A new measurement of kaonic hydrogen X-rays

The (K) over barN 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 DA Phi NE 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 Is 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 epsilon(1s) = -283 +/- 36(stat) +/- 6(syst) eV and Gamma(1s) = 541 +/- 89(stat) +/- 22(syst) eV, respectively. The new values will provide vital constraints on the theoretical description of the low-energy (K) over barN interaction.