First experimental evidence for heavy hyperhydrogen

The FINUDA experiment, carried out at the Frascati φ-factory DAΦNE, allowed to find the first experimental evidence for the existence of the neutron-rich hypernucleus ΛH by measuring in coincidence (π+,π−) pairs from the K− stop + Li → ΛH + π production reaction, followed by the ΛH → He + π− mesonic weak decay. The production rate of ΛH undergoing such a two-body π− decay turned out to be (2.9 ± 2.0) × 10−6/K− stop. A binding energy value BΛ( 6 ΛH) = (4.0 ± 1.1) MeV was evaluated with respect to the (H + Λ) threshold by combining the information from both production and decay processes. A systematic difference of (0.98 ± 0.74) MeV observed between BΛ values when derived separately from ΛH formation and from its decay was tentatively assigned to the ΛH 0 + g.s. → 1 excitation.