Direct Coupling of the Higgs Boson to the Top Quark


An observation made by the Compact Muon Solenoid experiment at CERN, reported in the June 2018 issue of the academic journal Physical Review Letters, connects for the first time the two heaviest elementary particles of the Standard Model of physics. The RWTH Institutes of Physics have made important contributions to this achievement.


On July 4, 2012, the two large-scale particle physics experiments ATLAS and CMS at CERN, the European Organization for Nuclear Research in Geneva, reported on the discovery of the Higgs Boson. The groundbreaking discovery made headlines worldwide, as it confirmed the existence of the last missing piece of the Standard Model of physics half a century after it was theoretically predicted. Moreover, the discovery marked the start of a new experimental program to determine the characteristics of the newly discovered particle.

The article published in the Physical Review Letters reports that a major research milestone has now been achieved: new results reveal how strongly the Higgs boson interacts with the heaviest known elementary particle, the top quark. RWTH’s Experimental Physics institutes have contributed to reaching this milestone.

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. 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 program.

“In our analyses, we applied state-of-the art deep learning methods, which are typically used for automatic speech and handwriting recognition, among other applications. This helped us to achieve a very high sensitivity in our data analyses,” says Professor Martin Erdmann from the High Energy Physics Group at RWTH’s Experimental Physics Department.

RWTH professors Thomas Hebbeker, head of the particle physics section of the German Physical Society DPG, and Lutz Feld, spokesperson for the German CMS research groups, agree that this milestone has been achieved considerably earlier than expected. According to Professor Achim Stahl, this is due to the availability of excellent experimental data as collected by the high-quality detectors built at RWTH and other contributing research institutions. As Professor Alexander Schmidt explains, “highly sophisticated methods were employed in the analysis, ensuring that the required statistical precision would be reached.”

With the observation of the coupling between the two heaviest elementary particles of the Standard Model, the LHC physics program to characterize and more fully understand the Higgs boson has taken an important step forward. While the strength of the measured coupling is consistent with the Standard Model expectation, there is still room for contributions from the 'new physics.' In the coming years, much more data will be collected and the precision will be further improved to see whether there will be further evidence of a physics beyond the Standard Model.

Source: Press and Communcations