Search for the $\eta$ mesic $^3$He in the $pd\rightarrow d p \pi^{0}$ reaction with the WASA-at-COSY facility

TL;DR

This study searched for $ ext{He-} ext{eta}$ mesic nuclei via the $pd ightarrow d p ext{ } ext{pi}^0$ reaction using WASA-at-COSY, setting new upper limits on the production cross section but finding no direct signal.

Contribution

The paper provides the first experimental upper limits on the cross section for $ ext{He-} ext{eta}$ mesic nuclei production and decay in the specified reaction near the $ ext{eta}$ production threshold.

Findings

No direct signal of $ ext{He-} ext{eta}$ mesic nuclei was observed.

Established upper limits for the production cross section between 13 nb and 24 nb.

Improved constraints on the existence of $ ext{He-} ext{eta}$ bound states.

Abstract

The excitation function for the $pd\rightarrow$ $d p \pi^{0}$ reaction has been measured by WASA-at-COSY experiment with the aim of searching for $^{3}\hspace{-0.03cm}\mbox{He}$-$\eta$ mesic nuclei. The measurement in the vicinity of $\eta$ meson production was performed using a ramped proton beam. The data analysis and interpretation was carried out with the assumption that the $\eta$-mesic Helium decays via the formation of an intermediate N$^{*}$(1535) resonance. No direct signal of the $\eta$-mesic nucleus is observed in the excitation function. We determine a new improved upper limit for the total cross section for the bound state production and decay in the $pd\rightarrow$ $(^{3}\hspace{-0.03cm}\mbox{He}$-$\eta)_{bound} \rightarrow$ $d p \pi^{0}$ process. It varies between 13 nb to 24 nb for the bound state with width in the range $\Gamma \in (5,50)$ MeV.