Search for a promising tetraquark candidate $X(ud\bar{s}\bar{s})$ in   $pn\to \Lambda\Lambda X$
  We propose to search for a tetraquark candidate $X(ud\bar{s}\bar{s})$ in $pn\to \Lambda\Lambda X(ud\bar{s}\bar{s})\to \Lambda\Lambda K^+K^0$ or $\Lambda\Lambda KK^*$. The existence of tetraquark state $X(ud\bar{s}\bar{s})$ with $J^P=0^+$, $1^-$ or $1^+$ was predicted in the literature based on specific diquark effective degrees of freedom inside hadrons. In order to understand the underlying dynamics for exotic hadrons, a search for the tetraquark $X(ud\bar{s}\bar{s})$ is strongly recommended. The proposed reaction involves two $\Lambda$ production, of which the narrow widths make it a great advantage in the analysis of the final state missing mass spectrum. We make an estimate of the production rate of $X(ud\bar{s}\bar{s})$ in an effective Lagrangian theory and find that for $J^P=1^-$ the sample events of $\sim 2200 \ nb^{-1}$ will be able to identify $X(ud\bar{s}\bar{s})$ with five standard deviations at a width of 10 MeV to $K^+K^0$ near threshold. For $J^P=1^+$ with a width of 20 MeV to $KK^*$, the sample events of $\sim 130 nb^{-1}$ will be needed. Large production cross sections are expected in a kinematic region beyond the threshold. We emphasize the advantage of low background in this transition channel, and in the meantime caution the large uncertainties in the present estimate due to lack of knowledge about the $X(ud\bar{s}\bar{s})$ state. Implications for its heavy-flavored partners $qq\bar{c}\bar{c}$ and $qq\bar{b}\bar{b}$ are briefly discussed. 
Xiao-Hai Liu and Qiang Zhao