An experiment demonstrating the production of double- $\Lambda$ hypernuclei in $\left(K^{-}{,K}^{+}\right)$ reactions on ${}^9\mathrm{Be}$ was carried out at the D6 line in the BNL alternating-gradient synchrotron. The technique was the observation of pions produced in sequential mesonic weak decay, each pion associated with one unit of strangeness change. The results indicate the production of a significant number of the double hypernucleus ${}_{\Lambda \Lambda }^4\mathrm{H}$ and the twin hypernuclei ${}_{\Lambda }^4\mathrm{H}$ and ${}_{\Lambda }^3\mathrm{H}$. The relevant decay chains are discussed and a simple model of the production mechanism is presented. An implication of this experiment is that the existence of an $S=-2\mathrm{}$ dibaryon more than a few MeV below the $\Lambda \Lambda$ mass is unlikely.

• Received 14 May 2001

DOI:https://doi.org/10.1103/PhysRevLett.87.132504