At the forefront of modern particle physics, the Higgs boson holds a pivotal role in explaining how elementary particles acquire mass through the mechanism of electroweak symmetry breaking. This fundamental particle, first hypothesized in the 1960s and confirmed in 2012, is not just a theoretical construct but a key to unlocking the mysteries of the universe’s subatomic structure. The ATLAS collaboration, one of the premier experiments at the Large Hadron Collider (LHC), is dedicated to investigating the properties of the Higgs boson, particularly its interactions with fermions and bosons. By honing in on these interactions, researchers aim to measure their strength with greater precision, which is essential for testing the predictions of the Standard Model of particle physics.
During the recent International Conference on High-Energy Physics (ICHEP) 2024, the ATLAS collaboration showcased significant advancements in their research methodology. These improvements stemmed from a thorough reanalysis of data collected during LHC Run 2 between 2015 and 2018. Employing cutting-edge analysis techniques—including refined jet tagging methods—researchers successfully enhanced their ability to detect the main decay channels of the Higgs boson.
But what exactly entails jet tagging? When the Higgs boson decays into a pair of quarks, the resulting quarks do not emerge in isolation; instead, they produce a collimated spray of particles known as jets, chiefly comprising hadrons. Jet tagging is the process of discerning the flavor of the quarks (top, bottom, charm) that generate these jets. The more accurately researchers can tag these jets, the more reliable their measurements of the Higgs boson’s properties become. ATLAS researchers introduced specialized jet tagging techniques, particularly for bottom and charm quarks, thereby amplifying the sensitivity of their analyses significantly.
The results of these methodological enhancements have been profound. For instance, the sensitivity of measurements related to the Higgs decays into bottom quarks (denoted as H→bb) and charm quarks (H→cc) improved by 15% and three-fold, respectively. Among the most notable findings from ATLAS was the first definitive observation of the WH, H→bb process, which was confirmed with an impressive significance level of 5.3σ. Similarly, the ZH, H→bb process was recorded with a significance of 4.9σ. These milestones represent not only a leap forward in Higgs research but also align closely with the Standard Model’s predictions, reinforcing our theoretical understanding of particle interactions.
Despite the exciting advances, challenges remain—specifically in observing the decay into charm quarks, where the decay rate is suppressed significantly in comparison to bottom quark decay. The ATLAS team has set an upper limit on the rate of the VH, H→cc process, which is estimated to be 11.3 times lower than what Standard Model calculations suggest.
Another area of focus for the ATLAS collaboration is the interaction between the Higgs boson and the elusive top quark. This research specifically targets the associated production of a Higgs boson alongside two top quarks—an inherently complex scenario fraught with noise from substantial background processes. Thanks to enhanced recognition of these backgrounds, the sensitivity to this interaction was doubled, allowing the team to measure the production signal strength for the process ttH, H→bb. The results yielded a signal strength of 0.81 ± 0.21, which provides invaluable data in relation to the Standard Model predictions.
The continuous efforts of the ATLAS collaboration foreshadow a future rich with possibilities. As they harness improved analysis techniques and integrate data from ongoing LHC Run 3, the prospects for new insights into Higgs boson interactions are incredibly promising. The anticipated High-Luminosity LHC (HL-LHC) phase will further enhance these capabilities, enabling researchers to probe rare processes like H→cc more deeply.
The latest findings from the ATLAS collaboration not only sharpen our understanding of the Higgs boson’s role within the Standard Model but also pave the way for future breakthroughs in particle physics. The interplay of rigorous data analysis, innovative techniques, and cutting-edge technology will likely yield even more profound insights into the fabric of our universe.