Precision spectroscopy of kaonic helium-3

We propose to measure the strong-interaction induced energy shift of the 3d 2p X-ray transition of kaonic helium-3 atoms with a precision better than 2 eV at the J-PARC 50-GeV PS. The proposed experiment, analyzed together with that for kaonic helium 4 atoms measured by the KEK-PS E570 collaboration [1], will provide crucial information on the isospin-dependent antikaon-nucleus strong interaction at the low energy limit, and will provide decisive data to understand the basis of the prediction of deeply-bound kaonic nuclei, and to clarify the nature of the strange multibaryon candidates recently reported at KEK [3], DATNE [4] and BNL [5]. The major parameters of the proposed experiment are summarized below: i) reaction : stopped K- + 3He hv(~ 6.4keV) + X, ii) primary beam: 30 GeV, 9 A proton, iii) secondary beam: 0.75 GeV/c K- , iv) J-PARC beamline: K1.8BR or K1.1, v) target: Liquid He-3, X-ray detectors: 8 100 mm silicon drift detectors (SDD). The anticipated beam time consists of 10 days for commissioning and 3.5 days at K1.8BR (assuming full PS intensity) and 35 days at K1.8BR (with 10% of the design intensity) respectively. The present proposal is closely related to another proposal to the J-PARC 50-GeV PS, "A search for deeply-bound kaonic nuclear states by in-flight 3He(K-, n) reaction"[6]. Both experiments will use the same beamline and the same 3 He target. In view of the relative simplicity of the X-ray measurement and its modest beam requirement, we believe it is appropriate to execute the present X-ray proposal before the "in-flight" experiment. The proposed experiment will be ready on DAY-1, and can quickly deliver crucial physics results. It is hence most suited as the J-PARC DAY-1 experiment.

The project resulted in the first ever measurement of the strong-interaction induced shift and width of the X-ray transition from the 3d to the 2p sate of kaonic helium-3, which together with earlier measurements of the same group in helium-4 solved a long standing discrepancy between early experimental results obtained in 1970-1980 and theoretical calculations. The results completely solved the so-called "kaonic helium puzzle". Exotic hadronic atoms, where the electron is replaced by a particle feeling the strong interaction, a hadron, can be used to investigate the strong interaction of this hadron with the nucleons. The strong interaction leads to a shift of the energy levels of the exotic atom as compared to the values given by the electromagnetic interaction, and a broadening due to the absorption of the hadron by the nucleon. By measuring the transition energy between low- lying states, which due to the larger mass of the hadron lies in the X-ray region, and comparing it to calculations using only the electro-magnetic interaction, the effect of the strong interaction can be precisely determined. This is of particular interest since the atomic energies are much lower than those possible in typical scattering experiments, and thus the results of exotic atom spectroscopy give precise values of the strong interaction between hadrons and nucleons at almost zero energy. A kaon is the lightest hadron containing a strange quark, the lightest quark beyond the up and down quarks present in the nucleons neutron and proton. In the case of kaonic helium, most theoretical calculations predict very small shift and width, which was in disagreement with old measurements in helium-4. Some newer calculations, which predicted a very strongly attractive kaon-nucleon interaction leading to the prediction of nuclear bound states of kaons with few nucleons, also predicted higher shifts and widths for kaonic helium. The results of earlier measurements in kaonic helium-4 and the result of these measurements in helium-3 showed, that the shift and width in both cases is in the order of eV at transition energies of ~6 keV and are thus in agreement with all calculations. This result has a high impact on the understanding of the strong interaction between kaons and nucleons and has been very positively received by the community of theoretical physicists.