$\pi\Sigma$ Spectra in the Kaon-Induced Reaction on Deuteron

$\Lambda(1405)$ is a hyperon resonance with a spin-parity of $1/2^-$, located just below the $\bar{K}N$ mass. There is a longstanding question if $\Lambda(1405)$ is a $\bar{K}N$ molecular state [1].

We carried out an experiment to observe missing mass spectra of $\pi^{\pm,0}\Sigma^{\mp,0}$ in the $K^-d\rightarrow n{\pi^{\pm,0}\Sigma^{\mp,0}}$ reactions at the J-PARC K1.8BR beam line [2]. A kaon beam of 1 GeV/$c$ was irradiated on a deuterium target. A neutron scattered at a very forward angle off deuteron was detected. A recoiled $\bar{K}$ is expected to react with a residual nucleon. An S-wave $\bar{K}N\rightarrow\pi\Sigma$ scattering is expected to be dominant since the recoiled momentum of $\bar{K}$ is as low as $\sim$250 MeV/$c$. In particular, the scattering process can take place even below the $\bar{K}N$ mass threshold. Therefore, we could access $\Lambda(1405)$ directly though the scattering process. We will present and discuss line shapes of the $\pi\Sigma$ spectra around the $\bar{K}N$ mass threshold in relation to the nature of $\Lambda(1405)$.

Reference

1. J. M. M. Hall $et\ al.$, Phys. Rev. Lett. 114, 132002 (2015).
2. J-PARC E31 proposal, H. Noumi $et\ al$, \verb*+http://www.j-parc.jp/researcher/Hadron/en/Proposal_e.html#0907+