The quest to understand the Higgs boson and its interactions with fundamental particles lies at the heart of modern particle physics. The ATLAS collaboration, one of the key experiments at the Large Hadron Collider (LHC), is dedicated to uncovering the intricacies of these interactions in order to validate and expand upon the Standard Model of particle physics. At the International Conference on High-Energy Physics 2024 (ICHEP 2024), ATLAS unveiled groundbreaking measurements that push the boundaries of our knowledge about the Higgs boson’s role in the universe.
The Higgs boson, discovered in 2012, is not only a fundamental part of the Standard Model but also essential in explaining the origin of mass for elementary particles through the mechanism of electroweak symmetry breaking. Understanding how the Higgs interacts with various particles, particularly fermions and bosons, provides crucial insights into why particles have mass and how they interact. ATLAS aims to measure these interaction strengths with increasing precision, leading to a deeper comprehension of the fundamental forces that govern our universe.
During ICHEP 2024, the collaboration reported enhanced measurements of the Higgs boson’s interactions with the heaviest quarks: top, bottom, and charm. These findings are based on a refined analysis of data collected during the LHC’s Run 2, which spanned from 2015 to 2018. The team employed advanced analysis techniques, including improved methods for identifying jet formations, critical for deriving interaction strengths accurately.
A key concept in this research is that of “jets.” When the Higgs boson decays into a pair of quarks, these quarks rapidly fragment into a spray of particles, creating what are known as jets. The challenge for physicists is to identify the specific types of quarks—designated as “flavors”—that produce each jet. This process involves extensive analysis of the jet’s characteristics.
The ATLAS team introduced sophisticated jet (or “flavor”) tagging techniques specific to bottom and charm quarks. These targeted methods have significantly enhanced the capability to detect signals from Higgs decays into b-quarks (H→bb) and c-quarks (H→cc). Remarkably, the sensitivity to H→bb events improved by 15%, while sensitivity to H→cc events soared by a factor of three. These improvements are pivotal, considering the theoretical complexities and rare occurrences associated with these decays.
Recent Findings and Their Implications
In a major breakthrough, ATLAS reported the first observation of the Higgs boson production in association with a W boson, followed by its decay into a pair of bottom quarks, achieving an impressive significance of 5.3σ. Furthermore, a measurement involving the decay into bottoms from Higgs production associated with a Z boson yielded a 4.9σ significance. These data sets represent the most precise investigations into these interactions, aligning closely with Standard Model predictions.
However, the challenge remains with the Higgs decay into charm quarks (H→cc), which is suppressed compared to H→bb decays, making it an elusive target. While ATLAS has determined an upper limit for the associated production of Higgs with a vector boson and subsequent decay into charm quarks, the interaction rate remains below Standard Model expectations, indicating that new physics might still be hiding just beyond the current observational limits.
The analysis of Higgs interactions, particularly with top quarks, proved to be complex, characterized by substantial background noise from competing processes. ATLAS’s latest study, which examined Higgs production linked to two top quarks and its subsequent decay into bottom quarks, enhanced sensitivity remarkably—doubling the previous capabilities. Here, ATLAS measured a signal strength for the process of ttH with H→bb, garnering findings consistent with theoretical predictions.
As the LHC embarks on its Run 3 and prepares for the High-Luminosity LHC (HL-LHC) phase, the prospects for further advancements in understanding Higgs physics seem promising. The heightened sensitivity to Higgs decays, particularly H→cc, is becoming increasingly feasible. This ongoing research will continue to refine our understanding of particle interactions, helping to uncover the underlying secrets of the universe and perhaps leading to discoveries beyond the current paradigm of particle physics. The journey to fully elucidate the role and influence of the Higgs boson in the cosmos is far from over.