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Register a new userObservation of Spin-Dependent Charge Symmetry Breaking in $Lambda N$ Interaction: Gamma-Ray Spectroscopy of $^4_{Lambda }$He
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The energy spacing between the ground-state spin doublet of $^4_Lambda $He(1$^+$,0$^+$) was determined to be $1406 pm 2 pm 2$ keV, by measuring $gamma$ rays for the $1^+ to 0^+$ transition with a high efficiency germanium detector array in coincidence with the $^4$He$(K^-,pi^-)$ $^4_Lambda $He reaction at J-PARC. In comparison to the corresponding energy spacing in the mirror hypernucleus $^4_Lambda $H, the present result clearly indicates the existence of charge symmetry breaking (CSB) in $Lambda N$ interaction. It is also found that the CSB effect is large in the $0^+$ ground state but is by one order of magnitude smaller in the $1^+$ excited state, demonstrating that the $Lambda N$ CSB interaction has spin dependence.
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Level structure of the $^{12}_{Lambda}$C hypernucleus was precisely determined by means of $gamma$-ray spectroscopy. We identified four $gamma$-ray transitions via the $^{12}$C$(pi^{+},K^{+}gamma)$ reaction using a germanium detector array, Hyperball2. The spacing of the ground-state doublet $(2^{-}_{1},1^{-}_{1})$ was measured to be $161.5pm0.3text{(stat)}pm0.3text{(syst)}$,keV from the direct $M1$ transition. Excitation energies of the $1^{-}_{2}$ and $1^{-}_{3}$ states were measured to be $2832pm3pm4$,keV and $6050pm8pm7$,keV, respectively. The obtained level energies provide definitive references for the reaction spectroscopy of $Lambda$ hypernuclei.
The missing mass spectroscopy of the $^{7}_{Lambda}$He hypernucleus was performed, using the $^{7}$Li$(e,e^{prime}K^{+})^{7}_{Lambda}$He reaction at the Thomas Jefferson National Accelerator Facility Hall C. The $Lambda$ binding energy of the ground state (1/2$^{+}$) was determined with a smaller error than that of the previous measurement, being $B_{Lambda}$ = 5.55 $pm$ 0.10(stat.) $pm$ 0.11(sys.) MeV. The experiment also provided new insight into charge symmetry breaking in p-shell hypernuclear systems. Finally, a peak at $B_{Lambda}$ = 3.65 $pm$ 0.20(stat.) $pm$ 0.11(sys.) MeV was observed and assigned as a mixture of 3/2$^{+}$ and 5/2$^{+}$ states, confirming the gluelike behavior of $Lambda$, which makes an unstable state in $^{6}$He stable against neutron emission.
Background: Models to calculate small isospin-symmetry-breaking effects in superallowed Fermi decays have been placed under scrutiny in recent years. A stringent test of these models is to measure transitions for which the correction is predicted to be large. The decay of 32Cl decay provides such a test case. Purpose: To improve the gamma yields following the beta decay of 32Cl and to determine the ft values of the the beta branches, particularly the one to the isobaric-analogue state in 32S. Method: Reaction-produced and recoil-spectrometer-separated 32Cl is collected in tape and transported to a counting location where beta-gamma coincidences are measured with a precisely-calibrated HPGe detector. Results: The precision on the gamma yields for most of the known beta branches has been improved by about an order of magnitude, and many new transitions have been observed. We have determined 32Cl-decay transition strengths extending up to E_x~11 MeV. The ft value for the decay to the isobaric-analogue state in 32S has been measured. A comparison to a shell-model calculation shows good agreement. CONCLUSIONS: We have experimentally determined the isospin-symmetry-breaking correction to the superallowed transition of this decay to be (delta_C-delta_NS)_exp=5.4(9)%, significantly larger than for any other known superallowed Fermi transition. This correction agrees with a shell-model calculation, which yields delta_C-delta_NS=4.8(5)%. Our results also provide a way to improve the measured ft values for the beta decay of 32Ar.
Three candidate events of the neutron-rich hypernucleus 6{Lambda}H were uniquely identified in the FINUDA experiment at DA{Phi}NE, Frascati, by observing {pi}+ mesons from the (K-stop,{pi}+) production reaction on 6Li targets, in coincidence with {pi}-mesons from 6{Lambda}H rightarrow 6He+{pi}- weak decay. Details of the experiment and the analysis of its data are reported, leading to an estimate of (2.9pm2.0)cdot10-6/K- stop for the 6{Lambda}H production rate times the two-body {pi}- weak decay branching ratio. The 6{Lambda}H binding energy with respect to 5H + {Lambda} was determined jointly from production and decay to be B{Lambda} = (4.0 pm 1.1) MeV, assuming that 5H is unbound with respect to 3H + 2n by 1.7 MeV. The binding energy determined from production is higher, in each one of the three events, than that determined from decay, with a difference of (0.98 pm 0.74) MeV here assigned to the 0+g.s. rightarrow 1+ excitation. The consequences of this assignment to {Lambda} hypernuclear dynamics are briefly discussed.
We study the central part of Lambda N and Lambda Lambda potential by considering the correlated and uncorrelated two-meson exchange besides the omega exchange contribution. The correlated two-meson is evaluated in a chiral unitary approach. We find that a short range repulsion is generated by the correlated two-meson potential which also produces an attraction in the intermediate distance region. The uncorrelated two-meson exchange produces a sizeable attraction in all cases which is counterbalanced by omega exchange contribution.
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