RWTH Researchers Contribute to Spectacular Discovery

29/08/2018

Six years after the discovery of the Higgs boson, researchers at CERN succeeded for the first time in proving that Higgs boson particles decay to short-lived b quarks.

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Lutz Feld

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Over the past years physicists, engineers and technicians at RWTH Aachen University have contributed to the development of the CMS particle detector, which was instrumental in achieving this success. They have participated in designing the CMS experiment, and essential components of the detector were built in the labs of the Aachen physics institutes.

About 200 institutes from 46 countries have joined forces to undertake the large-scale CMS experiment at the Large Hadron Collider, LHC for short. As RWTH physics professor Lutz Feld, currently the spokesperson for the German CMS groups, explains, “The German teams constitute the third-largest group to participate in the experiment – after the teams from the US and Italy. Aside from three RWTH physics institutes, there are groups from Universitšt Hamburg, the Karlsruhe Institute of Technology (KIT), and from the DESY accelerator center in Hamburg.”

The Higgs Boson – A Key Component in the Standard Theory of Physics

The Higgs boson, whose existence was confirmed in 2012, plays a central role in the Standard Model of particle physics. Only the Higgs mechanism can explain how elementary particles receive their mass, that is, by coupling to the all-pervasive Higgs field. In order to detect the Higgs particle, the researchers had to register the products of its decay. As Lutz Feld points out, “The problem is that other particle reactions can create such decay particles as well. The challenge is to differentiate the Higgs decay processes from all other reactions. This is enormously difficult to achieve, but the more extraordinary the reaction results are, the higher are the chances of success.”

As b quarks are generated very frequently in proton collisions, the researchers succeeded only now in confirming the most frequent Higgs decay as posited by theory, the decay into a b quark and a b antiquark. In order to detect the b quarks of the Higgs decay, the curves of the particles resulting from the collision had to be exactly measured, as the b quarks themselves decay after a few millimeters of flight. With the help of the CMS pixel and tracking detector, in whose development the Aachen group participated, the researchers succeeded in identifying the resulting vertices and thus the relevant b quarks.

Highly Complex Data Analysis Process

Differentiating Higgs decays from the other, much more frequent reactions, required a highly complex analysis of data. Only now that a very large dataset and highly sophisticated analysis techniques are available was it possible to identify the Higgs decay. Again, together with a group from DESY, the Aachen physicists have contributed to this success, as RWTH professor Alexander Schmidt explains: “By applying new deep learning methods, it was possible significantly to increase the efficiency of the data analysis process and thus to pass the threshold for discovery.”

The Higgs decay into b quarks as well as the direct coupling of the Higgs bosons to top quarks have been confirmed by both LHC experience, ATLAS and CMS. As Feld points out, “This independent confirmation of results significantly increases out trust in these discoveries.”

Currently about 1,000 doctoral candidates from all over the world, including about 25 from RWTH, are working on the CMS experiment. For further discoveries surrounding the Higgs boson, many more proton collisions will be required. Starting in 2026, the particle accelerator will be set to generate more than 5 billion collisions per second. In order to achieve this ambitious goal, both the accelerator and the CMS detector must be significantly improved. Development work is already underway in the Aachen physics labs.