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

TL;DR

This study reports the experimental observation of a $K^-pp$-like structure in the $d(^+,K^+)$ reaction at 1.69 GeV/$c$, indicating a possible bound state with specific binding energy and width.

Contribution

First experimental evidence of a $K^-pp$-like structure in the specified reaction at this energy, with detailed measurements of its binding energy and width.

Findings

Observed a broad enhancement around 2.27 GeV/$c^2$ in missing mass spectra.

Estimated $K^-pp$ binding energy of approximately 95 MeV.

Measured the width of the structure to be about 162 MeV.

Abstract

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 $\Lambda^*p\rightarrow K^-pp$ 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.