Remember how difficult it was to find one Higgs boson? Try finding two at the same place at the same time. Known as di-Higgs production, this fascinating process can tell scientists about the Higgs boson self-interaction. By studying it, physicists can measure the strength of the Higgs boson's "self-coupling", which is a fundamental aspect of the Standard Model that connects the Higgs mechanism and the stability of our Universe.
Searching for di-Higgs production is an especially challenging task. It's a very rare process, about 1000 times rarer than the production of a single Higgs boson. During the entire Run 2 of the Large Hadron Collider (LHC), only a few thousand di-Higgs events are expected to have been produced in ATLAS, compared with the 40 million collisions that happened every second. So how can physicists find these rare needles in the data haystack? One way to make it easier to look for di-Higgs production is to search for it in multiple places. By looking at the different ways di-Higgs can decay (decay modes) and putting them together, physicists are able to maximise their chances of finding and studying di-Higgs production.
Researchers at the ATLAS collaboration have now released the most sensitive search for di-Higgs production and self-coupling yet, achieved by combining five di-Higgs studies of LHC Run 2 data. This new result is their most comprehensive search so far, covering over half of all possible di-Higgs events in ATLAS.
The five individual studies in this combination each focused on different decay modes, each of which has its pros and cons. For example, the most probable di-Higgs decay mode is into four bottom quarks. However, Standard Model QCD processes are also likely to create four bottom quarks, making it difficult to differentiate a di-Higgs event from this background process. The di-Higgs decay to two bottom quarks and two tau leptons has moderate background contamination but is five times less common and has neutrinos that escape undetected, complicating physicists' ability to reconstruct the decay. The decay to multiple leptons, while not too rare, has complex signatures. Other di-Higgs decays are even more rare, such as the decay to two bottom quarks and two photons. This final state accounts for only 0.3% of total di-Higgs decays but has a cleaner signature and much smaller background contamination.
By combining the results from searches for each of these decays, the researchers were able to find that the probability that two Higgs bosons are produced excludes values more than 2.9 times the Standard-Model prediction. This result is at 95% confidence level, with an expected sensitivity of 2.4 (assuming that this process is not present in nature). They were also able to provide constraints on the strength of the Higgs boson self-coupling, achieving the best-yet sensitivity on this important observable. They found that the magnitude of the Higgs self-coupling constant and the interaction strength of two Higgs bosons and two vector bosons are consistent with Standard Model predictions.
This combined result sets a milestone in the study of di-Higgs production. Now, ATLAS researchers have set their sights on data from the ongoing LHC Run 3 and upcoming High-Luminosity LHC operation. With this data, physicists may be able to observe the elusive Higgs-boson-pair production at last.
