Evidence of Soft Dipole Resonance in ${}^{11}\mathrm{Li}$ with Isoscalar Character

The first conclusive evidence of a dipole resonance in ${}^{11}\mathrm{Li}$ having isoscalar character observed from inelastic scattering with a novel solid deuteron target is reported. The experiment was performed at the newly commissioned IRIS facility at TRIUMF. The results show a resonance peak at an excitation energy of $1.03\pm 0.03\mathrm{MeV}$ with a width of $0.51\pm 0.11\mathrm{MeV}$ (FWHM). The angular distribution is consistent with a dipole excitation in the distorted-wave Born approximation framework. The observed resonance energy together with shell model calculations show the first signature that the monopole tensor interaction is important in ${}^{11}\mathrm{Li}$. The first ab initio calculations in the coupled cluster framework are also presented.

Figure 1

(a) A schematic layout of the experiment. (b) Particle identification (PID) spectrum of light ejectiles using the Si(YY1)$(\mathrm{\Delta }E)$ silicon array and the stopping ($E$) CsI(Tl) array. (c) PID spectrum of heavy reaction residues in coincidence with deuterons using the $\mathrm{S}3\mathrm{d}1(\mathrm{\Delta }E)$ and the $\mathrm{S}3\mathrm{d}2$, stopping ($E$) silicon arrays.

Figure 2

Elastic scattering angular distribution for $d({}^{11}\mathrm{Li},d)$ at beam energy 55.3 MeV. The cross section with filled (blue) squares [open (blue) triangles] is from deuterons detected in the Si(YY1)-CsI(Tl) array by using efficiencies from simulation (elastic) data. The efficiency of the Si(YY1) array is shown in the inset, where filled squares are from simulation and open triangles are from elastic data. The cross section with filled red circles is from ${}^{11}\mathrm{Li}$ detected in the $\mathrm{S}3$ detector array. The curve shows distorted wave Born approximation (DWBA) predictions.

Figure 3

(a) Inelastic scattering excitation energy spectrum for deuterons in coincidence with ${}^9\mathrm{Li}$. Inset: Curve shows fitting with Gaussian plus exponential function. (b) The inelastic scattering angular distribution data for the resonance peak at $E_{ex}=1.03\mathrm{MeV}$. The curves are DWBA calculations for $L=0$ (pink dashed line), $L=1$ (red solid line), and $L=2$ (blue dotted line).

Figure 4

The experimental excitation energy compared with different theoretical model predictions: (1) experimental data; shell model with (2) SFO and (3) SFO-tls interactions; (4) coupled cluster; (5)–(7) three-body model with $2s_{1/2}$, 10%, 30%, and 50%, respectively; (8) Ref. [24] with ${}^{10}\mathrm{Li}(2^{-})$; (9) Ref. [24] with ${}^{10}\mathrm{Li}(1^{-})$; (10) Ref. [25]; and (11) Ref. [4]. The red (squares), blue (circles), and green (triangles) lines represent states with spin $3/2^{+}$, $5/2^{+}$, and $1/2^{+}$, respectively.