Measurement of kaonic deuterium X-ray transitions

The world, us included, is made up of atoms, which in turn contain a nucleus made up of protons, neutrons and electrons orbiting it. The simplest atom, hydrogen, is composed of an electron and a proton, while carbon, so important for life, is made out of six electrons and a nucleus with six protons and six neutrons. Protons and neutrons belong to the group of particles called hadrons and are subject to the strong interaction, one of the four fundamental forces of nature. Hadrons are built up of elementary particles named quarks. In the case of the proton, two "up" quarks and one "down" quark are involved. "Up" and "down" quarks are the two lightest quarks in the 6-member quark family, containing the "up", "down", "strange", "charm", "bottom", "down" and "top" quarks. The strong interaction still raises many questions, especially in the low-energy limit, that need to be answered: in particular the question of how the mass of hadrons is generated and has the value we measure and the role of the strong interaction in that. For example, the proton is made up of three "light" quarks, but the sum of their mass is only a few percent of the proton mass. What is the rest? Therefore, we plan to investigate the strong interaction at low energies by investigating so-called exotic atoms, in which an electron is replaced, in our case, by a negatively charged kaon. The kaon, unlike the electron, belongs to the group of hadrons and is therefore subject to the strong interaction. While the proton and neutron are formed from three quarks each, the kaon consists of only two quarks: a quark - antiquark pair. The special feature of the kaon is that it is made up of a "light" quark and the much heavier "strange" quark. The strange quark contained in the kaon allows us to study the strong interaction as seen through a magnifying glass. The SIDDHARTA-2 and E57 experiments will use negatively charged kaons to form kaonic deuterium atoms. Deuterium is a hydrogen isotope whose nucleus consists of a proton and a neutron. In contrast to the electron, which is bound to the atomic nucleus by the electromagnetic interaction, the kaon additionally feels the strong interaction, which causes a change in the binding energy between the kaon and the nucleus. This means that the energy levels the kaon would occupy in the ground state of a kaonic atom are displaced by the strong interaction as compared to the "pure" electromagnetic interaction, which is responsible for the binding energy of "normal" atoms. Both experiments plan to precise measure the change in the binding energy in the ground state of kaonic deuterium atoms, which allows us to study the influence of the strong interaction very closely. These measurements lead to a better understanding of the basic processes of the strong interaction, one of the fundamental phenomena of nature, with implications going from particle and nuclear physics all along to the neutron stars, whose core may contain strange quarks.