Search for evidence of ${}_{\Lambda }^{3}n$ by observing $d+{\pi }^{-}$ and $t+{\pi }^{-}$ final states in the reaction of ${}^6$Li+${}^{12}$C at $2A$ GeV

The experimental data obtained from the reaction of ${}^6$Li projectiles at 2$A$ GeV on a fixed graphite target were analyzed to study the invariant mass distributions of $d+{\pi }^{-}$ and $t+{\pi }^{-}$. Indications of a signal in the $d+{\pi }^{-}$ and $t+{\pi }^{-}$ invariant mass distributions were observed with significances of 5.3 $\sigma$ and 5.0 $\sigma$, respectively, when including the production target, and 3.7 $\sigma$ and 5.2 $\sigma$, respectively, when excluding the target. The estimated mean values of the invariant mass for $d+{\pi }^{-}$ and $t+{\pi }^{-}$ signal were 2059.3 $\pm$ 1.3 $\pm$ 1.7 MeV/c${}^2$ and 2993.7 $\pm$ 1.3 $\pm$ 0.6 MeV/c${}^2$ respectively. The lifetime estimation of the possible bound states yielding to $d+{\pi }^{-}$ and $t+{\pi }^{-}$ final states were deduced to be as ${181}_{-24}^{+30}\pm 25$ ps and ${190}_{-35}^{+47}\pm 36$ ps, respectively. Those final states may be interpreted as the two-body and three-body decay modes of a neutral bound state of two neutrons and a $\Lambda$ hyperon, ${}_{\Lambda }^{3}n$.

Figure 1

Invariant mass distributions of $d+{\pi }^{-}$ final-state candidate in panels (a1) and (a2) and of $t+{\pi }^{-}$ in panels (b1) and (b2). Panels (a1) and (b1) are for $-$10 cm $<Z<$ 30 cm, and panels (a2) and (b2) are for $-$2 cm $<Z<$ 30 cm. Observed distributions are represented by the filled-in circles. The hashed orange (gray) region represents one standard deviation of the fitted model centered at the solid blue (gray) line of the total best fit. The black and colored dotted lines respectively show the separate contributions of the signal and the background. The open triangle represents the data corresponding to invariant mass distribution of the mixed event analysis.

Figure 2

Profiled likelihood ratio for interval estimation of the lifetime of $d+{\pi }^{-}$ (a1) and $t+{\pi }^{-}$ (b1) final states. Interval estimation for one standard deviation is shown on each profiled likelihood ratio. Binned decay length distributions of the signal region of $d+{\pi }^{-}$ (a2) and $t+{\pi }^{-}$ (b2) are shown with the fitted model. The model includes the exponential function that resulted from the unbinned maximum likelihood fit and the background contribution estimated by the sidebands. The fitted model is represented by the black line, while the contribution of the exponential function is represented by the dotted blue (gray) line.

Figure 3

Invariant mass distribution for (a) $d+{\pi }^{-}$ and (b) $t+{\pi }^{-}$ from the geant4 Monte Carlo simulation data set, with the events produced by UrQMD represented by black symbols. Red (gray) crosses represent distributions with the mixed event analysis. The ratios between the both two distributions are shown in the bottom panel.