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

While the existence of kaonic nuclei has been intensively studied both theoretically and experimentally, there is no conclusive result establishing its existence. Here, we have searched for the $K^-pp$, a bound state of a $K^-$ with two protons, in the $d(\pi^+, K^+)$ reaction at 1.69 GeV/$c$ at J-PARC K1.8 beam line with a missing-mass resolution of 2.7 MeV/$c^2$(FWHM). In this reaction, the $K^-pp$ is assumed to be produced as $\Lambda(1405)$ as a doorway such as $\pi^+n \to K^+\Lambda^*$, $\Lambda^*p \to K^-pp$. Since the sticking probability of the $\Lambda^*$ on proton would not be so large, 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. We have obtained an inclusive $(\pi^+, K^+)$ spectrum in a wide missing-mass range from $\Lambda$, $\Sigma$ to $\Lambda(1405)$/$\Sigma(1385)$, for the first time. A proton coincidence spectrum shows a large proton-emission probability at around 2.27 GeV/$c^2$ as a broad bump. It might be attributed to the $K^-pp$ production. A study of decay branch suggests non-mesonic decays of $\Lambda N$ and $\Sigma N$ are dominant rather than mesonic decays. We will report the results both on inclusive and coincidence analyses.