Akaishi and Yamazaki predicted strongly bound kaonic nuclear states with narrow width^a. In the E549 and E570 experiments, data of the ⁴He(stopped K,N) reactions with N being a proton or a neutron were accumulated to increase the statistics obtained by the previous experiment (E471), with upgrades of the detector systems^b,^c . Compared to E471, about 20 times of statistics for the proton inclusive data and 5 times for the neutron data were accumulated. The time resolution was improved from 300 to 120 ps for the proton data, and from 300 to 200 ps for the neutron data.

The narrow (~20 MeV) peak identified as the S⁰(3115) state in Ref^d was not reproduced in both the inclusive ⁴He(stopped K^-, p) and semi-inclusive ⁴He(stopped K^-, pX^±) reaction channels^c, where X^± denotes one of the decay charged particles. Detailed comparison of the experimental data and simulations showed that the narrow peak structure previously reported as the S⁰(3115) state was caused by an analysis procedure which used an approximation function for the slewing correction of the time-of-flight measurement. With a slewing correction function taking into account higher-order terms, it was found that the peak reported in^d has a much larger width, if it exists at all. The upper limit of the probability for the S⁰ state to have a narrow width (< 20 MeV) was found to be only 10^-4.

For the neutron spectra obtained by the ⁴He(stopped K-, n) reaction, an indication for a peak-like structure named later S⁺(3140) was reported previously^e, although it was never published. A recent analysis including the E549 and E570 data shows that in contradiction to the previous result, no significant narrow peak structure can be seen in the spectra (see Fig. 1)^b.

The RIKEN group is now analyzing the proton and neutron data assuming a wider peak than was theoretically predicted. The determination of the background components, which is important to find a wide peak, has been performed with detail studies of the hyperon-decay channels. In particular, data analysis based on the invariant mass, which is similar to the analysis performed by FINUDA, could be helped to understand our spectra.

 

^a Y. Akaishi and T. Yamazaki, Phys. Rec. C 65 (2002) 04405.
^b H. Yim et al., Proc. of HYP06, Mainz 2006.
^c M. Iwasaki et al., Proc. of HYP06, Mainz 2006.
^d T. Suzuki et al., Phys. Lett. B 597 (2004) 263.
^e M. Iwasaki et al., nucl-ex/0310018 (2004).