Precise muon momentum calibration and Z mass measurement with the ATLAS experiment, using pp collisions at √s =13 TeV
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Abstract
Standard Model is the foundation of modern particle physics. Precise measurements are in the center of major studies. The Higgs boson discovery probes for further Electro-Weak measurements, to test the consistency of the model.
ATLAS is a multi-purpose detector and one of the experiments at the CERN LHC. With a dedicated tracking and magnetic system, the experiment achieves high precision measurements of the muon momentum at the final state.
In that context, the Z mass measurement in ATLAS is part of these efforts. The Z boson mass has been measured with excellent accuracy in the LEP experiment at CERN, without delving into reconstruction systematic uncertainties.
In ATLAS, the measurement involves the reconstruction of final muon objects introducing corresponding biases. The comparison between the two measurements is of high scientific interest and a test for ATLAS capabilities.
The thesis investigates muon momentum calibration in the ATLAS experiment, with pp collisions, at center of mass energy $\sqrt{s}=13$ TeV.
ATLAS uses "standard candles" resonances for the calibration, $\Jpsi$ meson and Z boson. By comparing di-muon mass spectrum in simulation and data, for the two resonances, the calibration is derived.
The calibration in this thesis is in the context of official ATLAS recommendations but also for Z mass measurement primarily, but also other Electro-Weak measurements, such as the W mass measurement.
The work focuses on calibration challenges using official ATLAS tools, with results divided into two main sections.
The first section addresses calibration work performed for the ATLAS experiment. Specifically, the relative momentum resolution of muons in ATLAS is investigated for the
Inner Detector (ID), the Muon Spectrometer (MS), and the Combined (CB) tracks. From these studies, maps of the muon relative momentum uncertainty are generated for both simulation and data.
These maps reveal systematic deficiencies in the detector subsystems and mis-modelings in the simulations.
The maps are then integrated into the official ATLAS calibration framework to assess their potential for improving calibration precision, with a comparison of results both with and without this additional information.
Lastly, the process and outcomes of deriving the official ATLAS calibration recommendations are presented. These recommendations are developed separately for the ID, MS, and CB tracks.
The second part of this thesis focuses on calibration in the context a the Z mass measurement, including a detailed analysis of a Z mass measurement with the applied calibration.
The calibration process is altered so it does not include the Z boson resonance. Calibration corrections are derived with the $\Jpsi$ meson.
Using official ATLAS tools, the calibration is compared for Prompt and Non-Prompt $J/\psi$ mesons. Kinematic distributions for both types are analyzed, and the differences between their calibrations are discussed.
The calibration parameters are derived as a function of detector pseudorapidity and muon transverse momentum.
Lastly, an assessment of the calibration uncertainties on the Z boson mass is extracted using a likelihood fit, with the di-muon channel.