The KN 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 ǫ 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 KN interaction. * Corresponding authors.
Kaonic hydrogen atoms provide a unique laboratory to probe the kaon-nucleon
strong interaction at the energy threshold, allowing an investigation of the
interplay between spontaneous and explicit chiral symmetry breaking in
low-energy QCD. The SIDDHARTA Collaboration has measured the $K$-series X rays
of kaonic hydrogen atoms at the DA$\Phi$NE electron-positron collider of
Laboratori Nazionali di Frascati, and has determined the most precise values of
the strong-interaction induced shift and width of the $1s$ atomic energy level.
This result provides vital constraints on the theoretical description of the
low-energy $\bar{K}N$ interaction.Comment: 11 pages, 7 figures, Nuclear Physics A (in press) Special Issue on
Strangeness Nuclear Physic
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