UK particle physicists celebrate first observation of the long sought decay of the Higgs boson

29 Aug 2018 11:14 AM

UK particle physicists are celebrating that the ATLAS Collaboration experiment 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% of the Higgs boson decays. It has taken over seven years to accomplish this observation but it could ultimately provide the first hints of new physics beyond our current theories.

“ATLAS is proud to announce the observation of this important and challenging Higgs boson decay," says Professor 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.”

For Dr Andy Mehta from the University of Liverpool this announcement is the culmination of over seven years work. “We started to think about this ATLAS search even before the LHC started and it’s wonderful to finally to see this decay.”

Over the last seven years, each new result has built upon the last one, so it is fantastic for all those people who have been involved over the years to finally observe this critical decay”, before adding "Finding the Higgs in its favoured decay channel was one of the key missing items in our knowledge of the Higgs boson. It opens a new window into our understanding of this intriguing particle and could ultimately provide the first hints of new physics beyond our current theories.”

UK groups including the Universities of Birmingham, Glasgow, Liverpool, Queen Mary, Oxford and UCL have played critical roles in the analysis of the data over the past seven years. This includes work on the key detector elements, reconstruction algorithms, data collection and cutting-edge analysis techniques, with UK researchers also leading the group for two key periods (Mehta and Scanlon), all of which culminated in this historic achievement that marks a crucial step forward in our understanding of the Higgs boson. Just six years after the Higgs boson was first detected.

The recent announcement is a new confirmation of the so-called “Yukawa couplings”. Similar to the Higgs mechanism, these couplings to the Higgs field provide mass to charged fermions (quarks and leptons), which are the building blocks of matter. Combined analyses of the Run-1 and Run-2 datasets have resulted in the first measurements of these couplings, as seen in the recent ATLAS observation of Higgs boson production in association with a top-quark pair and the observation of the Higgs boson decaying into pairs of tau leptons.

The recent result also establishes, for the first time, the production of a Higgs boson in association with a vector boson above five standard deviations. ATLAS has now observed all four main production modes of the Higgs boson, of which only two this year. These observations mark a new milestone in the study of the Higgs boson, as 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,” concludes Professor Jakobs.

Professor Daniela Bortoletto from the University of Oxford adds: “I believe that this measurement will improve our understanding of the mechanism of mass generation and its possible connections with cosmology and astrophysics.”

Images and the full CERN release can be found on the CERN website.

The ATLAS Collaboration first presented a preliminary result of this observation on 9 July 2018 at the International Conference on High-Energy Physics (ICHEP) in Seoul. On August 28, in a seminar together with the CMS Collaboration at CERN, ATLAS presented results which have been submitted for publication to Physics Letters B. They are based on combined Run-2 and Run-1 data, and utilise machine learning technology and new analysis techniques to reach a significance of 5.4 standard deviations.[1]

[Physicists consider five standard deviations (or “sigma”) the significance threshold past which they claim a discovery. There is only a one in a 3.5 million chance that such a signal originates from a statistical fluctuation of the background.]