Measurement of the Higgs boson mass and production cross section and calibration of the muon momentum with the ATLAS detector at the LHC

Thesis

The ATLAS and CMS experiments at the Large Hadron Collider, located in CERN in Geneva, Switzerland announced the discovery of the Higgs boson on the 4th of July, 2012. Measurements of the Higgs boson properties provide stringent consistency tests for the Standard Model, and offer critical input parameters for theoretical models both within and beyond the Standard Model. Such measurements therefore require a precise understanding of the underlying detector throughout its operation. This thesis contributes to both aspects: measurements of the Higgs boson mass and cross-section, and studies of ATLAS detector performance through muon momentum calibration.

As the Higgs can decay to muons, improvements to the muon detector performance directly translate to the corresponding systematic uncertainties in the measurement of Higgs properties. This motivates the inclusion of new techniques in muon momentum calibration, including the addition of negative log-likelihood scans and generator reweighting. The calibration procedure is performed for both 2015-2018 and 2022 datasets, and reduces the leading systematic uncertainty in the Higgs mass measurement by 50\%.

The measurements of Higgs boson properties are performed for the Higgs decaying into a pair of Z bosons, and subsequently decaying into two pairs of electrons or muons. This is a fully reconstructible final state with a high signal-to-background ratio, thus an ideal final state for precision measurements. The Higgs boson mass measurement utilises proton-proton collision data collected between 2015 - 2018 at the centre-of-mass energy $\sqrt{s}$ = 13 TeV. A number of techniques, including per-event mass uncertainty, Z-mass constraint, and kernel estimation for background modelling, are included to reduce the statistical uncertainty of the measurement. The measured Higgs mass is $124.99 \pm 0.18 \textrm{ (stat.)}\pm 0.04\textrm{ (sys.)}$ GeV using the 2015-2018 data. The Higgs boson fiducial cross-section measurement utilises proton-proton collision data collected in 2022 at the centre-of-mass energy $\sqrt{s}$ = 13.6 TeV. The fiducial cross-section for its decay into a pair of Z bosons is measured to be $\sigma_{fid} = 2.80 \pm 0.74$ fb, and $\sigma_{tot} = 46 \pm 12$ pb once extrapolated to the full phase-space.

Authors: Yan, S

• DOI: 10.5287/ora-ag8yaobgm
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Higgs physics; collider physics; particle physics; muon physics; detector performance