Direct coupling of the Higgs boson to the top quark observed

Brussels, Monday 4 June 2018:

An observation made by the Compact Muon Solenoid experiment at the Large Hadron Collider at CERN, published in the Physical Review Letters today, connects for the first time the two heaviest elementary particles of the Standard Model. Members of the CMS collaboration, including the Excellence of Science programme “be.h: The H boson gateway to physics beyond the Standard Model”, which includes researchers from the Université Catholique de Louvain, Université Libre de Bruxelles, Vrije Universiteit Brussel, Universiteit Gent and Universiteit Antwerpen were involved with this ground-breaking result.

On 4 July 2012, two of the experiments at the CERN’s Large Hadron Collider (LHC), ATLAS and CMS, reported independently the discovery of the Higgs boson. The announcement created headlines worldwide: the discovery confirmed the existence of the last missing elementary particle of the Standard Model, half a century after the Higgs boson was predicted theoretically. At the same time the discovery marked also the beginning of an experimental programme aimed to determine the properties of the newly discovered particle. Reporting today in Physical Review Letters, and already submitted in April 2018, the CMS collaboration announces a milestone in that programme.


Event displayIn the Standard Model, the Higgs boson can couple to the particles that all matter is made of, called fermions, with a coupling strength proportional to the fermion mass. While associated decay processes have been observed, the decay into top quarks, the heaviest known fermion, is kinematically impossible. Therefore, alternative routes to directly probing the coupling of the Higgs boson to the top quark are needed. One is through the production of a Higgs boson and a top quark–antiquark pair (see the figure). This is the production mechanism that has now been observed for the first time, and in doing so, the CMS collaboration accomplished one of the primary objectives the LHC during this decade.

“That milestone has been passed considerably earlier than expected”, says UCL professor and EOS be.h spokesperson Fabio Maltoni, who is involved with the theoretical predictions that are necessary to understand this discovery. 

The data analysis was partially performed in Brussels by Kirill Skovpen, FWO postdoc at the Vrije Universiteit Brussel. Skovpen mentions that this effort represents a lively and world-wide collaboration of many excellent physicists coming from the very different expertise fields within the collaboration and the expertise in Belgium is an enormous asset in pursuing an extremely challenging task. Skovpen contributed to the multilepton analysis, the most sensitive part of the data, which made it possible to claim the evidence for this process for the very first time.

Figure_003There is substantial expertise in the modelling and understanding of the most important input to the analysis in Belgium at UCL, UA, VUB, UGent and ULB, the background from top quarks (without Higgs bosons) and Higgs bosons (without top quarks). These Belgian scientists are working together in the Excellence of Science programme which aims to promote joint research between researchers in the Flemish and French-speaking community by funding joint fundamental research projects. “With the analysis of new data from the ongoing experiments at the LHC we are firm on our feet to start with the detailed exploration of the interplay between the top quark and the Higgs boson in order to scrutinise the intrinsic properties of this fundamental interaction. These two heaviest fundamental particles of the standard model may open heavy doors to search for possible answers to yet unresolved questions of the our universe!”, says Skovpen.

That this discovery has finally happened, is due to the availability of excellent experimental data that Belgian physicists have access to in the CMS collaboration at CERN, and also to a good part thanks to the use of sophisticated analysis methods, ensuring that the required statistical precision could be reached. One of the challenges  was to prepare the real-time selection (called trigger) of interesting collision events. Pascal Vanlaer from the ULB has been in charge of this selection and says “We thought of clever ways of using the detector data and, when I saw that they actually worked with the first 13 TeV collisions started, I had a big smile on my face.”
Maltoni adds: “and now the way is paved for new exciting experimental and theoretical explorations. Personally, I am also excited because I have started working on the predictions of this process 16 years ago. Actually, back then I proposed (together with two very good friends) looking at 2,3,4 leptons in the final state and…that’s exactly what happened: among all channels considered in the current analyses the multi­lepton final state is the one which provides the largest significance and has a clear Belgian flavour! It is just one of the many contributions to a huge effort, yet I feel proud and happy! It’s a great day.”

The present achievement is a case in point. With the observation of the coupling between the two heaviest elementary particles of the Standard Model, the LHC physics programme to characterise and more fully understand the Higgs boson has taken an important step. While the strength of the measured coupling is consistent with the Standard Model expectation, the precision of the measurement still leaves room for contributions from new physics. In the coming years, much more data will be collected and the precision will be improved, in order to see if the Higgs reveals the presence of physics beyond the Standard Model.

Further information can be found in a press release issued by CERN today, on the occasion of the opening of the Sixth Annual Large Hadron Collider Physics Conference 2018 (LHCP2018) conference in Bologna (Italy), where also the ATLAS collaboration is presenting their latest results for the first time.

The scientists involved in this report receive funding from F.R.S.-FNRS and FWO under the “Excellence of Science – EOS” – be.h project n. 3082081

For contacts at all universities involved:

For more information see:

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