.6 pb and 0.81 ± 0.02 pb, respectively, for a Higgs boson with a mass of 125 GeV. The results for each of the STXS framework categories, as well as for the CP-sensitive measurements, are all also consistent with the Standard Model predictions, albeit sometimes with large uncertainties.
A smaller part of this thesis describes my contribution to the luminosity measurement. Accurate luminosity measurements rely on various detectors and algorithms and are essential for the ATLAS physics program. The track counting algorithm is a key element for the offline luminosity measurements. It determines the luminosity by counting the average number of charged particle tracks produced at the interaction point during collisions. In 2018, the LHC began performing a new type of beam scans called emittance scans. These scans can be used as a tool to monitor the stability of the track-counting algorithm over time and ensure that the calibration does not drift