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