Since the Large Hadron Collider (LHC) began operations, it has fueled groundbreaking research into the properties of Higgs bosons and opened avenues in the search for new physics beyond the established Standard Model of particle physics. At CERN’s ATLAS detector, scientists have recently combined both of these pursuits, yielding insights into Higgs boson interactions and placing tighter constraints on the search for phenomena outside the Standard Model.
The LHC’s discovery of the Higgs boson in 2012 marked a monumental success, completing the particle roster predicted by the Standard Model, which describes fundamental particles and their interactions. The Higgs boson, through its interactions, is what gives other elementary particles their mass, making it a cornerstone of the model. However, scientists had hoped that the LHC might also uncover hints of new physics that could explain phenomena left unanswered by the Standard Model, such as dark matter and dark energy. Yet, despite years of experimentation and analysis, definitive evidence for physics beyond the Standard Model remains elusive.
The ATLAS collaboration, one of the primary experiments at the LHC, recently pursued a line of inquiry that brings together high-precision studies of the Higgs boson with a search for evidence of new physics. The research, published in the Journal of High Energy Physics, focuses on rare events where two Higgs bosons are produced simultaneously. These Higgs boson pairs then decay into particles from the lepton family, including electrons and muons. Studying these events provides physicists with a dual opportunity: to refine their understanding of Higgs boson properties within the Standard Model and to seek traces of new physical processes.
Within the Standard Model, the production of Higgs boson pairs is theoretically possible, but it is extremely rare, making it challenging to detect. To date, no direct evidence for this phenomenon has been observed in the data collected. However, alternative theoretical models, which extend beyond the Standard Model, predict that Higgs boson pairs could be produced with a higher frequency. Detecting these pair-production events would suggest the presence of physical processes beyond what the Standard Model describes. Thus, for the ATLAS team, exploring Higgs boson pair-production events became a high-priority investigation.
Dr. Bartlomiej Zabinski, a physicist at the Institute of Physics of the Polish Academy of Sciences (IPJ PAN) and coordinator of the analysis, explained the experimental challenge: Higgs bosons appear so rarely in proton collisions at the LHC that not a single direct event of Higgs boson pair production has been recorded thus far. “How, then, can we study a phenomenon that has not yet been observed?” he asks. The answer lies in sophisticated statistical modeling and simulations that predict what these rare events would look like if they were detected. By comparing observed data with theoretical simulations, scientists hope to identify even subtle deviations from expected outcomes.
For this analysis, the ATLAS researchers operated strictly within the Standard Model framework, simulating the signals that would appear in the detectors if two Higgs bosons were produced, alongside simulated background noise. They then normalized these results based on the expected data collected by the detectors. By comparing these values with the data gathered, the team could check for discrepancies that might suggest the presence of new physics. Machine learning techniques, particularly decision-tree algorithms, played a critical role in identifying these rare events amidst large volumes of background data.
“Our analysis of double Higgs boson production events in the final state with multiple leptons complements the studies already carried out on other final states,” said Dr. Zabinski. Despite these efforts, no unexpected signals have emerged, meaning that all observations to date align with Standard Model predictions. While this result does not entirely rule out the existence of new physics, it indicates that if such phenomena do affect Higgs boson pair production, their influence is likely too subtle to be detected with the current dataset.
Looking ahead, there are promising developments on the horizon. Over the coming years, the LHC will undergo significant upgrades, with plans to increase beam intensity tenfold. This increase in collision frequency will result in a substantial boost in the data available for analysis, enhancing the potential to observe rare events like Higgs boson pair production. This upgraded phase of the LHC, known as the High-Luminosity LHC (HL-LHC), is anticipated to generate sufficient data to possibly capture the first direct observations of double Higgs boson production by the early 2030s.
These forthcoming observations could allow physicists to directly confirm or refine the current theoretical predictions surrounding Higgs boson interactions. If any discrepancies between theoretical predictions and observed data were to emerge, they would provide a strong indication of new physics, possibly illuminating the path beyond the Standard Model. Until then, the refined constraints set by ATLAS and similar experiments continue to shape the frontier of particle physics, narrowing the possible parameters for phenomena that may one day revolutionize our understanding of the universe at the most fundamental level.
Source: Polish Academy of Sciences