Measurement of the transverse momentum of Z bosons with ATLAS Detector

The Standard Model (SM) of particle physics is so far the best theory to describe the properties and interactions of fundamental particles. It successfully predicted the existence of the $W$ and $Z$ bosons, discovered in 1983 at CERN's SPS, and the Higgs boson, discovered in 2012 by the A Toroidal LHC Apparatus (ATLAS) and the Compact Muon Solenoid~(CMS) experiments at the Large Hadron Collider (LHC). It is however well known that many questions remain unexplained by the SM, hence physicists have been for decades searching for new physics evidence, but so far no solid signals have been established. Improving the precision of Standard Model parameters will constraint the properties of possible new phenomena, and hence provides a new handle to explore for new physics. This thesis presents precise measurements of the $Z$ boson transverse momentum using a special dataset collected by the ATLAS experiment from proton-proton collisions at 5 TeV and 13 TeV at the Large Hadron Collider. The transverse momentum of the $W$ boson is then calibrated using this $Z$ boson transverse momentum measurement. These measurements are key ingredients for the next $W$ mass measurement from the ATLAS experiment that is planned to be released later. In addition, the thesis covers work done for the ATLAS detector upgrade, both in the vertex pixel detector (ITk) and the High Granularity Timing Detector (HGTD), for the High-Luminosity-LHC planned to start in 2029. These detectors are essential to allow the continuation of these precision physics measurement in ATLAS in the longer term. The $W$ boson mass, $m_{\mathrm{W}}$, is one of the most sensitive parameters in the standard model. Significant deviation between the prediction and measurement of the $m_{\mathrm{W}}$, would be a potential indication of new physics. Recently, the precision measurement from the CDF II collaboration raised a significant tension between the experimental observations and the SM theoretical expectations. A key ingredient of the $W$ mass measurement is the modeling of the $W$ boson transverse momentum, hence its precise measurement is of the utmost importance and one of the main challenges in the $W$ mass measurement. Given that the $W$ boson decays into a neutrino, that cannot be directly observed, the transverse momentum of the $W$ boson is measured through the hadronic recoil produced simultaneously with the $W$. The hadronic recoil is calibrated through the measurement of the $Z$ transverse momentum in the dilepton decay channel. Thus the $Z$ transverse momentum measurement is a crucial input to the $W$ transverse momentum and $W$ mass measurements. The $Z$ and $W$ boson transverse momenta measurements are also great probes to explore parton distribution functions of the proton and quantum chromodynamics, especially in the low $p_{\mathrm{T}}$ region, where QCD physics become non-perturbative. The $W$ boson recoil measurement, and hence the $W$ mass, is harder to make in a high instantaneous luminosity environment, where hard collisions occur simultaneously. Hence, ATLAS collected the dataset, used in this thesis, with very low instantaneous luminosity specifically to perform this measurement. The dataset corresponds to an integrated luminosity of 335 pb$^{−1}$ at 13 TeV and 257 pb$^{−1}$ at 5 TeV. $W$ and $Z$ boson events decaying in either electrons or muons are selected. Top and electroweak background processes are estimated using Monte Carlo, while QCD multi-jet backgrounds are estimated directly from data. Detector efficiency, acceptance and response are estimated using Monte Carlo. The transverse momentum distributions obtained from the $Z$ boson dilepton decay products and from the $Z$ boson recoil are retrieved from the corresponding reconstructed measurements using a Bayesian unfolding technique. The two measurements are compared to each other to validate the recoil calibration, which in turn, is applied to the $W$ boson sample to extract the $W$ transverse momentum measurement. The precision of the $Z$ boson transverse momenta measurements is around 1% in the low $p_{\mathrm{T}}$ region, dominated by data statistical uncertainty. In turn, $W$ boson transverse momenta measurements at low $p_{\mathrm{T}}$ ~also achieve a precision below 1%, after the calibration with $Z$ boson events. The precision is dominated by the hadronic recoil calibration and unfolding uncertainties. The measurements from the $Z$ transverse momenta are compared to various theoretical predictions. The agreement between measurements and predictions is better at 5 TeV than 13 TeV, especially for those predictions that were tuned with 7 TeV $Z$ boson production data. Starting in 2029, the LHC will be upgraded to much higher luminosities. This will produce significantly larger datasets that can be used for SM precision physics, but require an upgraded ATLAS detector. As part of my ATLAS authorship qualification task, I worked on the upgrade of the endcap pixel vertex detector, studying assembly techniques at Rutherford-Appleton Laboratory in UK. In addition, I measured the electronics properties of Low-Gain Avalanche Diode sensors developed at IHEP for the HGTD. These detectors improve the momenta measurement and suppress pileup effects, facilitating precision measurements of Standard Model parameters at the High-Luminosity-LHC. This is a rough translation to English, not the final version in Chinese.