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

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 ${}_{\Lambda }^3$H+${}_{\Lambda }^4$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 ${}_{\Lambda }^3$n nucleus would cause a striking structure in the momentum correlation matrix which is clearly inconsistent with the observation of E906.

Comments: 4 figures