H-dibaryon and hypernucleus formation in the Ξ^-12C reaction at rest

Stopped-Ξ^-12C reactions have been investigated with respect to H-dibaryon and hypernucleus formation with a scintillating fiber detector. Out of 5.5×10³ tagged Ξ^- particles via the (K^-,K⁺) reaction we obtained 72 stopping Ξ^- events with 6.7^+1.1_-1.4 background events after imposing kinematic constraints. Hypernuclear production has been studied by observing free Λ decays in the stopped-Ξ^- reactions. From the observed Λ momentum spectrum, we have found the hypernuclear production rates to be 18±13, 65±6, and 17±12 % for double/twin-A hypernucleus, single-Λ hypernucleus, and no hypernucleus cases, respectively. With the same sample, we set upper limits on R (stopped-Ξ^-12C→HX /stopped-Ξ-¹²C→ all) for H decaying into Λn, Σ^-p, and Λpπ^-, for H masses just below ΛΛ mass threshold and for H lifetime range 10^-11≤τ_H≤10^-8 s. The upper limits on R are ∼0.1-0.2, assuming all of the H decay into each decay mode. Our results give a tight constraint on the existence of the bound H, in comparison with the theoretical estimates of Aerts and Dover, albeit with different target nuclei.