Study of ZZ Production at ps = 13 TeV with the 2`2 Final State at the LHC with the ATLAS Detector

Abstract

This thesis presents a physics study of vector boson $ZZ$ production, focusing on precision measurements of the integrated and differential cross-sections with the physics process of $pprightarrow ZZrightarrow2l2nu $ through proton-proton collisions at $sqrt{s}$ = 13 TeV at the Large Hadron Collider. It utilizes data collected by the ATLAS experiment during the LHC Run 2 program.

With an integrated luminosity of 140 fb$^{-1}$ collected during 2015 - 2018 by ATLAS, the events originating from $ZZ$ production involve final states where one Z boson decays into a pair of same-flavor opposite-sign leptons ($e^- e^+$ or $mu^- mu^+$), while the other Z boson decays into a pair of neutrinos. This $ZZ$ decay final state has a branching fraction a factor of 6 higher than the four-charged lepton final state. However, the background is also significantly higher, which poses challenges in data analysis. Through meticulous examinations of $Z$ decays, this measurement provides a stringent test of the Standard Model electroweak theory, scrutinizing its predictions with high precision. Additionally, it facilitates the exploration of anomalous neutral vector boson self-interactions, offering valuable insights into potential deviations from the SM framework.

In previous analyses of this final state produced at $sqrt{s}= 13$~GeV, only partial Run-2 data corresponding to 36 fb$^{-1}$ were utilized. The measured production cross-section was approximately 13% higher than the SM prediction, with a measurement uncertainty of 7.5%, primarily driven by large statistical uncertainty. This thesis work employs data with almost four times higher statistics, anticipating a substantial enhancement in measurement precision and sensitivity for probing anomalous couplings.

The data analysis techniques employed in this study heavily rely on detector calibrations of lepton triggering, energy and momentum measurements, and event selection optimizations. Dedicated background control regions are defined to estimate the background contamination in the signal region. Signal modeling and detection efficiency rely heavily on Monte Carlo event generators and detector simulations. The final cross-section measurements are extracted from a sophisticated statistical fitting model. Ten kinematic variables are unfolded to make the differential cross-section measurements and compared with SM predictions.

The total $ZZ$ production cross-section is calculated to be $sigma_{SM} = 20.08 pm 1.11$ fb at an accuracy of the next-leading-order QCD and electroweak theory within the experimental fiducial phase-space. The cross-section is expected to be measured in terms of signal strength, $mu_s = 1.00pm 0.04$, where $mu_s$ is defined as the ratio of the measured cross-section normalized to the SM predicted cross-section, $mu_s = {sigma_{measured} over sigma_{SM}}$.

Based on these measurements, upper limits on the anomalous couplings within the framework of EFT are derived with a 95% confidence level.

This work relies heavily on precision identification and measurements of muon tracks and momentum. This is achieved at the ATLAS experiment by consistent hardware calibration and upgrades, both necessary parts of the ATLAS physics research program. Additional research is presented involving construction and testing of the sMDT muon detector, for ATLAS Muon Spectrometer upgrade for the high-luminosity LHC program. A total of 50 sMDT chambers were constructed at the University of Michigan, all satisfying stringent precision and performance requirements.