We have measured the shift and width of the kaonic hydrogen 1s state due to the KN strong interaction. We have observed, for the first time, distinct K-series kaonic hydrogen x rays with good signal-to-noise ratio in the energy spectrum. The measured energy shift and width were determined to be DE͑1s͒ 2323 6 63͑stat͒ 6 11͑syst͒ eV (repulsive) and G͑1s͒ 407 6 208͑stat͒ 6 100͑syst͒ eV, respectively. [S0031-9007(97)02992-X] PACS numbers: 13.75. Jz, 25.80.Nv, 29.30.Kv, 36.10.Gv The determination of the strong-interaction energy level shift and width of the kaonic hydrogen x rays is one of the most important subjects for the understanding of the KN interaction. It is strongly affected by the presence of the L͑1405͒ subthreshold resonance. The study of the KN interaction is also relevant to the important question of K 2 condensation in dense matter [1,2].The observation of the shift and width of the kaonic hydrogen K a ͑2p ! 1s͒ x rays gives direct information about the KN s-wave interaction at the K 2 p threshold energy in a fairly model independent way [3]. The status of the study was quite puzzling due to the contradiction between the signs of the scattering lengths obtained by the previous x-ray measurements [4-6] and those extracted from the analyses of the low energy KN data, e.g., , as shown in Fig. 1. This contradiction is known to be almost impossible to reconcile within the conventional theoretical framework. Moreover, the x-ray signals of the previous experiments are very difficult to identify in their spectra. Therefore, a definitive experiment has been long awaited.We accumulated data for 760 hours at KEK-PS K3. A detailed description of our experimental setup is given in a separate paper [10]. Here we present a short summary.Optimization of the target density is quite important for this experiment. As a compromise between kaon stopping yield and kaon loss during the atomic cascade due to the Stark effect, we chose to operate the hydrogen FIG. 1. The energy shift and width of 1s state. One-standarddeviation region of shift and width of the previous experiments are plotted together with theoretical calculations. The present result is shown in bold.

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Energy spectra of both protons and deuterons emitted following the capture of negative muons by He nuclei have been measured for energies above 15 MeV. A limited number of proton-neutron pairs emitted in coincidence were also observed. A simple plane wave impulse approximation (PWIA) model calculation yields fair agreement with the measured proton energy spectra, but underpredicts the measured rate of deuteron production above our energy threshold by a large factor. A more sophisticated PWIA calculation for the two-body breakup channel, based on a realistic three-body wave function for the initial state, is closer to the deuteron data at moderate energies, but still is signi6cantly lower near the kinematic end point. The proton-neutron coincidence data also point to the presence of signi6cant strength involving more than one nucleon in the capture process at high energy transfer. These results indicate that additional terms in the capture matrix element beyond the impulse approximation contribution may be required to explain the experimental data. Speci6cally, the inclusion of nucleon-nucleon correlations in the initial or 6nal state and meson exchange current contributions could bring calculations into better agreement with our data. A fully microscopic calculation would thus open the possibility for a quantitative test of multinucleon effects in the weak interaction.Present address: Stanford Linear Accelerator Center, Stanford, CA 94309.Nuclear muon capture Z+p m (Z -1)'+v"normally leads to the emission of an energetic neutrino carrying most of the rest energy of the muon, while the energy transfer to the nucleus q = m& -E"is relatively small.The transferred energy leads to low-lying excitations of the residual nucleus up to the giant resonant region [1] or to the emission of medium energy neutrons (see, e. g., Ref. [2] and references therein). These conditions can be understood in the impulse approximation (IA) picture, in 0556-2813/94/50(4)/

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