Case for quasinuclearN¯Nbound states

Recent experimental results on nucleon-antinucleon annihilation reactions of the type N¯N→πX have provided evidence for a new mesonic state ${\mathit{X}}_2$ with quantum numbers ${\mathit{J}}^{\mathrm{\pi }\mathit{C}}$(${\mathit{I}}^{\mathit{G}}$)=$2^{++}$($0^{+}$) which lies outside of the usual quark-antiquark (QQ¯) tensor meson nonet of flavor SU(3). We show that the observed production rates of ${\mathit{X}}_2$ from atomic p¯p states of orbital angular momentum L=0,1, as well as the decay characteristics of ${\mathit{X}}_2$, namely, its preference for ρρ over ππ decay and the absence of an observable K¯K mode, are consistent with an interpretation of ${\mathit{X}}_2$ as a quasinuclear bound state of the N¯N potential. The principal binding mechanism for such states is provided by the coherent tensor forces for isospin I=0 which arise from meson exchanges. We review the evidence for the $0^{++}$($0^{+}$) and $1^{\mathrm{-}\mathrm{-}}$($0^{\mathrm{-}}$) states predicted in this model and apply the ${}_{}{}^3{}_{}{}^{}$ ${\mathit{P}}_0$ strong-coupling quark model to estimate the decay branching ratios for a variety of N¯N bound states. Key experimental tests of our interpretation are discussed.