含手征三体力的第一性原理Gamow壳模型计算14O同中子素

Ab initio Gamow shell model with chiral three-nucleon force for 14O isotones

从手征有效场论N^(3)LO两体核力和N^(2)LO三体核力出发,利用第一性原理Gamow壳模型计算研究了质子滴线附近原子核15F~18Mg的基态和低激发态性质.通过计算1s1/2和0d5/2轨道的有效单粒子能与占据概率,分析了18Mg及其镜像核18C基态中的连续态耦合效应.同时考虑手征三体力、连续态耦合和多体关联,计算了18Mg及其镜像核18C 2^(+)态的能量,其镜像能差异相比实空间计算结果得到显著改善.计算给出18Mg 2^(+)激发态比20Mg 2^(+)态高这一现象,分析了其中的机理.基于实空间第一性原理壳模型,预言了20Si和22Si能谱中的2^(+)态位置,为今后实验提供了有用信息.

Proton-rich nuclei exhibit many novel phenomena, such as proton emission, proton halo, and Thomas-Ehrman shift. On one hand, due to the Coulomb force, outer protons have a lower nuclear separation threshold and are strongly coupled to the continuum. Such weakly bound nuclei are called the open quantum system. On the other hand, the Coulomb barrier restricts the spatial spread of the weakly-bound outer protons, resulting in that unbound resonance nuclei beyond the proton drip line may have certain lifetimes, e.g., a millisecond lifetime. The interplay of the continuum coupling and Coulomb forces produces a complex pattern in nuclei near the proton-rich drip line, which contains rich and unique structures and dynamic information. These properties lead to new insights into nuclear forces, symmetry breaking between mirror nuclei and many-body correlations.Recent experiment observed the ground state of18Mg via its decay into 4p+14O. The decay of the ground state of this nucleus is consistent with two sequential steps of 2p decay passing through the ground state of16Ne. And another excited state at 1.84 MeV was also observed, which may be the first 2+excited state. The excitation energy of this 2+state is significantly higher than that in18C, which shows a typical Thomas-Ehrman shift. In addition, this 2+excitation energy of18Mg exceeds that of20Mg. It is meaningful to study these novel phenomena through ab initio calculations. However,continuum coupling is important in these proton-rich nuclei, so the continuum degrees of freedom should be treated properly. Using the Berggren basis, the Gamow shell model provides a good description of the interplay between bound state, resonance, and continuum in a self-consistent many-body framework.In the present paper, we employ two-and three-nucleon forces from chiral effective theory, perform the Gamow shell model calculations of15–18C and15F–18Mg with continuum coupling included. The Gamow Hartree-Fock method with three-body force included was used to generate the Berggren basis, it can give a good description of the bound, resonant,and scattering states. The Hamiltonian is then normal ordered with respect to the Gamow Hartree-Fock reference state, and the normal ordered two-body approximation is adopted. We use the many-body perturbation theory to construct the effective Hamiltonian for the psd model space with s1/2and d5/2partial waves treated in Berggren basis. We calculate the S-box up to the third order and Q-box up to the second order. We use the Jacobi-Davidson method to diagonalize the complex-symmetric model-space Hamiltonian, and allow at most two valence particles in the continuum.The energies of the ground and first 2+excited states are calculated for15–18C and15F–18Mg. The calculated energies agree well with experimental data or evaluations. By calculating occupation probabilities and effective single-particle energies of the 1s1/2and 0d5/2orbitals, we analyze the continuum-coupling effect in the ground state of18Mg. The calculation of the mirror energy difference between the 2+states of18C and18Mg is improved with continuum coupling included. We have systematically calculated the first 2+states in Z=12 isotopes, supporting the experimental observation that the 2+excitation energy in18Mg is slightly higher than that in20Mg. There have been no experimental data for20Si and22Si. We predict that the 2+excitation energy in20Si is lower than that in22Si with the real-energy shell-model calculation.