The ATLAS Collaboration at CERN’s Large Hadron Collider (LHC) has – at long last – observed the Higgs boson decaying into a pair of bottom (b) quarks. This elusive interaction is predicted to make up almost 60 percent of the Higgs boson decays and is thus primarily responsible for the Higgs natural width. Yet it took over six years after the 2012 discovery of the Higgs boson to accomplish this observation.
This observation is among the most demanding analyses carried out by ATLAS so far. Siegfried Bethke, Director at the Max Planck Institute of Physics and former Chairman of the ATLAS Collaboration Board, explains why: “The LHC employs very high energies to force protons into collision. These collisions produce huge amounts of b quarks. It’s an enormous challenge to separate the quarks that Higgs boson decays generate from all the others in this sea of b quarks – it’s even worse than trying to find a needle in the famous haystack!”
The analysis teams therefore focused on specific signatures, above all the production of a Higgs boson in conjunction with a W or Z boson, the photon’s heavy associates, which together convey the electroweak force between the original building blocks of matter. This produced a particularly clear signal – an approach that proved to be very successful.
“ATLAS is proud to announce the observation of this important and challenging Higgs boson decay,” says Karl Jakobs, ATLAS Spokesperson. “While the result is certainly a confirmation of the Standard Model, it is equally a triumph for our analysis teams. During the early preparations of the LHC, there were doubts on whether this observation could be achieved. Our success is thanks to the excellent performance of the LHC and the ATLAS detector, and the application of highly sophisticated analysis techniques to our large dataset.”
The ATLAS Collaboration first presented a preliminary result of this observation (http://atlas.cern/updates/physics-briefing/higgs-observed-decaying-b-quarks) on 9 July at the 2018 International Conference on High-Energy Physics (ICHEP) in Seoul. Today, in a seminar together with the CMS, ATLAS presented results which have been submitted for publication to Physics Letters B. They are were achieved and utilising machine learning technology and new analysis techniques to reach a significance of 5.4 standard deviations. Physicists consider five standard deviations the significance threshold past which they claim a discovery.
These observations mark a new milestone in the study of the Higgs boson, complementing recent results on production and decay mechanisms of this elementary particle (https://www.mpp.mpg.de/en/what-s-new/news/detail/wissenschaftler-beobachten-kopplung-des-higgs-bosons-an-top-quarks/ as well as http://atlas.cern/updates/physics-briefing/higgs-couples-heaviest-lepton).
ATLAS transitions from observations to precise measurements of its properties. “We now have the opportunity to study the Higgs boson in unprecedented detail and will be able to further challenge the Standard Model of particle physics,” concludes Karl Jakobs.