M. S. Chung

Search for the H dibaryon in (K−, K+) reaction with scintillating fiber active target

Abstract We have studied ( K − , K + ) reactions at P K − = 1.66 GeV/c by using a newly developed active target made of plastic scintillating fibers. The H dibaryon ( S = −2, I ( J π ) = 0(0 + )) has been searched for through the direct process, K − + C → K + + H + X , by the analysis of the K + momentum spectrum with the help of visual data of the scintillating fiber target. No evidence for the H production was observed. Upper limits for the differential cross section for the H production in the forward direction of the K + is 0.04–0.6 μb/sr at the 90% confidence level for the H mass range from 1850 to 2215 MeV/c 2 .

Measurement of the Ξ−p scattering cross sections at low energy

Abstract In this Letter we report on the cross-section measurements for Ξ − p elastic and inelastic scatterings at low energy using an active scintillating fiber target. Upper limit for the total cross section of the elastic scattering was found to be 24 mb at 90% confidence level, and the total cross section for the Ξ − p → Λ Λ reaction was found to be 4.3 −2.7 +6.3 xa0mb . We compare these results with currently competing theoretical estimates.

Enhanced ΛΛ production near threshold in the 12C(K−,K+) reaction

Abstract Enhanced production of ΛΛ pairs, above levels predicted by a two-step process model, has been observed near threshold (in the mass range 2.23-2.26 GeV/c2) in the 12C(K−,K+) reaction at 1.66 GeV/c using a scintillating fiber target. The differential cross section for ΛΛ production in the momentum region 0.95≤pK+≤1.3 GeV/c averaged over the range 2.3o≤θK+≤14.7o was found to be 7.6±1.3 ±0.9 μ b/sr, and that for the enhancement was found to be approximately 3 μ b/sr.

Production of twin λ-hypernuclei from ξ- hyperon capture at rest

Abstract A hybrid emulsion experiment was carried out to study double-strangeness nuclei produced via Ξ− hyperon capture at rest with the expectation of ten times larger statistics than previous experiments. We have analyzed 5% of the total emulsion and found one “twin-hypernuclei” event involving the emission of two single-Λ hypernuclei and a nuclear fragment from a Ξ− hyperon stopping point. The event is interpreted as the decay of a Ξ − + 14 N atomic system to 5 Λ He + 5 Λ He + 4 He + neutron . The species of the Ξ−-atom and the fragmentation products are uniquely identified for the first time for twin-hypernuclei events. Combined with the results from a past hybrid-emulsion experiment, one double-Λ hypernucleus and three twin-hypernuclei events have been found from Ξ− captures on light emulsion nuclei. The ratio of the detected rate of double-Λ hypernuclei to that of twin-hypernuclei is compared with theoretical estimates.

Scalar and vector meson production and two-step processes in the (K-, K+) reaction on 12C

Abstract The double strangeness exchange reaction ( K − , K + ) is investigated with respect to the sub-threshold production of scalar and vector mesons ( f 0 / a 0 / φ ) decaying into K + K − and the two-step processes induced by intermediate mesons and Ξ − hyperons at p k − = 1.66 GeV/ c using a scintillating fiber active target. The differential cross section ( 〈 dσ dΩ L 〉) averaged over the angular interval (2.3° ⩽ θ K + L ⩽ 14.7°) for the sub-threshold f 0 / a 0 / φ meson production with the K + K − decay is 11 ± 6 μ b/sr at 0.6 ⩽ p K 1 c . The present result differs significantly from the theoretical calculation which predicts the contribution of the f 0 / a 0 / φ meson production to be predominant in the ( K − , K + ) reaction below p K + = 0.95 GeV/ c . We found a sizable contribution from two-step ( K − , K + processes, characterized by production of two S = −1 hyperons, consistent with the result of the intra-nuclear cascade (INC) model calculation with respect to the meson-induced hyperon (or hyperon resonance) pair production in the momentum region 0.6 ⩽ p K + c . The observed enhancement of the cross section for the two-step ΛΛ production beyond the prediction of the INC model at p K + ⋍ 1.1 GeV /c could be due to the Ξ − p → ΛΛ reaction in 12 C.

Doubly strange nuclei by a hybrid-emulsion experiment E373 at KEK

A hybrid emulsion experiment E373 at KEK has been carried out to study doubly strange nuclear system. By the 8% data analysis of all, one twin single-Λ hypernuclei event and two events of double-Λ hypernucleus have been successfully detected. The twin single-Λ event is uniquely interpreted as Ⅺ−+14N→Λ5He+Λ5He+4He+n for the first time. In the first double-Λ hypernucleus event, “Demachi-Yanagi,” it is interpreted as Ⅺ−+12C→ΛΛ10Be* (or ΛΛ10Be*)+t. The second double-Λ hypernucleus event, “NAGARA,” shows a clearly recognized topology and the interpretation is unique as Ⅺ−+12C→ΛΛ6He+4He+t, ΛΛ6He→Λ5He+π−+p, Λ5He→p+d+2n etc. By the preliminary result of the ΛΛ6He, the attractive ΛΛ interaction has been confirmed with the ΔBΛΛ∼1u2009MeV.

Observation of double-hypernuclei and Λ-Λ interaction

Abstract A hybrid-emulsion experiment has been carried out at KEK-PS to study S = −2 nuclei. With the analysis of about 35% of total emulsion, we have successfully found three events of sequential weak decay of a double-hypernucleus. The results of these E373 events are summarized together with those of past experiments. The first experimental study of the branching ratios of weak decay modes of double-hypernuclei has been performed.

Search for H dibaryon by scintillating fiber track detector

The H dibaryon search experiment using a scintillating-fiber target was performed at KEK-PS. The preliminary results are reported. No evidence of the H production via C(K−, K+) reaction has been found. The upper limit of the ratio of the H production to the quasi-free process, K−+(p)→K++Ξ− is 0.05–0.6% forMH⩽2200 MeV/c2. For 2200 MeV/c2⩽MH⩽2230 MeV/c2, the upper limit for the production of the H which decays into Σ−+p is 0.6–07%.

Enhanced production of ΛΛ pairs near threshold in the (K-,K+) reaction on 12C

Abstract Enhanced production of ΛΛ pairs, above the prediction of a two-step process model, is observed near threshold (around the masses of 2.23 – 2.26 GeV/c2) in the12C(K−,K+) reaction at PK− = 1.66GeV/c using a scintillating fiber target. The differential cross section for the ΛΛ production averaged over 2.3° ≤