Physicists from the ATLAS Collaboration at the Large Hadron Collider (LHC) at CERN have announced the results of the most sensitive search to date for double Higgs production and self-coupling, achieved by combining five double Higgs studies from LHC Run 2 data.
Event display of a double Higgs candidate event, photographed in 2017. Image courtesy of ATLAS Collaboration / CERN.
Remember how hard it was to find one Higgs boson? Now try and find two of them in the same place at the same time.
This intriguing process, known as double Higgs production, can teach scientists about the Higgs particle's self-interaction.
By studying it, physicists can measure the strength of the Higgs particle's self-binding, a fundamental aspect of the Standard Model that links the Higgs mechanism to the stability of the universe.
Searching for the creation of double Higgs particles is a particularly challenging task.
This is an extremely rare process, about 1,000 times rarer than the creation of a single Higgs particle.
While LHC Run 2 produced 40 million collisions per second, ATLAS is expected to produce just a few thousand double Higgs events.
So how can physicists find these rare needles in a mountain of data?
One way to make it easier to find double Higgs production is to search in multiple locations.
By investigating the different ways in which the double Higgs decay (decay modes) and combining them, physicists can maximise their chances of discovering and studying the creation of the double Higgs.
The new results from the ATLAS collaboration are the most comprehensive search to date, covering more than half of all possible double Higgs events with ATLAS.
Each of the five individual studies in this combination focuses on a different mode of damping, each with its own strengths and weaknesses.
For example, the most likely double-Higgs decay mode is the decay into four bottom quarks.
However, the Standard Model QCD process likely also produces four bottom quarks, making it difficult to distinguish this background process from a double Higgs event.
The double-Higgs decay into two bottom quarks and two tau leptons involves moderate background contamination, but it occurs five times less frequently and there are neutrinos that escape undetected, complicating physicists' efforts to recreate the decay.
Decays into multiple leptons are not uncommon, but they have complex characteristics.
Other double Higgs decays are even rarer, such as the decay into two bottom quarks and two photons.
This final state accounts for only 0.3% of all double Higgs decays, but has a cleaner signature and much smaller background contamination.
Combining their findings for each of these decays, ATLAS physicists were able to find that the probability of producing two Higgs particles rules out more than 2.9 times the Standard Model prediction.
This result has a confidence level of 95% and an expected sensitivity of 2.4 (assuming this process does not exist in nature).
They were also able to provide constraints on the strength of the Higgs particle's self-coupling, achieving the highest sensitivity to date for this important observable.
They found that the magnitude of the Higgs self-coupling constant and the strength of the interaction between two Higgs particles and two vector particles are consistent with the Standard Model predictions.
“This overall result marks a milestone in the study of double Higgs particle production,” the researchers said.
their result will be published in journal Physics Review Letter.
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ATLAS Collaboration. 2024. Combined search for Higgs pair production in pp collisions at s√=13 TeV with the ATLAS detector. Physiotherapy Rev Lett,in press; arXiv:2406.09971
Source: www.sci.news
