Search Results (10 total)

The bound-state level structures of the _Λ¹⁶O and _Λ¹⁵N hypernuclei were studied by γ-ray spectroscopy using a germanium detector array (Hyperball) via the ¹⁶O (K^-,π^-γ) reaction. A level scheme for _Λ¹⁶O was determined from the observation of three γ-ray transitions from the doublet of states (2^-,1^-) at ~6.7 MeV to the ground-state doublet (1^-,0^-). The _Λ¹⁵N hypernuclei were produced via proton emission from unbound states in _Λ¹⁶O. Three γ rays were observed, and the lifetime of the 1/2⁺;1 state in _Λ¹⁵N was measured by the Doppler shift attenuation method. By comparing the experimental results with shell-model calculations, the spin dependence of the ΛN interaction is discussed. In particular, the measured _Λ¹⁶O ground-state doublet spacing of 26.4±1.6±0.5 keV determines a small but nonzero strength of the ΛN tensor interaction.

The H-dibaryon resonance was sought by a ΛΛ invariant mass spectrum that was obtained by the ¹²C (K^-,K⁺ΛΛX) reactions. We observed a bump near the ΛΛ threshold, as reported by the previous experiment (KEK E224), with better statistics. Data were compared with results of a cascade model calculation including ΛΛ final state interactions consistent with the newly measured binding energy of _ΛΛ⁶He. No significant enhancements above levels of the model predictions were observed. The resulting upper limit for the production cross section of the H with a mass range between the ΛΛ and ΞN threshold is found to be 2.1 ± 0.6 (stat.) ± 0.1 (syst.) μb/sr at a 90% confidence level.

In a γ-ray spectroscopy experiment with the (K^-,π^-) reaction on a ¹⁰B target, we have observed the 7/2⁺→5/2⁺ spin-flip M1 transition in _Λ⁷Li via γ-γ coincidence with the 5/2⁺→1/2⁺E2 transition. This is the first successful γ-γ coincidence measurement for hypernuclei. The measured M1 γ-ray energy is 470.8±1.9(stat)±0.6(syst) keV. The spacing of the 7/2⁺,5/2⁺ doublet is consistently explained using the strengths of the spin-spin, Λ-spin-orbit, and tensor interactions obtained from previous γ-spectroscopy experiments.

We have observed two γ-ray transitions in _Λ¹⁶O from the 6.6 MeV excited 1₂^- state to both ground-state spin-doublet members (1₁^-,0^-) by the (K^-,π^-γ) reaction. We have obtained the ground-state doublet spacing to be 26.4±1.6(stat)±0.5(syst)  keV and the excitation energy of the 1₂^- state to be 6561.7±1.1(stat)±1.7(syst)  keV. The ground-state doublet spacing provides a small but nonzero strength of the tensor interaction between a Λ and a nucleon. This is the first experimental result on the ΛN tensor interaction.

The _Λ¹³C hypernucleus was studied by measuring γ rays in coincidence with the ¹³C(K^-,π^-) reaction. γ rays from the 1/2^- and 3/2^- states, which are the partners of the spin-orbit doublet states with a predominant configuration of [¹²C_g.s.(0⁺)⊗p_Λ], to the ground state were measured. The splitting of the states was found to be ΔE(1/2^--3/2^-)=+152±54(stat)±36(syst) keV. This value is 20–30 times smaller than that of single particle states in nuclei around this mass region. The j_Λ=l_Λ-1/2[(p_1/2)_Λ] state appeared higher in energy, as in normal nuclei. The value gives new insight into the YN interaction. The excitation energies of the 1/2^- and 3/2^- states were obtained as 10.982±0.031(stat)±0.056(syst) and 10.830±0.031(stat)±0.056(syst) MeV, respectively. We also observed γ rays from the 3/2⁺ state, which has a [¹²C(2⁺)⊗s_Λ] configuration, to the ground state in _Λ¹³C. The excitation energy of the 3/2⁺ state was obtained as 4.880±0.010(stat)±0.017(syst) MeV. Nuclear γ rays with energies of 4.438 and 15.100 MeV had similar yields, which suggests that a quasifree knockout of a Λ particle is dominant in highly excited regions.

With a germanium detector array (Hyperball), we observed two γ-ray peaks corresponding to the two transitions ( 5/2⁺→1/2⁺ and 3/2⁺→1/2⁺) in the _Λ⁹Be hypernucleus which was produced by the ⁹Be(K^-,π^-) reaction. The energies of the γ rays are 3029±2±1 keV and 3060±2±1 keV. The energy difference was measured to be 31.4_-3.6^+2.5 keV, which indicates a very small Λ-spin-dependent spin-orbit force between a Λ and a nucleon. This is the smallest level splitting by far ever measured in a hypernucleus.

A double-hyperfragment event has been found in a hybrid-emulsion experiment. It is identified uniquely as the sequential decay of _ΛΛ ⁶He emitted from a Ξ^- hyperon nuclear capture at rest. The mass of _ΛΛ ⁶He and the Λ-Λ interaction energy ΔB_ΛΛ have been measured for the first time devoid of the ambiguities due to the possibilities of excited states. The value of ΔB_ΛΛ is 1.01±0.20_-0.11^+0.18  MeV. This demonstrates that the Λ-Λ interaction is weakly attractive.

An experiment demonstrating the production of double- Λ hypernuclei in (K^-,K⁺) reactions on ⁹Be 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 _ΛΛ⁴H and the twin hypernuclei _Λ⁴H and _Λ³H. 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 dibaryon more than a few MeV below the ΛΛ mass is unlikely.

We report on the first measurement of a hypernuclear γ-transition probability. γ rays emitted in the E2(5/2⁺→1/2⁺) transition of _Λ⁷Li were detected by a large-acceptance germanium detector array (Hyperball), and the lifetime of the parent state ( 5/2⁺) was determined by the Doppler shift attenuation method. The obtained result, 5.8_-0.7^+0.9±0.7 ps, was then converted into the reduced transition probability [ B(E2)] to be B(E2;5/2⁺→1/2⁺)  =  3.6±0.5_-0.4^+0.5 e² fm⁴. Compared with the B(E2) of the corresponding E2(3⁺→1⁺) transition in the ⁶Li nucleus, our result gives evidence that the size of the ⁶Li core in _Λ⁷Li is smaller than the ⁶Li nucleus in the free space.

Using a large-acceptance germanium detector array (Hyperball), we have observed a spin-flip M1 γ transition between the ground-state spin doublet of _Λ⁷Li ( 3 / 2⁺→1 / 2⁺). The observed energy of 691.7±0.6(stat)±1.0(syst) keV provides crucial information on the strength of the spin-spin interaction between a Λ and a nucleon. This is the first observation of well-identified hypernuclear γ transitions using germanium detectors.