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Register a new userObservation of the $K^-pp$-like structure in the $d(pi^+, K^+)$ reaction at 1.69 GeV/$c$
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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.
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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.
A search for $K^-pp$ bound state (the lightest kaonic nucleus) has been performed using the $gamma d rightarrow K^+ pi^- rm{X}$ reaction at E$_gamma$=1.5-2.4 GeV at LEPS/SPring-8. The differential cross section of $K^+ pi^-$ photo-production off deuterium has been measured for the first time in this energy region, and a bump structure was searched for in the inclusive missing mass spectrum. A statistically significant bump structure was not observed in the region from 2.22 to 2.36 GeV/$c^2$, and the upper limits of the differential cross section for the $K^-pp$ bound state production were determined to be 0.1$-$0.7 $mu$ b (95$%$ confidence level) for a set of assumed binding energy and width values.
We have studied the reaction K+ p -> K+ n pi+ using an 11 GeV/c K+ beam and the Large Acceptance Superconducting Solenoid (LASS) multiparticle spectrometer facility at SLAC. We put limits on the production of narrow theta+ baryons in this reaction.
We have analyzed data of the DISTO experiment on the exclusive pp -> p Lambda K+ reaction at 2.85 GeV to search for a strongly bound compact K-pp (= X) state to be formed in the pp -> K+ + X reaction. The observed spectra of the K+ missing-mass and the p Lambda invariant-mass with high transverse momenta of p and K+ revealed a broad distinct peak with a mass M_X = 2265 +- 2 (stat) +- 5 (syst) MeV/c2 and a width Gamma_X = 118 +- 8 (stat) +- 10 (syst) MeV.
The $Theta^+$ pentaquark baryon was searched for via the $pi^-pto K^-X$ reaction in a missing-mass resolution of 1.4 MeV/$c^2$(FWHM) at J-PARC. $pi^-$ meson beams were incident on the liquid hydrogen target with the beam momentum of 1.92 GeV/$c$. No peak structure corresponding to the $Theta^+$ mass was observed. The upper limit of the production cross section averaged over the scattering angle of 2$^{circ}$ to 15$^{circ}$ in the laboratory frame was obtained to be 0.26 $mu$b/sr in the mass region of 1.51$-$1.55 GeV/$c^2$.The upper limit of the $Theta^+$ decay width using the effective Lagrangian approach was obtained to be 0.72 MeV/$c^2$ and 3.1 MeV/$c^2$ for $J^P_{Theta}=1/2^+$ and $J^P_{Theta}=1/2^-$, respectively.
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