The study of the Higgs boson, often described as the cornerstone of the Standard Model of particle physics, continues to captivate researchers as they delve into its interactions with fundamental particles. The ATLAS collaboration, a major experiment at the Large Hadron Collider (LHC), aims to refine our understanding of these interactions, specifically those involving elementary fermions and bosons. As a vital aspect of the electroweak symmetry breaking theory, these interactions inform scientists about how particles acquire mass.
At the forefront of the ATLAS program’s objectives is the need to achieve greater precision in measuring the interaction strengths of the Higgs boson. Recent presentations at the International Conference on High-Energy Physics (ICHEP) 2024 revealed that the collaboration has made significant strides in this area. The groundwork laid during LHC Run 2, which spanned from 2015 to 2018, has been reassessed using cutting-edge analysis techniques including advanced jet tagging.
Jets, which are collimated sprays of particles resulting from quark fragmentation, play a critical role in Higgs boson decay studies. When a Higgs boson decays into quarks, it produces jets, and identifying the quark flavor that originated the jet is essential for accurate measurements. Enhanced jet tagging procedures for bottom and charm quarks have allowed ATLAS researchers to discern these jets with remarkable clarity. This refined technique markedly improves the sensitivity of analyses, particularly for decay channels like H→bb (Higgs to bottom quark pairs) and H→cc (Higgs to charm quark pairs).
With their innovative jet tagging strategies, ATLAS has observed a notable increase in the sensitivity of their experiments. The decay processes H→bb and H→cc showcased impressive improvements—specifically, a 15% enhancement for H→bb and a significant three-fold increase for H→cc. These advancements have culminated in first-time observations of the WH, H→bb process with a remarkable significance of 5.3σ (sigma), along with the ZH, H→bb process achieving 4.9σ.
The complexity of measuring the Higgs boson decay into charm quarks should not be understated, as this channel is suppressed compared to its bottom decay counterpart due to mass factors. Although the detection of H→cc remains elusive, ATLAS’s analyses have established an upper limit on the branching ratio of the VH, H→cc process, indicating that it may occur at a rate 11.3 times greater than predicted by the Standard Model.
The interaction of the Higgs boson with the top quark poses a unique set of challenges for physicists. ATLAS has targeted this interaction through measurements of Higgs production in association with two top quarks, followed by decay into bottom quarks. This complicated process, known as ttH, presents considerable background noise that complicates the analysis. Recent enhancements in understanding these massive background processes have significantly boosted sensitivity, effectively doubling the accuracy of measurements.
The resulting signal strength for ttH, H→bb production was determined to be 0.81 ± 0.21, relative to the Standard Model predictions. These results illuminate the intricate relationship between the Higgs boson and the top quark, allowing physicists to glean further insights into the broader landscape of particle interactions.
Looking ahead, the ongoing Run 3 of the LHC presents exciting opportunities for further exploration. With continual improvements in analysis techniques and accumulating data, there is optimism that better measurements of the Higgs boson’s interactions will emerge. The impending High-Luminosity LHC (HL-LHC) promises to set the stage for enhanced experimental capabilities, where measuring processes like H→cc could transition from theoretical speculation to experimental reality.
The persistence and innovation demonstrated by the ATLAS collaboration signify a critical phase in Higgs boson research. As results align more closely with the predictions of the Standard Model, the community eagerly awaits future discoveries that promise to illuminate the foundational principles governing our universe.