We have measured the lifetimes of the weak decay of ${}_{\Lambda }^{12}\mathrm{C},$ ${}_{\Lambda }^{28}\mathrm{Si},$ and ${}_{\Lambda }^{56}\mathrm{Fe}$ hypernuclei which were produced via a $\left({\pi }^{+}{,K}^{+}\right)$ reaction at KEK 12-GeV PS. The lifetimes were obtained by direct measurement of the production and decay time of the hypernucleus using a fast timing counter system whose time resolution was about 83 ps. Our results for the lifetimes of ${}_{\Lambda }^{12}\mathrm{C},$ ${}_{\Lambda }^{28}\mathrm{Si},$ and ${}_{\Lambda }\mathrm{Fe}$ are $231\pm 15,$ $206\pm 11,$ and $215\pm 14\hspace{0.5em}\mathrm{ps},$ respectively. The lifetimes of ${}_{\Lambda }^{11}\mathrm{B}$ and ${}_{\Lambda }^{27}\mathrm{Al}$ with the application of the gate to the $p_{\Lambda }$ state of the formation spectra of ${}_{\Lambda }^{12}\mathrm{C}$ and ${}_{\Lambda }^{28}\mathrm{Si}$ were also determined to be $211\pm 13$ and $203\pm 10\hspace{0.5em}\mathrm{ps}.$ From the present work, it was found that the total decay width saturates according to the hypernuclear mass number even at $A\sim 12.$ This suggests the importance of the proper treatment of the short range nature of the nonmesonic decay of $\Lambda$ hypernuclei.

• Received 10 December 1999

DOI:https://doi.org/10.1103/PhysRevC.61.054004