Has J-PARC E07 observed a $\Xi^-_{1s}$ nuclear state?

Abstract

The Strangeness ${\cal S}=-2$ J-PARC E07 emulsion experiment published recently two new $\Xi^-$--$^{14}$N capture events, IRRAWADDY and IBUKI, identified by observing weak-decay sequences of pairs of single-$\Lambda$ hypernuclei and interpreted as $\Xi^{-}_{1s}$ and $\Xi^{-}_{1p}$ nuclear bound states, respectively, with binding energies $B_{\Xi^-}^{1s}$=6.27$\pm$0.27 MeV and $B_{\Xi^-}^{1p}$=1.27$\pm$0.21 MeV. $\Xi^-$ capture events in emulsion play a major role in determining the $\Xi$-nuclear and the $\Lambda\Lambda$ potential strengths. Here we question the assignment of a $\Xi^{-}_{1s}$ nuclear bound state to IRRAWADDY and offer an alternative assignment as a near-threshold $\Xi^0_{1p}$--$^{14}$C nuclear bound state, slightly admixed with a $\Xi^{-}_{1p}$--$^{14}$N nuclear bound state component corresponding to IBUKI. We also question using IBUKI's $B_{\Xi^-}^{1p}$ value as is to determine the $\Xi$-nuclear potential depth. Altogether, $\Xi^-$ capture events in $^{12}$C and $^{16}$O should prove less ambiguous than in $^{14}$N.