Kaonic hydrogen is an exotic atom consisting of a negatively charged kaon orbiting a proton.
Such particles were first identified, through their X-ray spectrum, at the KEK proton synchrotron in Tsukuba, Japan in 1997.
At DAFNE, kaons are produced by the decay of φ mesons which are in turn created in collisions between electrons and positrons.
In kaonic hydrogen this strong contribution was found to be repulsive, shifting the ground state energy by 283 ± 36 (statistical) ± 6 (systematic) eV, thus making the system unstable with a resonance width of 541 ± 89 (stat) ± 22 (syst) eV[3] (decay into Λπ and Σπ[4]).
Kaonic hydrogen is studied mainly because of its importance for the understanding of kaon-nucleon interactions and for testing quantum chromodynamics.