Multi-Boson Production and Interactions in pp Collisions with the ATLAS Experiment at the LHC

Abstract

The Standard Model (SM) of particle physics is one of the most successful theoretical frameworks for describing elementary particles and their interactions. While it has demonstrated remarkable agreement with experimental results, certain rare processes predicted by the SM remain unexplored. This thesis focuses on measuring the cross-sections of rare processes involving massive vector boson production and interactions as a rigorous test of the SM.

One of the SM’s key achievements is the unification of the electromagnetic and weak forces, marking a major milestone in our understanding of fundamental interactions. The W and Z bosons, as mediators of the weak force, play a central role in this framework. Precise measurements of multi-boson production processes provide critical tests of the SM and may offer insights into potential new physics beyond its current formulation.

The Large Hadron Collider (LHC) provides a unique opportunity at the energy frontier to study multi-boson production and interactions with unprecedented precision. This thesis focuses on measurements of the production and interaction of multiple bosons, using data collected by the ATLAS experiment from proton-proton collisions at √s = 13 TeV during the second run (Run 2) of the LHC (2015 - 2018), except for the Higgs boson production cross-section measurement, which utilizes data from the third run (Run 3) at √s = 13.6 TeV in 2022. Two physics topics are included in this thesis: The first one searches for triboson (VVZ) production, where V represents either a W or a Z boson, and the second concentrates on the measurements of diboson (ZZ) production. The production and interactions of tribosons and dibosons are crucial for a comprehensive check of the electroweak sector of the SM and for testing potential deviations from SM predictions.

The first part of the physics analysis reports the first observation of VVZ production with the ATLAS experiment. The analysis uses three final states originating from VVZ production and decay modes: 3ℓ+jets, 4ℓ, and 5ℓ, where ℓdenotes electron or muon. Boosted Decision Trees (BDTs) are employed in each channel to distinguish VVZ processes from background events. The combined observed (expected) significance of 6.4 (4.7) standard deviations over the background-only hypothesis model is obtained in the analysis. The VVZ production cross-section (σ_VVZ) is measured to be σ_VVZ = 655+94−93(stat.)+98−87(syst.) FB.

The second part includes three measurements to study the diboson ZZ production and interaction using the 4ℓfinal state. The first one is the differential cross-section for ZZ production associated with two forward/backward hadronic jets (denoted as ZZjj production). The measured differential cross-sections are used to search for anomalous weak-boson self-interactions induced by dimension-8 Effective Field Theory (EFT) operators. The second is the production of two longitudinally polarized Z bosons (Z_L Z_L). The longitudinally polarized Z_L Z_L production cross-section is measured for the first time in ATLAS to be 2.45 ±0.60 fb in the experimental fiducial phase space with an observed (expected) significance of 4.3 (3.8) standard deviations over the background-only hypothesis. And the third one is the Higgs boson production at √s = 13.6 TeV with ZZ→4ℓ fiducial phase space. The measured cross-section of σfid = 2.80 ±0.74 fb is in agreement with the corresponding SM prediction of σSM = 3.67 ±0.19 fb. This fiducial measurement is extrapolated to the full phase space, yielding a total cross-section of σ(pp →H)= 46 ±12 pb at 13.6 TeV.