The intricacies of particle physics often hinge on understanding the Higgs boson, a fundamental particle essential for explaining how other particles acquire mass. At the core of the ATLAS collaboration’s Higgs physics endeavors is a critical goal: to meticulously evaluate the interaction strengths between the Higgs boson and various elementary fermions and bosons. The framework of electroweak symmetry breaking posits that these interactions play a pivotal role in generating the masses of fundamental particles. Consequently, the ATLAS team focuses on deciphering the interactions through precise measurements of Higgs production and decay.

At the recent International Conference on High-Energy Physics (ICHEP) 2024, significant breakthroughs emerged from the ATLAS collaboration regarding the interactions of the Higgs boson with the heaviest quarks: top, bottom, and charm. These results stem from a revised analysis of data collected during the Large Hadron Collider (LHC) Run 2, conducted between 2015 and 2018. Employing state-of-the-art analysis techniques, specifically upgraded methods for jet tagging, researchers drastically improved their findings.

But what exactly are jets, and why is their tagging so crucial? When a Higgs boson decays into a pair of quarks, these quarks fragment and produce a distinct spray of particles—known as jets—within the detector’s confines. Tagging these jets is essential to identify the specific flavor of quark responsible for producing the jet. With innovative jet tagging methodologies targeting charm and bottom quarks, the ATLAS researchers amplified their analysis sensitivity. Specifically, they observed a 15% increase in sensitivity for the H→bb decay process and an astounding threefold enhancement for H→cc decays.

The ATLAS collaboration’s latest findings have yielded promising results, especially in the context of Higgs boson production entwined with W or Z bosons, alongside decays into charm or bottom quarks. Notably, the team achieved the first observation of the WH, H→bb process with a remarkable significance of 5.3σ, alongside a measurement of ZH, H→bb at 4.9σ significance. However, the decay of the Higgs boson into c quarks remains elusive, primarily due to a 20-fold suppression in its mass relative to b quarks, rendering it too rare for direct observation with current techniques. As such, ATLAS has established an upper limit on the frequency of the VH, H→cc process, which stands at 11.3 times the predictions established by the Standard Model.

In a complementary investigation, researchers honed in on the Higgs boson interaction with the top quark, particularly emphasizing Higgs production associated with two top quarks that subsequently decay into bottom quarks. This analysis presents formidable challenges due to the intricate final states and prominent background noise overwhelming the genuine signals. However, with advancements in knowledge regarding the significant background processes involving top quarks, the ATLAS collaboration succeeded in doubling the sensitivity of their measurements, yielding a signal strength for the ttH, H→bb production of 0.81 ± 0.21, in relation to what the Standard Model would predict.

The ongoing Run 3 at the LHC holds exciting potential for refining these measurements further. As analysis techniques evolve and richer datasets become available, scientists in the ATLAS collaboration are poised to achieve even greater precision in examining the complexities of Higgs boson interactions, especially with respect to the elusive H→cc decay. The anticipation surrounding the future capabilities of the High-Luminosity LHC (HL-LHC) is palpable, as the ability to detect this process inches closer to reality.

The ATLAS collaboration’s recent advancements in measuring Higgs boson interactions mark significant milestones in the field of particle physics. These efforts not only reinforce the accuracy of the Standard Model but also pave the way for new discoveries that may transform our understanding of the universe’s fundamental components. The road ahead is fraught with challenges, yet the prospects of enhanced technology and refined methodologies keep the scientific community eagerly looking toward the future.