| Results are presented for BNL-E813, an H-dibaryon search conducted at the Alternating Gradient Synchrotron (AGS) of Brookhaven National Lab using a 1.8-GeV/c K- beam line. About 100,000 xi - hyperons were produced by the p(K-, K +)xi- reaction on a liquid hydrogen target. A liquid deuterium vessel was situated above the hydrogen target so that some xi -'s would stop in the deuterium, form atomic states, then fuse with the deuteron to form the H-dibaryon through (xi-, d)atom → Hn if the H exists. Two efficient neutron detector arrays were present to detect the signature neutron of H production in (delayed) coincidence with the K + from xi- production. The energy of the hypothetical neutron from this two-body decay would unambiguously reveal the H-dibaryon mass. The technique is independent of the H-dibaryon lifetime and decay mode and covers a considerable range mass range, from an H deeply-bound by about 100 MeV with respect to the ΛΛ strong decay threshold to a unbound (resonant) H about 20 MeV above threshold.;The xi- hyperons most likely to stop in the LD 2 were tagged by their large energy deposition in silicon detectors located between the two cryogenic vessels. The neutron beta-1 spectrum of roughly 1200 tagged xi- events has been searched for a signature peak of H-dibaryon production. A two-parameter fit has been performed to determine the number of neutrons in a hypothetical peak at each point in the beta-1 spectrum. The H-branching fraction nuR, or fraction of xi- atoms decaying strongly to the H-dibaryon (as opposed to strong background such as ΛΛn and Sigma0 nn or weak decay) has been determined from the number of neutrons found in the fit for the corresponding H mass. As the fit values are close to the unphysical region of nuR < 0, an ordering parameter has been employed to construct 90% C.L. intervals of nuR to avoid unphysical upper limits, which are possible with conventional approaches.;This 1995 data analyzed here has been combined with that of two previous runs in 1992 and 1993. No statistically significant enhancement above background is seen in either the 1995 or the combined data set. Upper limits are set at the level of nuR ≈ 0.4 over the range of 2--10 in neutron beta-1 or 2150--2250 MeV/c 2 in H mass (100 MeV > BH > -20 MeV). A marginal enhancement, a non-zero 90% C.L. lower limit, in the region of low-binding, BH ≈ 5 MeV, may be conservatively attributed to background fluctuations. Upper limits are generally comparable to theoretical predictions. The experimental results for this low binding regime complement (K-, K +) measurements at the AGS on solid targets which are sensitive to a more deeply bound H-dibaryon. |
