ATLAS and CMS experiments at CERN describe for the first time the interaction of the two heaviest elementary particles of the Standard Model. Members of the Department of Physics of the Swiss institutes: the University of Geneva, the University of Zurich (UZH) and ETH Zurich have been involved in the analysis in a leading manner.
The top quark and the Higgs boson are the most massive particles in the Standard Model. As the Higgs boson interacts only with massive particles and stronger with heavier particles, the interaction between the top quark and the Higgs boson is expected to be large. Unfortunately the decay of the Higgs into top-anti top quarks is impossible since the mass of the Higgs is smaller than the top. Therefore it’s necessary to find an alternative route to directly probing the coupling of the Higgs boson to the top quark. Quantum mechanics allows the Higgs to fluctuate for a very short time into a top quark and a top anti-quark, which shortly after annihilate each other into a photon pair. This mechanism has now been observed for the first time.
This measurement allows us to explore new physics, an undiscovered heavy new-physics particles could infact participate in this type of process and alter the result. This is why the Higgs boson is seen as a portal to new physics.
These results are a great step forward in our knowledge of the properties of the Higgs boson. The findings of the two experiments are consistent with one another and with the Standard Model.
The extraction of these events from the LHC data is challenging as there are many mundane type of events that can mimic them. Identifying these events requires measurements from all CMS subdetectors, which makes the reconstruction quite complex. The CMS groups at ETH and UZH have developed sophisticated techniques that improve the sensitivity to these events. As consequence, this milestone has been passed considerably earlier than expected, says ETH professor and CMS Deputy Spokesperson Günther Dissertori. The development of these techniques also opens the possibility of increasing the sensitivity in many other areas of research at the LHC, says UZH professor Florencia Canelli, who is also co-leading the physics group that studies top quarks.
Dr. Stefan Gadatsch from the group of Prof. Tobias Golling at the University of Geneva was one of the key contributors to the ATLAS result. Among other things he was responsible for the combination of all channels, he was co-editor of the journal publication and was one of the major analyser of one of the main channels.
A. Benelli reports news from ATLAS and CMS Swiss Teams