Has the neutral double hypernucleus $_{\Lambda\Lambda}^{4}$n been observed?

TL;DR

This paper investigates the possible observation of the neutral double hypernucleus $_{ ext{ΛΛ}}^{4}$n, analyzing experimental data and theoretical models to determine its existence and distinguish it from other hypernuclear states.

Contribution

The study provides evidence supporting the existence of $_{ ext{ΛΛ}}^{4}$n and challenges previous interpretations of experimental structures as other hypernuclei.

Findings

The structure at (133,114) MeV/c can be explained by $_{ ext{ΛΛ}}^{4}$n decays.

Production scenarios for $_{ ext{ΛΛ}}^{4}$n are consistent with experimental data.

A bound $^{3}_{ ext{Λ}}$n nucleus is inconsistent with the observed data.

Abstract

The BNL-AGS E906 experiment was the first fully electronic experiment to produce and study double hypernuclei with large statistics. Two dominant structures were observed in the correlated $\pi^-$--$\pi^-$ momentum matrix at (p$_{\pi-H}$,p$_{\pi-L}$) = (133,114) MeV/c and at (114,104) MeV/c. In this work we argue that the interpretation of the structure at (133,114) MeV/c in terms of $^3_{\Lambda}$H+$^4_{\Lambda}$H pairs is questionable. We show, that neither a scenario where the hypernuclei are produced after capture of a stopped $\Xi^-$ by a $^9$Be nucleus nor interactions of energetic $\Xi^-$ with $^9$Be nuclei in the target material can produce a sufficient amount of such twin pairs. We have therefore explored the conjecture of Avraham Gal that decays of the $_{\Lambda\Lambda}^{4}$n may be responsible for the observed structure. Indeed, the inclusion of $_{\Lambda\Lambda}^{4}$n with a two-body $\pi^-$ branching ratio of 50 % in a statistical multifragmentation model allows to describe the E906 data remarkably well. On the other hand, a bound $^{3}_{\Lambda}$n nucleus would cause a striking structure in the momentum correlation matrix which is clearly inconsistent with the observation of E906.

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