DEAR pins down kaonic hydrogen

The DEAR (DAFNE Exotic Atoms Research) experiment at the DAFNE φ factory at Frascati has performed the most accurate determination of the effect of the strong interaction on the binding energy of kaonic hydrogen.

Kaonic hydrogen is an exotic atom where the electron is replaced by a K^–, and it turns out to be an excellent laboratory for studies of quantum chromodynamics. Especially interesting is the determination of the strangeness content of the nucleon, which has traditionally been determined from low-energy kaon-nucleon scattering amplitudes. A significantly more accurate approach has now been discovered, which involves measuring the ground-state X-ray transitions in kaonic hydrogen atoms.

The DEAR collaboration took advantage of the low-energy monoenergetic kaons from the decay of φ mesons resonantly produced by e⁺e^– collisions at one of the two interaction points at DAFNE. The kaons travelled through the thin beam pipe of DEAR and stopped in a gaseous hydrogen target. CCD detectors with a pixel size of 22.5 x 22.5 μm² cooled to 165 K detected the X-rays emitted.

The DEAR experiment follows the steps of the KpX experiment at KEK in Japan, which first measured the ground-state X-ray peak of kaonic hydrogen. DEAR’s values are about a factor of two more accurate, and roughly 40% lower than those of the Japanese collaboration. The ground-state shift, ε_1s, was measured to be -193 ± 37 (stat.) ± 6 (syst.) eV, with a 1s strong interaction width of Γ_1s = 249 ± 111 (stat.) ± 30 (syst.) eV.

DEAR has also become the first experiment to observe transitions from different excited states, clearly identifying K_α, K_β and K_γ lines.