Lifetimes of Light Hyperfragments

Abstract

We have studied the lifetimes of light hyperfragments, making use of two stacks of nuclear-emulsion pellicles exposed to ${K}^{\ensuremath{-}}$-meson beams at the Bevatron and the Brookhaven A. G. S. In these two stacks we obtained 18 mesonic decays in flight from a total of 258 mesonic decays of hyperfragments. The values we found for the lifetimes are the following: For $_{\ensuremath{\Lambda}}\mathrm{He}^{4}$, $_{\ensuremath{\Lambda}}\mathrm{He}^{5}$, and $_{\ensuremath{\Lambda}}\mathrm{He}^{4,5}$ events combined (8 in flight, 117 at rest), we obtained the value $\ensuremath{\tau}(_{\ensuremath{\Lambda}}\mathrm{He}^{4,5})=({{2.2}_{\ensuremath{-}0.6}}^{+1.5})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ sec. By suitably apportioning the $_{\ensuremath{\Lambda}}\mathrm{He}^{4,5}$ events between $_{\ensuremath{\Lambda}}\mathrm{He}^{4}$ and $_{\ensuremath{\Lambda}}\mathrm{He}^{5}$ we obtain the values $\ensuremath{\tau}(_{\ensuremath{\Lambda}}\mathrm{He}^{5})=({{1.8}_{\ensuremath{-}0.6}}^{+1.5})\ifmmode\times\else\texttimes\fi{}{10}^{{}^{\ensuremath{-}10}}$ sec and $\ensuremath{\tau}(_{\ensuremath{\Lambda}}\mathrm{He}^{4})\ensuremath{\ge}1.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ sec. For $_{\ensuremath{\Lambda}}\mathrm{H}^{3}$, using only two-body decays (3 in flight, 4 at rest), we obtain $\ensuremath{\tau}(_{\ensuremath{\Lambda}}\mathrm{H}^{3})=({{0.8}_{\ensuremath{-}0.3}}^{+1.9})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ sec. Using both two-body and three-body decays (5 in flight, 18 at rest), we obtain $\ensuremath{\tau}(_{\ensuremath{\Lambda}}\mathrm{H}^{3})=({{3.4}_{\ensuremath{-}1.4}}^{+8.2})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ sec. This value may be an overestimate owing to $_{\ensuremath{\Lambda}}\mathrm{H}^{4}$ contamination in the three-body $_{\ensuremath{\Lambda}}\mathrm{H}^{3}$ events. This point is discussed in the text. For $_{\ensuremath{\Lambda}}\mathrm{H}^{4}$ we obtain $\ensuremath{\tau}(_{\ensuremath{\Lambda}}\mathrm{H}^{4})=({{2.4}_{\ensuremath{-}1.0}}^{+6.0})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ sec using only three-body decays (4 in flight, 14 at rest), and $\ensuremath{\tau}(_{\ensuremath{\Lambda}}\mathrm{H}^{4})=({{3.6}_{\ensuremath{-}1.3}}^{+4.9})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ sec using all $_{\ensuremath{\Lambda}}\mathrm{H}^{4}$ decays (5 in flight, 40 at rest). This last value may suffer from a bias against finding two-body $_{\ensuremath{\Lambda}}\mathrm{H}^{4}$ decays in flight which simulate small-angle scatterings under low magnifying power, although all scatterings were examined closely. The above results are compared with theoretical estimates of hyperfragment lifetimes.