A New Measurement of Kaonic Hydrogen X rays

Abstract

The $\bar{K}N$ 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 DA$\hat{I}¦$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 $1s$ 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 $\hat{I}{\mu }_{1s}=-283\pm 36\pm 6(syst)$ eV and $\hat{I}{“}_{1s}=541\pm 89(stat)\pm 22(syst)$ eV, respectively. The new values will provide vital constraints on the theoretical description of the low-energy $\bar{K}N$ interaction.

5 figures, submitted to Physics Letters B