The Higgs boson, a fundamental particle in the Standard Model of particle physics, plays a pivotal role in explaining how other particles acquire mass through the mechanism of electroweak symmetry breaking. Understanding the interactions of the Higgs boson with elementary particles, including both fermions and bosons, is crucial for advancing our knowledge of particle physics and refining the Standard Model. The ATLAS collaboration has dedicated extensive efforts to analyze these interactions, ultimately aiming to achieve precise measurements. Such achievements not only enhance our understanding of fundamental interactions but also help to explore potential physics beyond the Standard Model.
At the recent International Conference on High-Energy Physics (ICHEP) 2024, the ATLAS collaboration unveiled significant advancements in their measurement of Higgs boson interactions, particularly with the three heaviest quarks: top, bottom, and charm. These measurements draw from a reanalysis of data gathered during Run 2 of the Large Hadron Collider (LHC) from 2015 to 2018. Employing refined analysis techniques, including advanced jet tagging methods, the ATLAS team has made substantial progress in their research.
An essential aspect of studying Higgs boson decays involves understanding jets, which are sprays of particles that emerge when quarks fragment following their production. When the Higgs boson decays into a quark-antiquark pair, each quark creates a jet that is predominantly composed of hadrons. Jet tagging is therefore vital, as it allows researchers to identify the flavor of quark that generated each jet by analyzing various jet properties. Improved tagging techniques for bottom and charm quarks have significantly bolstered the sensitivity of measurements related to Higgs decays into these quarks.
The recent analysis yielded groundbreaking results in the measurement of the Higgs boson’s decay into pairs of bottom and charm quarks. Specifically, the sensitivity to decay channels H→bb (Higgs decay into bottom quarks) and H→cc (Higgs decay into charm quarks) was enhanced by 15% and a remarkable factor of three, respectively. The team successfully observed the WH, H→bb process with a significance of 5.3σ, indicating a robust finding, as well as ZH, H→bb with a 4.9σ significance. However, the decay of the Higgs boson into charm quarks remains elusive due to its relatively low probability of occurrence, effectively suppressed by a mass factor compared to bottom quark decays.
Additionally, an emphasis on understanding the Higgs boson’s interactions with top quarks has led to new measurements focusing on associated Higgs production with two top quarks followed by decay into bottom quarks. This analysis, characterized by its complexity and significant background noise, revealed an improved sensitivity for the associated process (ttH, H→bb). By refining the understanding of the dominant background processes, the team achieved an insightful signal strength measurement of 0.81 ± 0.21 relative to Standard Model expectations.
As the LHC progresses into its Run 3 phase, ongoing data collection and analysis efforts are expected to provide even more refined measurements of Higgs boson interactions. The future looks promising, particularly with the anticipated arrival of the High-Luminosity LHC (HL-LHC), where the detection of more elusive processes, such as H→cc, may soon become feasible. These advances represent a critical step forward in our understanding of Higgs physics and the fundamental forces that govern particle interactions.
The pursuit of precision in measuring the Higgs boson’s interactions is a testament to the collaborative efforts of researchers in the ATLAS collaboration. Each new result adds a piece to the puzzle of particle physics, reinforcing the current framework while sowing the seeds for the exploration of potential new physics. As our methodologies, technologies, and data evolve, the quest for knowledge about the universe’s most basic building blocks continues, propelled by the milestones achieved in Higgs boson studies.
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