Observation of the $K^-pp$-like structure in the $d(pi^+, K^+)$ reaction at 1.69 GeV/$c$

We have observed a $K^-pp$-like structure in the $d(pi^+,K^+)$ reaction at 1.69 GeV/$c$. In this reaction $Lambda(1405)$ hyperon resonance is expected to be produced as a doorway to form the $K^-pp$ through the $Lambda^*prightarrow K^-pp$ process. However, most of the produced $Lambda(1405)$s would escape from deuteron without secondary reactions. Therefore, coincidence of high-momentum ($>$ 250~MeV/$c$) proton(s) in large emission angles ($39^circ<theta_{lab.}<122^circ$) was requested to enhance the signal-to-background ratio. A broad enhancement in the proton coincidence spectra are observed around the missing-mass of 2.27 GeV/$c^2$, which corresponds to the $K^-pp$ binding energy of 95 $^{+18}_{-17}$ (stat.) $^{+30}_{-21}$ (syst.) MeV and the width of 162 $^{+87}_{-45}$ (stat.) $^{+66}_{-78}$ (syst.) MeV.

Inclusive spectrum of the $d(pi^+, K^+)$ reaction at 1.69 GeV/c

We have measured an inclusive missing-mass spectrum of the $d(pi^+, K^+)$ reaction at the pion incident momentum of 1.69 GeV/$c$ at the laboratory scattering angles between 2$^circ$ and 16$^circ$ with the missing-mass resolution of 2.7 $pm$ 0.1 MeV/$c^2$ (FWHM) at the missing mass of 2.27 GeV/$c^{2}$. In this Letter, we first try to understand the spectrum as a simple quasi-free picture based on several known elementary cross sections, considering the neutron/proton Fermi motion in deuteron. While gross spectrum structures are well understood in this picture, we have observed two distinct deviations; one peculiar enhancement at 2.13 GeV/$c^2$ is due to the $Sigma N$ cusp, and the other notable feature is a shift of a broad bump structure, mainly originating from hyperon resonance productions of $Lambda(1405)$ and $Sigma(1385)^{+/0}$, by about 22.4 $pm$ 0.4 (stat.) $^{+2.7}_{-1.7}$ (syst.) MeV/$c^2$ toward the low-mass side, which is calculated in the kinematics of a proton at rest as the target.