Search for the neutron-rich hypernucleus ${}_{\Lambda }^9$He

Search for the neutron-rich hypernucleus ${}_{\Lambda }^9\mathrm{He}$ is reported by the FINUDA experiment at DA$\Phi$NE, INFN-LNF, studying $({\pi }^{+},{\pi }^{-})$ pairs in coincidence from the $K_{\mathrm{stop}}^{-}+{}^9\mathrm{Be}\to {}_{\Lambda }^9\mathrm{He}+{\pi }^{+}$ production reaction followed by ${}_{\Lambda }^9\mathrm{He}\to {}^9\mathrm{Li}+{\pi }^{-}$ weak decay. An upper limit of the production rate of ${}_{\Lambda }^9\mathrm{He}$ undergoing this two-body ${\pi }^{-}$ decay is determined to be $(2.3\pm 1.9)\times {10}^{-6}/K_{\mathrm{stop}}^{-}$ at the 90$\%$ confidence level.

Figure 1

Anticipated ${}_{\Lambda }^9$He energy level scheme below the lowest neutron emission threshold, together with higher neutron emission thresholds. Note the schematically marked ${}_{\Lambda }^9$He excited doublet that is based on ${}^8$He (particle-unstable) first excitation $2^{+}$ at $\approx$3.1 MeV [4].

Figure 2

${\pi }^{+}$ momentum vs ${\pi }^{-}$ momentum for ${}^9$Be target events with $T_{\mathrm{sum}}=(194.5$–197.5) MeV. The shaded (red) rectangle indicates the position of events with $p_{{\pi }^{+}}=(253.5$–259) MeV/$c$ and $p_{{\pi }^{-}}=(114.5$–122) MeV/$c$.

Figure 3

${\pi }^{+}$ momentum (a) and ${\pi }^{-}$ momentum (b) distributions for ${}^9$Be target events with $T_{\mathrm{sum}}=(194.5$–197.5) MeV. The shaded (red) rectangles highlight pion momenta $p_{{\pi }^{+}}=(253.5$–259) MeV/$c$ and $p_{{\pi }^{-}}=(114.5$–122) MeV/$c$, corresponding to $B_{\Lambda }\left({}_{\Lambda }^9\mathrm{He}\right)=5$–10 MeV.