The exploration of fundamental particles and their interactions continues to be a critical focus within the realm of particle physics, particularly in the context of the Higgs boson. Understanding how the Higgs field interacts with elementary fermions and bosons not only elucidates the origin of mass but also serves as a cornerstone for testing the predictions of the Standard Model. The recent findings from the ATLAS collaboration presented at the International Conference on High-Energy Physics (ICHEP) 2024 demonstrate significant advancements in this domain, providing deeper insights into the intricate mechanisms of particle interaction.
The Higgs boson, discovered in 2012, remains a pivotal element in explaining why particles possess mass. According to the electroweak symmetry breaking theory, the interactions between the Higgs boson and other fundamental particles guide the mass-generation process. As researchers work to refine measurements of these interactions, they seek to bridge the gap between theoretical predictions and observable phenomena in particle collisions.
The ATLAS experiment at the Large Hadron Collider (LHC) offers a robust framework for examining these properties. By harnessing extensive datasets from LHC Run 2 (2015-2018), the ATLAS collaboration has employed advanced analysis techniques, including enhanced jet tagging, to gain improved sensitivity in measuring the strength of the Higgs boson’s interactions with different quarks.
The reanalysis of LHC data signifies a monumental step forward in measuring the Higgs boson’s interaction strengths with the top, bottom, and charm quarks. Central to this improvement is the process of jet tagging, which involves identifying and classifying the flavors of quarks that result from the decay of the Higgs boson. Upon decay, quarks produce jets—collimated sprays of particles that can be tracked and analyzed in sophisticated detectors.
Utilizing bespoke techniques for distinguishing between charm and bottom quarks has yielded significant gains in sensitivity—15% for bottom quarks and an impressive factor of three for charm quarks. This refined capability allows physicists to observe decay processes with greater accuracy, paving the way for high-precision measurements that could align further with the Standard Model predictions.
Among the notable achievements presented in ATLAS’s latest report is the observation of the WH, H→bb process with a significance level of 5.3σ. This finding is complemented by a detection of ZH, H→bb with a significance of 4.9σ. These accomplishments mark significant milestones in the ongoing pursuit of understanding particle interactions facilitated by the Higgs boson.
However, not all interactions are equally observable. The decay of the Higgs boson into charm quarks (c quarks) remains a challenge due to its relatively suppressed rate—20 times less probable than that of decay into bottom quarks (b quarks). Nevertheless, the ATLAS collaboration has established an upper limit for the VH, H→cc process, showing it could be 11.3 times higher than predicted by the Standard Model.
One of the most complex interactions to analyze involves the Higgs boson’s coupling with the top quark, especially in processes where it produces two top quarks. ATLAS has made strides in refining the methodology to discern these interactions by improving the understanding of the substantial background noise inherent in such measurements. The latest analysis has doubled the previous sensitivity, yielding a signal strength of ttH, H→bb production at 0.81 ± 0.21—an important metric that aligns with the Standard Model prediction.
These results not only enhance the current body of knowledge but also set the stage for even more precise measurements as we advance into Run 3 of the LHC. The upcoming High-Luminosity LHC (HL-LHC) aims to intensify particle collision rates, making it increasingly feasible to detect more elusive processes like H→cc, which would deepen our understanding of the Higgs mechanism.
The work conducted by the ATLAS collaboration reflects a vibrant landscape of discovery and innovation in particle physics. As research methodologies continue to evolve, the potential for revealing new physics also grows. The pursuit of more precise measurements of the Higgs boson’s interactions will not only test the boundaries of the Standard Model but could also unveil unexpected phenomena that challenge existing theoretical frameworks.
By fostering advancements in detector technologies, analysis techniques, and experimental frameworks, the field is poised for significant breakthroughs in the years ahead, continuing to unravel the mysteries of the universe and the fundamental forces that govern it.