The first evidence of a deeply bound state of Xi--14N system

We have observed a deeply bound state of the |Ξ ⁻|–|¹⁴ N| system that decayed into twin single-hypernuclei in nuclear emulsion exposed in the E373 experiment at KEK-PS. The process is uniquely identified as |Ξ ⁻ + ¹⁴ N → ¹⁰Λ Be + ⁵Λ He|⁠. We have measured the binding energy of the |Ξ ⁻|–|¹⁴ N| system, |B_Ξ ⁻|⁠, to be |4.38 ± 0.25|MeV, which is significantly larger than that of the |Ξ ⁻|–|¹⁴ N| 3|D| atomic state (0.17 MeV), if both single-hypernuclei are emitted in the ground state from at-rest capture of a |Ξ ⁻| hyperon. If the |¹⁰Λ Be| nucleus is produced in an excited state, the |B_Ξ ⁻| value mentioned above decreases by the excitation energy. Model calculations based on known values for |⁹ Be| excited states have predicted two excited states in the bound region. Even in the case of |¹⁰Λ Be| production in the highest excited state, the |B_Ξ ⁻| value is far from the 3|D| atomic level of the |Ξ ⁻|–|¹⁴ N| system by more than 3.7 standard deviations. The event provides the first clear evidence of a deeply bound state of the |Ξ ⁻|–|¹⁴ N| system by an attractive |Ξ|N interaction.