Search for diffractive dissociation of a long-livedHdibaryon

We have searched for long-lived $H$ dibaryons (six-quark $\mathrm{uuddss}$ states) in a neutral beam produced by 24.1 GeV/c $p$-Pt collisions. The signature was exclusive ${\Lambda }^0{\Lambda }^0$ production from diffractive dissociation of $H\text{'}\mathrm{s}$ striking a plastic scintillator. We observed 40 ${\Lambda }^0{\Lambda }^0$ events with a background of 3.2 events, but see no evidence of $H$ dissociation. Using our additional observation of 187±39 ${\Lambda }^0{K}_S^0$ events produced by coherent diffractive dissociation of neutrons from carbon for normalization, we place an upper limit of 1 mb/sr on the production of $H\text{'}\mathrm{s}$ with lifetimes ≳${10}^{-8\mathrm{}}$ s.