Experimental study of the reaction ${\Xi }^{0}n\to \Lambda \Lambda X$ using ${\Xi }^0$-nucleus scattering

Abstract

Using $(10087\pm 44)\times 10^6$$J/\psi$ events collected with the BESIII detector operating at the BEPCII storage ring in $2009$, $2012$, $2018$, and $2019$, we perform a search for the reaction ${\Xi }^{0}n\to \Lambda \Lambda X$, where $X$ denotes any additional final particles. Given the highly suppressed phase space for producing extra pions, the $X$ consists of either nothing or a photon, corresponding to the processes ${\Xi }^{0}n\to \Lambda \Lambda$ and ${\Xi }^{0}n\to \Lambda {\Sigma }^0\to \Lambda \Lambda \gamma$. The ${\Xi }^0$ comes from the decay of $J/\psi \to {\Xi }^0{\bar{\Xi }}^0$, while the neutron originates from material of the beam pipe. A signal is observed for the first time with a statistical significance of 6.4$\sigma$. The cross section for the reaction ${\Xi }^0+{}^9\mathrm{Be}\to \Lambda +\Lambda +X$ is measured to be $(43.6\pm 10.5_{\text{stat}}\pm 11.1_{\text{syst}})$ mb at $P_{{\Xi }^0}\approx 0.818$ GeV/$c$, where the first uncertainty is statistical and the second systematic. No significant $H$-dibaryon signal is observed in the $\Lambda \Lambda$ final state.

9 pages, 3 figures