Nonmesonic Decay of a Helium Hyperfragment

Abstract

In an emulsion stack exposed in an equatorial balloon flight, we have observed an event which is most simply interpreted as the disintegration of a ${\mathrm{He}}^{4*}$ hyperfragment according to the scheme ${\mathrm{He}}^{4*}\to p+d+n+Q$, where $Q=167.6\mathrm{}\pm 2.9$ Mev. The fragment, emerging from a star of type $11+7\alpha$, traveled 633 μ before coming to rest. All the measurements, including those on the fragment track, are consistent with the foregoing decay scheme. Other interpretations are considered and shown to be untenable or very unlikely. From the $Q$-value and the relevant masses, the binding energy of the ${\Lambda }^0$ in the fragment is 1.9±2.9 Mev, in agreement with values from mesonic disintegrations of ${\mathrm{He}}^{4*}$. This low binding energy may be compared to the much larger value, 20.6 Mev, with which the "last neutron" is bound in normal ${\mathrm{He}}^4$.