The ATLAS Collaboration has made a significant breakthrough in understanding the Higgs boson, a fundamental particle in the Standard Model of physics. By studying the production of the Higgs boson in association with top quarks, researchers have gained insight into the interaction between these particles, known as the “top Yukawa coupling.” This process, although rare, provides a unique opportunity to directly measure this interaction.
The ATLAS team has refined their analysis methods and boosted their experiment’s precision, achieving a factor of two better expected signal significance compared to previous results. They used advanced machine-learning techniques, including neural networks, to identify jets of particles originating from bottom quarks and improve their description of top-quark background processes.
The measured signal strength for ttH production is 0.81±0.21, compatible with the Standard Model prediction of 1, corresponding to an observed significance at the 4.6 standard-deviation level. This result is in good agreement with the Standard Model and brings researchers closer to understanding the origin of mass.
Precise Measurement of Higgs Boson Production in Association with Top Quarks
The ATLAS Collaboration has made a significant breakthrough in understanding the interaction between top quarks and the Higgs boson, known as the “top Yukawa coupling.” This is achieved by studying the production of the Higgs boson together with a top-quark pair, also referred to as “ttH production.” Although this process accounts for only about 1% of all Higgs bosons produced, it provides a unique opportunity for researchers to directly measure the interaction between the top quark and the Higgs boson.
The collaboration has studied ttH production across various Higgs-boson decays, including its most common decay into bottom quark pairs (the “ttH(bb) process”). In 2022, researchers measured the ttH(bb) production rate using the full LHC Run-2 dataset collected in 2015–2018. The measured signal was lower than predicted, but still compatible with the Standard Model within uncertainties.
Challenges and Refinements
When performing ttH(bb) measurements, researchers face several challenges. One major issue is that other top-quark-pair-production processes produce signals similar to those of the ttH(bb) process, making them difficult to distinguish. Developing accurate theoretical descriptions of these background processes is also very challenging.
To overcome these challenges, ATLAS physicists are continuously refining their analysis methods and boosting their experiment’s precision. At the recent International Conference on High-Energy Physics (ICHEP) 2024, the ATLAS Collaboration presented a new measurement of the ttH(bb) process using the same Run-2 dataset, achieving a factor of two better expected ttH signal significance compared to the previous result.
Advanced Machine-Learning Techniques and Refined Reconstruction
Researchers used advanced machine-learning techniques to achieve the most precise individual measurement of ttH production. They incorporated refined reconstruction and calibration of physics objects, enhancing their ability to identify “jets” of particles originating from bottom quarks. Further, they worked with theorists to improve their description of top-quark background processes.
This allowed them to expand their event selection criteria and analyze a factor of three more ttH(bb) events. In addition, the ATLAS team used an advanced neural network to classify the selected collision events. This helped identify which jets likely originated from a Higgs-boson decay, enabling researchers to carry out additional measurements of the signal strength as a function of the Higgs boson transverse momentum.
Results and Implications
The results were found to be in good agreement with the Standard Model. The measured signal strength for ttH production is 0.81±0.21, compatible with the Standard Model prediction of 1, corresponding to an observed significance at the 4.6 standard-deviation level. This is just shy of the conventional 5 standard deviation requirement to claim an observation of ttH production in this decay channel.
The figure shows a simplified view summarizing all events in the analysis compared to the background prediction. It demonstrates an excess of data over the background prediction, compatible with events from ttH production. This new measurement is the most precise individual measurement of ttH production – but it is only one piece of the puzzle! Ultimate precision will be reached when combining this result with the measurements of other Higgs boson final states.
The implications of this study are far-reaching, as it provides a deeper understanding of the top quark and Higgs boson interaction. This can shed light on new physics beyond the Standard Model, particularly in events with high transverse momentum. The ATLAS Collaboration’s achievement is a significant step forward in the pursuit of uncovering the secrets of the universe.
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