A new measurement of kaonic hydrogen X-rays

The K¯N system at threshold is a sensitive testing ground for low energy QCD, especially for the explicit chiral symmetry breaking. Therefore, we have measured the K-series X-rays of kaonic hydrogen atoms at the DAΦNE electron-positron collider of Laboratori Nazionali di Frascati, and have determined the most precise values of the strong-interaction energy-level shift and width of the 1s atomic state. As X-ray detectors, we used large-area silicon drift detectors having excellent energy and timing resolution, which were developed especially for the SIDDHARTA experiment. The shift and width were determined to be ɛ_1s=-283±36(stat)±6(syst) eV and Γ_1s=541±89(stat)±22(syst) eV, respectively. The new values will provide vital constraints on the theoretical description of the low-energy K¯N interaction.