Neutron-rich hypernuclei: Evidence for ⁶_λ H and search for⁹_λ He

The FINUDA experiment at DAφNE, Frascati, has found evidence for the neutron-rich hypernucleus ⁶_λ H studying (π⁺, π^-) pairs in coincidence from the K ^-_stop+⁶Li → ⁶ _λ H + π^{+ -}production reaction followed by ⁶_λ H → ⁶He + π^- weak decay. The production rate of ⁶ _λH undergoing this two-body π^- decay has been found to be (2.9 ± 2.0) · 10^-6/K^- _stop Its binding energy has been evaluated to be Bλ( ⁶_λH) = (4.0 ± 1.1) MeV with respect to ⁵H + λ, jointly from production and decay. A systematic difference of (0.98 ± 0.74) MeV between B_λ values derived separately from decay and from production has been tentatively assigned to the ⁶_λH 0⁺_g.s→1 ⁺ excitation. A similar investigation has been carried out for the neutron-rich hypernucleus ⁹_λHe studying the K ^-_stop + ⁹Be → ⁹ _λ He + π⁺ reaction in coincidence with the ⁹_λHe → ⁹Li + π^- weak decay; an upper limit for the production rate of ⁹ _λHe undergoing the two-body π^- decay has been found to be 4.2 ·10^-6/K^-_stop(90% C.L.).