New Higgs boson process observed

The ongoing collaborative ATLAS experiment at the European Research Center for Elementary Particle Physics at CERN in Geneva, Switzerland, has announced the discovery of a new particle process – the production of a Higgs boson together with two top-quarks. "This measurement constitutes a landmark achievement in the exploration of the Higgs mechanism and the interaction of Standard Model particles with the Higgs boson. It provides direct evidence that the heaviest known particle, the top quark, gets its great mass via the Higgs mechanism," says Karl Jakobs, a physics professor at the University of Freiburg and the spokesman for of  the international ATLAS collaboration. The discovery of the Higgs boson at CERN in 2012 was just the first step in the investigation of the particle. Researchers are now particularly interested in finding out if the Higgs mechanism is truly responsible for the masses of  elementary particles.

"Only one percent of all Higgs bosons are produced in association with two top quarks. In order to discover this process, we investigated important Higgs boson decays and combined the individual measurements," says Dr. Andrea Knue, a leading ATLAS collaboration researcher in the area of investigating the properties of top quarks. At the University of Freiburg, Knue heads the analysis group on Higgs production in association with top quarks. She explains, "Our biggest problem is that there are other much more frequent physical processes which look very similar to the actual signal. That is why we use complex methods, such as machine learning, for example, in order to make us better able to filter the signal."

The result exploits the full dataset up to 2017 delivered to ATLAS by the Large Hadron Collider (LHC), the world’s largest particle accelerator. The analysis enabled the researchers to establish the signal with a statistical significance of 6.3 standard deviations (sigma). It concurs with a recent observation by the CMS Collaboration experiment, which used a smaller data set – from 2011 to 2016 – and arrived at a significance of 5.2 standard deviations. Both measurements agree with the predictions of the Standard Model, even though the measurement uncertainties are still relatively large.

Further, more precise measurements and advancing the understanding of the Higgs boson and the top quark are seen as being extremely significant. Deviations from the expected particle properties could indicate new, previously unknown reactions or the existence of new elementary particles. These experiments are at the center of the research training group "Mass and Symmetries after the Discovery of the Higgs Particle at the LHC" at the University of Freiburg, which has already been funded for three years by the German Research Foundation (DFG).

"Up to now, ATLAS has only carried out three percent of the measurements envisaged for the long term. We need a more exact detector and more precise methods of analysis for the full data set. We are developing new detector components in Freiburg which will markedly improve the existing ATLAS detector by 2024. In the future we will apply artificial intelligence to data analysis more intensively than we have until now in order to better separate the signal from the background," says Gregor Herten, a physics professor at the University of Freiburg. "The coming years will be challenging and exciting," he adds.    

CERN press release