Six years after its discovery, the Higgs boson has at last been observed decaying to fundamental particles known as bottom quarks. The finding, presented this week at CERN by the ATLAS and CMS collaborations at the Large Hadron Collider (LHC), is consistent with the hypothesis that the all-pervading quantum field behind the Higgs boson also gives mass to the bottom quark. Both teams have submitted their results for publication.
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 flavours of quarks. Testing this prediction is crucial because the result would either lend support to the Standard Model – which is built upon the idea that the Higgs field endows quarks and other fundamental particles with mass – 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, such as the decay to a pair of photons, 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 LHC, which involved collisions at energies of 7, 8 and 13 TeV. They then applied complex analysis methods to the data. The upshot, for both ATLAS and CMS, 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, both teams measured a rate for the decay that is consistent with the Standard Model prediction, within the current precision of the measurement.
This observation is a milestone in the exploration of the Higgs boson. It shows that the ATLAS and CMS experiments have achieved deep understanding of their data and a control of backgrounds that surpasses expectations. ATLAS has now observed all couplings of the Higgs boson to the heavy quarks and leptons of the third generation as well as all major production modes,” said Karl Jakobs, spokesperson of the ATLAS collaboration.
Members of the University of Birmingham ATLAS group have been working to find this decay since before the Higgs boson was discovered in 2012. Their work has centred on analysing decays where the Higgs particle is produced together with a Z boson. In these events the Higgs boson decays to bottom quarks, and the Z decays to two electrons or two muons, which can be measured precisely in the detector. Over the years the understanding of the detector and analysis techniques have evolved along with the increasing data sample.
Dr Paul Thompson from the University of Birmingham’s School of Physics and Astronomy, who was heavily involved in this research, said: "This result shows that the dominant decay of the Higgs boson really appears in nature - it's fantastic to have this measurement after so many years of hard work and painstaking analysis."
Professor Dave Charlton, also from Birmingham and former Spokesperson of ATLAS added: “This exquisite observation has taken years of study, yet the result is simple: we have now, after so much careful work, seen the path by which the Higgs boson decays most often! Physicists from Birmingham were centrally involved, just as we were in the Higgs boson discovery back in 2012.”
Kate Chapple, Press Office, University of Birmingham, tel +44 (0)121 414 2772 or + 44 (0)7789 921 164.
The ATLAS collaboration is a 3000-strong team. Six university teams from the UK were involved in this analysis. The Birmingham work has been a long-term engagement, working closely with physicists from the University of Liverpool, as well as more widely. Professor David Charlton of the Birmingham ATLAS team was the Spokesperson (head) of the ATLAS Collaboration from 2013-2017.
In addition to the hunt for Higgs boson decays to b-quarks, the group also searches for other, rarer, decays of the Higgs particle, including to lighter quarks, and was strongly involved in the Higgs-boson discovery in 2012
Physicists from the Birmingham particle physics group have been members of ATLAS since the experiment began. Currently more than thirty physicists - staff and PhD students - work on a range of physics topics, as well as on construction of upgrades to ATLAS. The work is supported by the Science and Technologies Facilities Council (STFC), as well as benefiting from investment from the University. Birmingham is unusual in a university group, in that we are also involved in two other experiments at the CERN LHC.