Search for $\eta$-bound Nuclei

TL;DR

This paper investigates the existence of eta-mesic nuclei through various reactions, providing experimental evidence and scattering length measurements that suggest possible bound states of eta mesons with nuclei.

Contribution

The study offers new experimental data on eta-nucleus interactions and reports potential evidence for eta-bound states with specific binding energies and widths.

Findings

Complex scattering lengths for eta interactions with helium isotopes.

Evidence of a possible eta-bound state with a binding energy of 13.3 MeV.

Observation of a significant bump in the missing mass spectrum indicating a bound state.

Abstract

The $\eta$ meson can be bound to atomic nuclei. Experimental search is discussed in the form of final state interaction for the reactions $dp\to{^3\text{He}}\eta$ and $dd\to{^4\text{He}}\eta$. For the latter case tensor polarized deuterons were used in order to extract the s-wave strength. For both reactions complex scattering lengths are deduced: $a_{{}\rm{^3He}\eta } = \left[ { \pm \left( {10.7 \pm 0.8_{ - 0.5}^{ + 0.1} } \right) + i \cdot \left( {1.5 \pm 2.6_{ - 0.9}^{ + 1.0} } \right)} \right]$ fm and $a_{^4\text{He}\eta } = \left[ { \pm \left( {3.1 \pm 0.5} \right) + i \cdot \left( {0 \pm 0.5} \right)} \right]$ fm. In a two-nucleon transfer reaction under quasi-free conditions, $p^{27}\text{Al}\to {^3\text{He}}X$, was investigated. The system $X$ can be the bound $^{25}\text{Mg}\otimes\eta$ at rest. When a possible decay of an intermediate $N^*/1535)$ is required, a highly significant bump shows up in the missing mass spectrum. The data give for a bound state a binding energy of 13.3$\pm$1.6 MeV and a width of $\sigma$=4.4$\pm$1.3 MeV.