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.

The cryogenic storage ring CSR Review of Scientific Instruments 87, 063115 (2016); https://doi

The total photoabsorption cross section for 7Li, C, Al, Cu, Sn, Pb has been measured in the energy range 300–1200 MeV at Frascati with the jet-target tagged photon beam. A 4π NaI crystal detector and a lead-glass shower counter were used, respectively, to measure hadronic events and to reject the electromagnetic background. Data above 600 MeV clearly indicate a broadening of higher nucleon resonance peaks in nuclei and a reduction of the absolute value of the cross section per nucleon with respect to the free-nucleon case. This large broadening suggests a strong influence of the nuclear medium in the resonance propagation and interaction, while the systematic reduction of the measured cross sections might be due to a depletion of the resonance excitation strength and to the onset of the shadowing effect around 1 GeV. Moreover, our systematic study indicates that also the Δ-resonance excitation parameters are not the same for all nuclei, being its mass and width increasing with the nuclear density

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