# Search for eta-mesic 3He in non-mesonic final states

## TL;DR

This study searched for eta-mesic helium-3 nuclei via various reactions near the eta production threshold but found no conclusive evidence, setting upper limits on the formation cross section.

## Contribution

First experimental search for eta-mesic 3He in non-mesonic final states using COSY, providing upper limits on the bound state formation cross section.

## Key findings

- No narrow eta-mesic 3He signal observed.
- Upper limit for formation cross section ranges from 2 to 15 nb.
- Indication of a possible bound state is within systematic error.

## Abstract

In May-June 2014, the experiment devoted to the search for the bound state of the $\eta$ meson and $^3$$He$ nucleus has been performed at COSY accelerator in Research Center J\"ulich in Germany with WASA-at-COSY facility. The excitation functions for $pd\rightarrow^3$$He2\gamma$, $pd\rightarrow^3$$He6\gamma$, $pd\rightarrow ppp\pi^{-}$, $pd\rightarrow ppn\pi^{0}$, $pd\rightarrow dn\pi^{-}$, $pd\rightarrow dp\pi^{0}$, $pd\rightarrow pd$, and $pd\rightarrow ppn$ reactions have been measured in the vicinity of the $\eta$ meson production threshold. The experiment was carried out using COSY ramped proton beam and deuterium pellet target. The beam momentum varied continuously in the range of $1.426-1.635~GeV/c$ corresponding to $^3$$He-\eta$ excess energy range from $-70$ to $+30~MeV$. The analysis of the obtained excitation functions for the $pd\rightarrow^3$$He\eta$ indicate slightly the signal from the bound state for $\Gamma>20~MeV$ and $B\in[0;15]~MeV$. However, the observed indication is in the range of the systematic error. Therefore the final conclusion of this thesis is that no narrow structure that could be interpreted as $\eta$-mesic nucleus was observed in both excitation curves. Thus, the upper limit for the total cross section of the bound state formation was estimated assuming that the $\eta$ decay branching ratios in the bound state is the same as in free space. The upper limit at the $90\%$ confidence level varies from $2$ to $15~nb$ within the binding energy range from $0$ to $60~MeV$ and the width from $2.5$ to $40~MeV$.