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 DEAR (DANE exotic atom research) experiment measured the energy of x rays emitted in the transitions to the ground state of kaonic hydrogen. The measured values for the shift and the width ÿ of the 1s state due to the K ÿ p strong interaction are 1s ÿ193 37 (stat) 6 (syst) eV and ÿ 1s 249 111 (stat) 30 (syst) eV, the most precise values yet obtained. The pattern of the kaonic hydrogen K-series lines, K , K , and K , was disentangled for the first time. DOI: 10.1103/PhysRevLett.94.212302 PACS numbers: 13.75.Jz, 25.80.Nv, 36.10.Gv Over 40 years, chiral symmetry breaking has been recognized as the essential aspect of nuclear low-energy phenomena. The outline of how the breaking plays a vital role is well known, yet its detailed dynamics is uncertain. The existence of the eight pseudoscalar mesons (; K; ) is believed to arise from spontaneous symmetry breaking of the flavor global symmetry represented by the group SU3 L SU3 R , which generates the mesons as Nambu-Goldstone bosons, leaving the vacuum only SU(3) symmetric [1]. Furthermore, the mass spectrum of these mesons reflects the explicit breaking of this symmetry [2]. In the quark model, the squares of the meson masses are proportional to the small current quark masses with the multiplicative factors of the chiral quark condensate in vacuum. The large mass difference between the mesons and the current quarks then suggests that the condensate is playing a significant role in the structure of the mesons [3].A similar situation is expected to occur in the structure of baryons and to be manifested in the baryon-pseudoscalar meson interaction [4]. In this case, the corresponding relation is that the baryon sigma terms are proportional to the current quark masses with the factors of the chiral quark condensate for the baryons [3]. The sigma terms thus serve as the measure of the significance of the condensate in the structure of the baryons. Especially of interest here is how the SU(3) flavor symmetry is realized in this aspect of the nucleon structure, but more specifically, how high is the strangeness content of the nucleon. The resolution of these issues depends quite sensitively on the value of the kaonnucleon (KN) sigma terms [5]. As the basic symmetry of QCD is SU(3), the KN sigma terms play the central role in various nuclear phenomena, such as strangeness production in heavy-ion collision and chiral restoration in nucleon matter, a topic of astrophysical interest [6].The KN sigma terms are closely related to the lowenergy KN and antikaon-nucleon (KN) scattering amplitudes [7], but the value of the KN sigma terms continues to remain with a large uncertainty [6,7] in spite of the recent efforts in lattice [8] and chiral perturbation [9] calculations, where the information on the KN and KN scattering lengths is vital. In this work, we report an accurate measurement of the ground-state x-ray transitions in kaonic hydrogen atoms. The shift and width of the atomic ground state is known to provide the most accurate information of the K ÿ -proton scattering l...

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We study kaonic hydrogen, the bound K − p state A Kp . Within a quantum field theoretic and relativistic covariant approach we derive the energy level displacement of the ground state of kaonic hydrogen in terms of the amplitude of K − p scattering for arbitrary relative momenta. The amplitude of low-energy K − p scattering near threshold is defined by the contributions of three resonances Λ(1405), Λ(1800) and Σ 0 (1750) and a smooth elastic background. The amplitudes of inelastic channels of low-energy K − p scattering fit experimental data on near threshold behaviour of the cross sections and the experimental data by the DEAR Collaboration. We use the soft-pion technique (leading order in Chiral Perturbation Theory) for the calculation of the partial width of the radiative decay of pionic hydrogen A πp → n+γ and the Panofsky ratio. The theoretical prediction for the Panofsky ratio agrees well with experimental data. We apply the soft-kaon technique (leading order in Chiral Perturbation Theory) to the calculation of the partial widths of radiative decays of kaonic hydrogen A Kp → Λ 0 +γ and A Kp → Σ 0 +γ. We show that the contribution of these decays to the width of the energy level of the ground state of kaonic hydrogen is less than 1%.