Has the neutral double hypernucleus nΛΛ4 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 π − -π − momentum matrix at (p π−H ,p π−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 Λ H+ 4 Λ H pairs is questionable. We show, that neither a scenario where these singe-Λ hypernuclei are produced after capture of a stopped Ξ − by a 9 Be nucleus nor interactions of energetic Ξ − 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 4 ΛΛ n may be responsible for the observed structure. Indeed, the inclusion of 4 ΛΛ n with a two-body π − branching ratio of 50% in a statistical multifragmentation model allows to describe the E906 data remarkably well. On the other hand, a bound 3 Λ n nucleus would cause a striking structure in the momentum correlation matrix which is clearly inconsistent with the observation of E906.

The onset of ΛΛ hypernuclear binding

“…Indeed, a slightly bound I = 0 4 ΛΛ H(1 + ) was found in ΛΛpn four-body calculations by Nemura et al [10,11] but not in a four-body calculation by Filikhin and Gal [12] who nonetheless got it bound as a ΛΛd cluster. Unfortunately, the AGS-E906 counter experiment [13] searching for light ΛΛ hypernuclei failed to provide conclusive evidence for the particle stability of 4 ΛΛ H [14,15]. Interestingly, the neutral four-body system 4 ΛΛ n has been assigned in Ref.…”

Novel method for producing very-neutron-rich hypernuclei via charge-exchange reactions with heavy ion projectiles

“…Additionally, interest in neutral hypernuclei such as Λnn has grown. A recent reanalysis of the BNL-AGS E906 experiment revealed the possibility of the existence of a bound double-strangeness neutral hypernucleus, denoted as 4 ΛΛ n [9]. Additional information regarding neutral hypernuclei and resonance states with a hyperon (hereafter called 'hyper-resonance states'), including Λnn and states with more neutrons, is required to understand the equation of state in the interiors of neutron stars.…”

“…In projectile residues, there are two good nuclei 9 C and 12 N, which are bound only by their ground states. These residues can be produced by stable primary beams 9 Be and 12 C via double-and single-charge-exchange reactions, (i.e., ( 9 Be, 9 C), and ( 12 C, 12 N)). By combining these charge-exchange reactions with ΛK + production in the target, several protons in the target nucleus are converted into neutrons and a Λ-hyperon, such as pp( 12 C, 12 N K + )nΛ and ppp( 9 Be, 9 C K + )nnΛ.…”

“…Although the proposed reactions involve 9 Be and 12 C projectiles impinged on different target nuclei, this reaction was selected because clustering for producing fragments and hypernuclei has already been well studied and optimised in this reaction for the HypHI Phase 0 experiment, which will be discussed later.…”

Neutral baryonic systems with strangeness

“…[3][4][5]10 Bound states of two neutrons and two Λ hyperons are another controversial subject. Recently, Bleser et al 11 have offered a new interpretation of the results of the BNL AGS-E906 experiment to produce and study double hypernuclei through a (K − , K + ) reaction on 9 Be. 12 Following a suggestion made by Avraham Gal, they explored the conjecture that decays of a 4 ΛΛ n double hypernucleus may be responsible for some of the observed structures in the correlated π − − π − momenta.…”