Evidence for Heavy Hyperhydrogen ${}_{\Lambda }{}^6\mathrm{H}$

Evidence for the neutron-rich hypernucleus ${}_{\Lambda }{}^6\mathrm{H}$ is presented from the FINUDA experiment at $\mathrm{DA}\Phi \mathrm{NE}$, Frascati, studying $({\pi }^{+},{\pi }^{-})$ pairs in coincidence from the $K_{\mathrm{stop}}^{-}+{}^6\mathrm{Li}\to {}_{\Lambda }{}^6\mathrm{H}+{\pi }^{+}$ production reaction followed by ${}_{\Lambda }{}^6\mathrm{H}\to {}^6\mathrm{He}+{\pi }^{-}$ weak decay. The production rate of ${}_{\Lambda }{}^6\mathrm{H}$ undergoing this two-body ${\pi }^{-}$ decay is determined to be $(2.9\pm 2.0)\times {10}^{-6}/K_{\mathrm{stop}}^{-}$. Its binding energy, evaluated jointly from production and decay, is $B_{\Lambda }({}_{\Lambda }{}^6\mathrm{H})=(4.0\pm 1.1)\mathrm{MeV}$ with respect to ${}^5\mathrm{H}+\Lambda$. A systematic difference of $(0.98\pm 0.74)\mathrm{MeV}$ between $B_{\Lambda }$ values derived separately from decay and from production is tentatively assigned to the ${}_{\Lambda }{}^6\mathrm{H}$ $0_{\mathrm{g}.\mathrm{s}.}^{+}\to 1^{+}$ excitation.

Figure 1

Distribution of raw total kinetic energy $T_{\mathrm{sum}}\equiv T({\pi }^{+})+T({\pi }^{-})$ for ${\pi }^{\pm }$ pair coincidence events from ${}^6\mathrm{Li}$ targets. The vertical (red) bar represents the cut $T_{\mathrm{sum}}=202–204\mathrm{MeV}$. The dashed (blue) histogram is a quasifree simulation of $K_{\mathrm{stop}}^{-}+{}^6\mathrm{Li}\to {\Sigma }^{+}+{}^4\mathrm{He}+n+{\pi }^{-};{\Sigma }^{+}\to n+{\pi }^{+}$ background, and the dotted (violet) histogram is a four-body phase space simulation of the same background. Their best fit to the data is shown by the solid (black) histogram; see the text.

Figure 2

${\pi }^{+}$ momentum vs ${\pi }^{-}$ momentum for ${}^6\mathrm{Li}$ target events with $T_{\mathrm{sum}}=202–204\mathrm{MeV}$ (LHS) and with $T_{\mathrm{sum}}=200–206\mathrm{MeV}$ (RHS). The shaded (red) rectangle on each side consists of a subset of events with $p_{{\pi }^{+}}=250–255\mathrm{MeV}/c$ and $p_{{\pi }^{-}}=130–137\mathrm{MeV}/c$.

Figure 3

${}_{\Lambda }{}^6\mathrm{H}$ mass (RHS) from three ${}_{\Lambda }{}^6\mathrm{H}$ candidate events, as related to several particle stability thresholds and theoretical predictions (LHS).