A new historic milestone in the study of the Higgs boson

28 August 2018

A new milestone in the study of the Higgs boson

ATLAS observes elusive Higgs boson decay to a pair of bottom quarks, a confirmation of the ‘Yukawa couplings’

By Prof Daniela Bortoletto. 28 August 2018, Geneva


A candidate event display for the production of a Higgs boson decaying to two b-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: ATLAS Collaboration/CERN)

Particle physicists are celebrating that 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% of the Higgs boson decays. Yet it took over seven years to accomplish this observation.

“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 on 9 July at the 2018 International Conference on High-Energy Physics (ICHEP) in Seoul. Today, in a seminar together with the CMS Collaboration, 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. UK groups including the Universities of Birmingham, Glasgow, Liverpool, Queen Mary, Oxford and UCL have made important contributions to this historic achievement.

Finding Higgs boson decaying into a pair of b quarks at the LHC is challenging. Since the LHC collisions produce b-quark pairs in great abundance it is essential to select events where the Higgs boson appears alongside a W or Z particle, which makes the events easier to tag. The Oxford team led by Professor Daniela Bortoletto, analysed elusive W and Z bosons decays where the decay products of these particles are not directly identified in the detector but are inferred from a large transverse energy imbalance in the event. Her postdoc, Elisabeth Schopf and her students, Cecilia Tosciri and Luca Ambroz, made significant contributions to the result. Elisabeth played a leading role in optimizing the sophisticated machine learning algorithms that allowed ATLAS to increase the sensitivity to these events. Luca established a new technique that used Monte Carlo events in a clever way and lead to a higher expected significance for the analysis. Cecilia upgraded a method to improve the resolution achieved in the measurements of the decay of the Higgs boson into b-quarks.


Distribution of b-quark pair mass from all search channels combined after subtraction of all backgrounds except for WZ and ZZ production. The data (points with error bars) are compared to the expectations from the production of WZ and ZZ (in grey) and of WH and ZH (in red). (Image: ATLAS Collaboration/CERN)

This is among the most demanding analyses carried out by ATLAS so far. “LHC collisions produce b-quark pairs in great abundance, making it hard to spot those originating from Higgs boson decays,” says Kerstin Tackmann, ATLAS Higgs working group convener. “The analysis teams therefore focused on signatures, in particular the production of a Higgs boson in association with a vector boson, which increased substantially the purity of the signal.” This technique proved highly successful.

For Bortoletto this is a very special moment and the culmination of an even longer wait. She started looking for the decays of the Higgs boson to b-quarks at the US proton-proton TEVATRON collider with the CDF detector in 2005. Four of her former students completed their thesis on searches for the Higgs in events with large transverse missing energy and b-quarks between 2007 and 2012. The results of this work were used in the final TEVATRON combination which reached about 3 standard deviations in 2012, not enough for a discovery. She is delighted that the LHC finally unveiled this important decay mode of the Higgs boson. “I did not have any doubt that at the end of this tour de force we will pass the significance of 5 standard deviations which is necessary to claim a discovery. The LHC is a more powerful accelerator than the TEVATRON and ATLAS is a superb detector.” says Bortoletto. She also adds: “I believe that this measurement will improve our understanding of the mechanism of mass generation and its possible connections with cosmology and astrophysics.”

Today’s 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.

Today’s result also establishes, for the first time, the production of a Higgs boson in association with a vector boson above 5 standard deviations. ATLAS has now observed all four main production modes of the Higgs boson, of which two only 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 Karl Jakobs.

For any specific enquiries, please contact Prof Daniela Bortoletto.