International Conference on Exotic Atoms and Related Topics - EXA2017

Lambda(1405) is a well-known hyperon resonance with the spin/parity of 1/2-. According to PDG, its mass and width are 1405.1(+1.3-1.0) MeV and 50 MeV [1], Its light mass, located at 27 MeV below the KbarN mass threshold, arises a basic idea of possible deeply bound kaonic nuclear states [2]. On the other hand, there is a longstanding argument if Lambda(1405) has a so-called double-pole structure, comprizes pi-Sigma and KbarN states [3,4,5]. In particular, a chiral unitary model calculation claims that the pole coupled to the KbarN state is located at about 1426 MeV [4], much closer to the KbarN mass threshold. In order to confirm the pole structure, it is desired to measure the S-wave KbarN scattering amplitude in the isospin equal to 0 channel below the KbarN threshold. We therefore proposed an experiment via the (K-,n) reaction on deuteron at the K1.8BR beam line of J-PARC [6]. In the reaction, an incident negative kaon of 1 GeV/c knocks out a neutron at a forward angle and a recoilled kaon reacts with a residual nucleon. The d(K-,n) reaction is expected to enhance the S-wave KbarN scattering even below the KbarN threshold due to a small momentum transfer of about 200 MeV/c. Missing mass spectra of pi+-Sigma-+,pi0Sigma0, and pi-Sigma0 in the d(K-,n) and d(K^-,p) reactions were measured. We will discuss the line shapes of the measured spectra to deduce information on a resonance coupled to the KbarN channel below the KbarN thresold. [1] C. Patrignani et al. (Particle Data Group), Chin. Phys. C40, 100001(2016). [2] Y. Akaishi and T. Yamazaki, Phys. Rev. C65, 044005(2002); Y. Akaishi and T. Yamazaki, Phys. Lett.. B535, 70(2002). [3] J.A. Oller and U.-G. Meissner, Phys. Lett. B500, 263(2001). [4] D. Jido, J. A. Oller, E. Oset, A. Ramos, and U.-G. Meissner, Nucl. Phys. A725, 181(2003). [5] T. Hyodo and W. Weise, Phys. Rev. C77, 035204(2008). [6] H. Noumi et al., J-PARC E31 Proposal, 2009