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 How many matter particles exist in nature? Particle physicists have been dealing with this question for a long time. The 12 matter particles contained in the standard model of particle physics? Or are there further particles with too high a mass to be produced by the experiments performed so far? These questions are now answered by researchers of KIT, CERN, and Humboldt University in the current issue of the Physical Review Letters.

Matter particles, also called fermions, are the elementary components of the universe. They make up everything we see on earth or through telescopes. "For a long time, however, it was not clear whether we know all components," explains Ulrich Nierste, Professor at KIT. The standard model of particle physics knows 12 fermions. Based on their similar properties, they are divided into three generations of four particles each. Only the first generation of particles occurs in appreciable amount outside of particle accelerators. Among these particles are the electron, the electron neutrino, and the up-quark and down-quark. Up- and down-quarks form heavier particles, such as protons and neutrons and, hence, all elements of the periodic system.

"But why does nature have second and third generations, if these are hardly needed? And are there maybe more generations of particles?," ask the main authors of the article, Martin Wiebusch and Otto Eberhardt. At least, the latter question is answered: "There are exactly three fermion generations in the standard model of particle physics!"

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