Study of the Lambda(1405) resonance via kaon-induced reactions*

 The Λ(1405) hyperon resonance has been attracting us for a long time if it is an "exotic" meson-baryon molecular state. Recent experimental results on extracted anti-kaon-nucleon (KbarN) scattering amplitude in isospin I = 0 and the pole mass of Λ(1405) at J-PARC will be presented.

 We measured πΣ invariant mass spectra below and above the KbarN mass threshold in the K^-d -> NπΣ in order to study the KbarN interaction and the Λ(1405) resonance [1]. For this purpose, a negatively-charged kaon (K^-) beam of 1 GeV/c was irradiated on a deuterium target at the K1.8BR beam line in the J-PARC Hadron Experimental Facility. In the experiment, a nucleon (N: neutron or proton) knocked out from a deuteron (d) by an incident K^- was detected at a very forward angle, and four different final states of π⁺Σ^-, π^-Σ⁺, π⁰Σ⁰, and π^-Σ⁰ were identefied by measuring the charged particles emitted around the target.

 This reaction can be described by the following two-step process: (i) K^-"N₁"->KbarN and (ii) Kbar"N₂"->πΣ, where "N₁" and "N₂" are nucleons bound in the deuteron.

Since the nucleon emitted at the forward angle carries away most of the collision energy in (i), the center-of-mass energy in (ii) can be lower, even below the KbarN mass threshold. Since the recoiled kaon is slow down below 250 MeV/c, one expects that the S-wave Kbar"N₂"->πΣ scattering is dominant around the KbarN mass region. In order to separate the I=0 and 1 amplitudes in (ii), we demonstrate that an isospin relation of the cross sections between the four final πΣ states is satisfied as π⁰Σ⁰ = [π⁺Σ^- + π^-Σ⁺ - π^-Σ⁰]/2.

 The scattering amplitudes of step (i) has been well analyzed by Gopal et al. [2]. Employing their amplitudes in step (i), we could deduce the S-wave scattering amplitudes of step (ii): KbarN->KbarN as well as KbarN->πΣ in I = 0 in the framework of the KbarN-πΣ coupled channel by reproducing the πΣ spectrum of the I=0 channel. We find a resonance pole at 1417.7^+6.0_-7.4(fitting error)^+1.1_-1.0(systematic error) -i[26.9^+6.0_-7.9(fitting error)^+1.7_-2.0(systematic error)] MeV in the deduced KbarN->KbarN amplitude [3].