First Determination of the Level Structure of an sd -Shell Hypernucleus, FΛ19

Abstract

We report on the first observation of γ rays emitted from an sd-shell hypernucleus, _{Λ}^{19}F. The energy spacing between the ground state doublet, 1/2^{+} and 3/2^{+} states, of _{Λ}^{19}F is determined to be 315.5±0.4(stat)_{-0.5}^{+0.6}(syst) keV by measuring the γ-ray energy of the M1(3/2^{+}→1/2^{+}) transition. In addition, three γ-ray peaks are observed and assigned as E2(5/2^{+}→1/2^{+}), E1(1/2^{-}→1/2^{+}), and E1(1/2^{-}→3/2^{+}) transitions. The excitation energies of the 5/2^{+} and 1/2^{-} states are determined to be 895.2±0.3(stat)±0.5(syst) and 1265.6±1.2(stat)_{-0.5}^{+0.7}(syst) keV, respectively. It is found that the ground state doublet spacing is well described by theoretical models based on existing s- and p-shell hypernuclear data.

Strangeness in nuclei and neutron stars

“…36 evidences that the measured energy spacing of 315 ± 0.4 (stat) +0.6 −0.5 (syst) keV is primarly determined by the ΛN spin-spin interaction. This energy line is reproduced to better than 20% by shell-model analyses that use its fitted strength in the p-shell [266]. This new measurement is of significant interest to test the theoretical framework for heavier hypernuclei.…”

“…This effect deals with the invariance of the strong interaction to a rotation in the isospin space (substituting protons with neutrons), and it is almost completely maintained in standard nuclei (70 keV difference between 3 H and 3 He). Figure 36: (Color online) γ-ray spectra obtained from the 19 F (K − , π − ) at at a beam momentum of 1.8 GeV/c, and level diagram for the 19 Λ F with the assigned γ ray transitions [266].…”

“…More recently, measurements of the γ-ray spectra obtained in 19 F (K − , π − ) at a beam momentum of 1.8 GeV/c allowed to pin down, for the first time, sd hypernuclei 4 [266], that can extend the study of the ΛN spin-spin interactions to a wide A-range of hypernuclei. Figure 36 shows the γ ray spectrum obtained gating the Λ binding energy with a selection of the missing mass for the 19 F (K − , π − ) reaction.…”

“…Figure 36: (Color online) γ-ray spectra obtained from the 19 F (K − , π − ) at at a beam momentum of 1.8GeV/c, and level diagram for the19 Λ F with the assigned γ ray transitions[266].…”

Simple model for a study of Λ hypernuclei in medium and heavy mass regions

“…Recently, theoretical and experimental studies on F 19 L have created opportunities for the analysis of sd-shell Λ hypernuclei [10,11]. The DWIA (Distorted-Wave Impulse Approximation) cross sections of 19…”