Remember how hard it was to find a single Higgs boson? Try to find two at the same place and time. This fascinating process, called Higgs pair production, can provide scientists with information about the self-interaction of the Higgs boson.
Remember how hard it was to find a single Higgs boson? Try to find two at the same place and time. This fascinating process, called Higgs pair production, can provide scientists with information about the self-interaction of the Higgs boson.
Thanks to the research, physicists can measure the self-coupling strength of the Higgs boson, which is a fundamental aspect of the Standard Model that links the Higgs mechanism to the stability of our universe.
Searching for Higgs pair production is a particularly challenging task. This is a very rare process, about 1,000 times rarer than the production of a single Higgs boson. During each cycle 2 of the Large Hadron Collider (LHC), only a few thousand Higgs pair events are expected to be produced at ATLAS, compared to the 40 million collisions occurring each second.
So how can physicists find these rare needles in the information pile? One way to make the search for Higgs pair production easier is to look for it in multiple places. By looking at the different ways in which Higgs pairs can decay (decay states) and combining them together, physicists can maximize their chances of finding and studying the production of Higgs pairs.
Researchers from the ATLAS collaboration have now released the results of the most sensitive search to date for Higgs pair production and self-coupling, achieved by combining five Higgs pair studies of LHC cycle 2 data. This new result is their most comprehensive search to date, covering over half of all possible Higgs pair events in ATLAS. The study was also published on the early publication server arXiv.
The five studies that are the focus of the present research each focused on different disintegration situations, each of them has advantages and disadvantages. For example, the most likely decay state of Higgs pairs is into four bottom quarks. However, Standard Model QCD processes also tend to create four bottom quarks, making it difficult to distinguish the Higgs pair event from this background process.
The decay of a Higgs pair into two bottom quarks and two tau leptons involves moderate background contamination, but is five times rarer and has neutrinos that escape undetected, complicating physicists' ability to reproduce the decay. The multilepton decay, while not too rare, has complex signs.
Other decay states of a Higgs pair are even rarer, such as the decay into two bottom quarks and two photons. This final state accounts for only 0.3% of all Higgs pair decays, but has a clearer sign and less background contamination.
By combining the results from the searches for each of these decays, the researchers were able to discover that the probability of producing two Higgs bosons exceeds the prediction of the standard model by more than 2.9 times. This result is obtained at a confidence level of 95%, with an expected sensitivity of 2.4 (assuming that this process does not exist in nature).
The researchers were also able to provide constraints on the self-coupling strength of the Higgs boson, achieving the best sensitivity yet for this important observation. They found that the magnitude of the Higgs self-coupling constant and the strength of the interaction of two Higgs bosons and two vector bosons match the predictions of the standard model.
This combined result sets a milestone in the study of Higgs pair production. Now, ATLAS researchers have set their sights on data from the ongoing LHC cycle 3 and the upcoming High-Luminosity LHC operation. With this data, physicists may finally be able to observe the production of the elusive Higgs boson pairs.
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