Physicists from the ATLAS and CMS collaborations at CERN’s Large Hadron Collider have observed the Higgs boson decaying into a pair of bottom quarks (H→ bb–). This elusive interaction is predicted to make up almost 60% of the Higgs boson decays. The finding is consistent with the hypothesis that the all-pervading quantum field behind the Higgs boson also gives mass to the bottom quark.
A candidate event display for the production of a Higgs boson decaying to two bottom quarks (blue cones), in association with a W boson decaying to a muon (red) and a neutrino. The neutrino leaves the detector unseen, and is reconstructed through the missing transverse energy (dashed line). Image credit: ATLAS Collaboration / CERN.
The Standard Model of particle physics predicts that about 60% of the time a Higgs boson will decay to a pair of bottom quarks, the second-heaviest of the six flavors of quarks.
Testing this prediction is crucial because the result would either lend support to the Standard Model or rock its foundations and point to new physics.
Spotting this common Higgs-boson decay channel is anything but easy, as the six-year period since the discovery of the boson has shown.
The reason for the difficulty is that there are many other ways of producing bottom quarks in proton-proton collisions. This makes it hard to isolate the Higgs-boson decay signal from the background ‘noise’ associated with such processes.
By contrast, the less-common Higgs-boson decay channels that were observed at the time of discovery of the particle are much easier to extract from the background.
To extract the signal, the ATLAS and CMS collaborations each combined data from the first and second runs of the Large Hadron Collider, which involved collisions at energies of 7, 8 and 13 TeV. They then applied complex analysis methods to the data.
The upshot was the detection of the decay of the Higgs boson to a pair of bottom quarks with a significance that exceeds 5 standard deviations.
Furthermore, the collaborations measured a rate for the decay that is consistent with the Standard Model prediction, within the current precision of the measurement.
“We are proud to announce the observation of this important and challenging Higgs boson decay,” said Dr. Karl Jakobs, spokesperson of the ATLAS Collaboration.
“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 Large Hadron Collider, 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.”
“Since the first single-experiment observation of the Higgs boson decay to tau-leptons one year ago, CMS, along with our colleagues in ATLAS, has observed the coupling of the Higgs boson to the heaviest fermions: the tau, the top quark, and now the bottom quark,” said Dr. Joel Butler, spokesperson of the CMS Collaboration.
“The superb performance of the Large Hadron Collider and modern machine-learning techniques allowed us to achieve this result earlier than expected.”
With more data, the ATLAS and CMS collaborations will improve the precision of these and other measurements and probe the decay of the Higgs boson into a pair of much-less-massive fermions called muons, always watching for deviations in the data that could point to physics beyond the Standard Model.
“The experiments continue to home in on the Higgs particle, which is often considered a portal to new physics,” said Dr. Eckhard Elsen, Director for Research and Computing at CERN.
“These beautiful and early achievements also underscore our plans for upgrading the Large Hadron Collider to substantially increase the statistics.”
“The analysis methods have now been shown to reach the precision required for exploration of the full physics landscape, including hopefully new physics that so far hides so subtly.”
The results will be published in the journals Physics Letters B (preprint) and Physical Review Letters (preprint).
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M. Aaboud et al (ATLAS Collaboration). 2018. Observation of H→bb– decays and VH production with the ATLAS detector. Phys. Lett. B, in press; arXiv: 1808.08238
A.M. Sirunyan et al (CMS Collaboration). 2018. Observation of Higgs boson decay to bottom quarks. Phys. Rev. Lett, in press; arXiv: 1808.08242
