Direct coupling of the Higgs boson to the top quark observed

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, the CMS collaboration announces a milestone in that programme.

In the Standard Model, the Higgs boson can couple to 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 of the Higgs physics programme.

That milestone has been passed considerably earlier than expected, says ETH professor and CMS Deputy Spokesperson Günther Dissertori. This is due to the availability of excellent experimental data, but also to a good part thanks to the use of sophisticated analysis methods, ensuring that the required statistical precision could be reached.

Ever since CMS started data taking in late 2009, the ETH groups of Dissertori and Rainer Wallny (and the former group of Felicitas Pauss), together with the group of Christoph Grab and further colleagues at the Institute for Particle Physics and Astrophysics, have taken a leading role in the analysis of those data. Among the analysis tools they have developed — in collaboration with colleagues from the University Zurich — is the so-called matrix element method, which in particular has become an invaluable statistical tool for the extraction of signals in the presence of challenging backgrounds. Such tools are at the interface of theory and experiment, and they are indispensable for extracting the physics hidden in the haystack of data produced in modern high-energy physics experiments.

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 external page press release issued by CERN today, on the occasion of the opening of the LHCP2018 conference in Bologna (Italy), where also the ATLAS collaboration is presenting their latest results for the first time.