Search for QCD instantons at the LHC using the ATLAS Detector

Thesis

This thesis presents the design of the first search for QCD instantons at the LHC, which is based on $1.693~\mathrm{nb}^{-1}$ of low-pileup data collected by the ATLAS Experiment in 2015. QCD instantons are semi-classical solutions of the Yang-Mills equations of motion and correspond to tunneling processes between topologically distinct vacuum sectors. Their very existence reveals the rich vacuum structure of theory of the strong interactions, where the different vacuum sectors are continually being mixed together. Instanton field configurations are deeply embedded in the Standard Model of particle physics, being already essential in explaining the heaviness of the $\eta'$ meson. Yet they have never been observed. The search presented here uses a Machine Learning technique based on Deep Neural Networks (DNNs) for concomitant signal discrimination and background estimation. Despite the deployment of DNNs, signal discrimination has proved to be difficult, with instanton event kinematics being very similar to those of the minimum-bias background events. Blinded results show that the analysis has an expected exclusion sensitivity to a signal strength value of $\mu = 412.6^{+151.9}_{ −110.3}$.

Authors: Harris, YT

• DOI: 10.5287/ora-pyzv6qm75
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Atlas experiment; instantons; standard model; CERN; quantum chromodynamics; Large Hadron Collider (LHC); particle physics
• Subjects: Particle Physics