Kaonic hydrogen
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Kaonic hydrogen (also called kaonium) is an exotic atom with a negatively charged kaon orbiting a proton.
Such particles were first seen, through their X-ray spectrum, at the KEK in Tsukuba, Japan. More detailed studies have been performed at DAFNE in Frascati, Italy. Kaonium has been created in very low energy collisions of kaons with protons. In the experimenta called DAFNE exotic atoms research (DEAR), kaons were produced by the decay of a φ meson at the φ factory in DAFNE at Frascati. The experiment saw X-rays from three states in the "atom". The ground state binding energy was measured to be 193 ± 37 (statistical) ± 6 (systematic) eV.
Unlike in the hydrogen atom, where the electron and proton interact only through quantum electrodynamics, kaons and protons interact also by nuclear forces. This results in the "atom" being metastable, with a very small lifetime. This lifetime is given as a width of 249 ± 111 (statistical) ± 30 (systematic) eV. The lifetime of kaonium is expected to be as low as 1,18 as.
