Coupled-cluster computations of atomic nuclei.

In the past decade, coupled-cluster theory has seen a renaissance in nuclear physics, with computations of neutron-rich and medium-mass nuclei. The method is efficient for nuclei with product-state references, and it describes many aspects of weakly bound and unbound nuclei. This report reviews the technical and conceptual developments of this method in nuclear physics, and the results of coupled-cluster calculations for nucleonic matter, and for exotic isotopes of helium, oxygen, calcium, and some of their neighbors.

Optimized chiral nucleon-nucleon interaction at next-to-next-to-leading order.

We optimize the nucleon-nucleon interaction from chiral effective field theory at next-to-next-to-leading order (NNLO). The resulting new chiral force NNLO(opt) yields χ(2)≈1 per degree of freedom for laboratory energies below approximately 125 MeV. In the A=3, 4 nucleon systems, the contributions of three-nucleon forces are smaller than for previous parametrizations of chiral interactions. We use NNLO(opt) to study properties of key nuclei and neutron matter, and we demonstrate that many aspects of nuclear structure can be understood in terms of this nucleon-nucleon interaction, without explicitly invoking three-nucleon forces.

Spectroscopy of 26F to probe proton-neutron forces close to the drip line.

A long-lived J(π) = 4(1)(+) isomer, T(1/2) = 2.2(1) ms, has been discovered at 643.4(1) keV in the weakly bound (9)(26)F nucleus. It was populated at Grand Accélérateur National d'Ions Lourds in the fragmentation of a (36)S beam. It decays by an internal transition to the J(π) = 1(1)(+) ground state [82(14)%], by ß decay to (26)Ne, or ß-delayed neutron emission to (25)Ne. From the ß-decay studies of the J(π) =1(1)(+) and J(π) = 4(1)(+) states, new excited states have been discovered in (25,26)Ne. Gathering the measured binding energies of the J(π) = 1(1)(+) -4(1)(+) multiplet in (9)(26)F, we find that the proton-neutron π0d(5/2)ν0d(3/2) effective force used in shell-model calculations should be reduced to properly account for the weak binding of (9)(26)F. Microscopic coupled cluster theory calculations using interactions derived from chiral effective field theory are in very good agreement with the energy of the low-lying 1(1)(+), 2(1)(+), 4(1)(+) states in (26)F. Including three-body forces and coupling to the continuum effects improve the agreement between experiment and theory as compared to the use of two-body forces only.

Continuum effects and three-nucleon forces in neutron-rich oxygen isotopes.

We employ interactions from chiral effective field theory and compute binding energies, excited states, and radii for isotopes of oxygen with the coupled-cluster method. Our calculation includes the effects of three-nucleon forces and of the particle continuum, both of which are important for the description of neutron-rich isotopes in the vicinity of the nucleus 24O. Our main results are the placement of the neutron drip line at 24O, the assignment of spins, parities and resonance widths for several low-lying states of the drip line nucleus, and an efficient approximation that incorporates the effects of three-body interactions.

Evolution of shell structure in neutron-rich calcium isotopes.

We employ interactions from chiral effective field theory and compute the binding energies and low-lying excitations of calcium isotopes with the coupled-cluster method. Effects of three-nucleon forces are included phenomenologically as in-medium two-nucleon interactions, and the coupling to the particle continuum is taken into account using a Berggren basis. The computed ground-state energies and the low-lying J(π) = 2+ states for the isotopes (42,48,50,52)Ca are in good agreement with data, and we predict the excitation energy of the first J(π) = 2+ state in (54)Ca at 1.9 MeV, displaying only a weak subshell closure. In the odd-mass nuclei (53,55,61)Ca we find that the positive parity states deviate strongly from the naive shell model.

Quenching of spectroscopic factors for proton removal in oxygen isotopes.

We present microscopic coupled-cluster calculations of the spectroscopic factors for proton removal from the closed-shell oxygen isotopes (14,16,22,24,28)O with a chiral nucleon-nucleon interaction at next-to-next-to-next-to-leading order. We include coupling-to-continuum degrees of freedom by using a Hartree-Fock basis built from a Woods-Saxon single-particle basis. This basis treats bound and continuum states on an equal footing. We find a significant quenching of spectroscopic factors in the neutron-rich oxygen isotopes, pointing to enhanced many-body correlations induced by strong coupling to the scattering continuum above the neutron emission thresholds.

Ab initio computation of the 17F proton halo state and resonances in A = 17 nuclei.

We perform coupled-cluster calculations of the energies and lifetimes of single-particle states around the doubly magic nucleus (16)O based on chiral nucleon-nucleon interactions at next-to-next-to-next-to-leading order. To incorporate effects from the scattering continuum, we employ a Gamow-Hartree-Fock basis. Our calculations for the J(pi)=1/2(+) proton halo state in (17)F and the 1/2(+) state in (17)O agree well with experiment, while the calculated spin-orbit splitting between 5/2(+) and 3/2(+) states is too small due to the lack of three-nucleon forces. Continuum effects yield a significant amount of additional binding energy for the 1/2(+) and 3/2(+) states in (17)O and (17)F.

Orbital dependent nucleonic pairing in the lightest known isotopes of tin.

By studying the (109)Xe→(105)Te→(101)Sn superallowed α-decay chain, we observe low-lying states in (101)Sn, the one-neutron system outside doubly magic (100)Sn. We find that the spins of the ground state (J=7/2) and first excited state (J=5/2) in (101)Sn are reversed with respect to the traditional level ordering postulated for (103)Sn and the heavier tin isotopes. Through simple arguments and state-of-the-art shell-model calculations we explain this unexpected switch in terms of a transition from the single-particle regime to the collective mode in which orbital-dependent pairing correlations dominate.

Medium-mass nuclei from chiral nucleon-nucleon interactions.

We compute the binding energies, radii, and densities for selected medium-mass nuclei within coupled-cluster theory and employ a bare chiral nucleon-nucleon interaction at next-to-next-to-next-to-leading order. We find rather well-converged results in model spaces consisting of 15 oscillator shells, and the doubly magic nuclei 40Ca, 48Ca, and the exotic 48Ni are underbound by about 1 MeV per nucleon within the coupled-cluster singles-doubles approximation. The binding-energy difference between the mirror nuclei 48Ca and 48Ni is close to theoretical mass table evaluations. Our computation of the one-body density matrices and the corresponding natural orbitals and occupation numbers provides a first step to a microscopic foundation of the nuclear shell model.

0(gs)+ -->2(1)+ transition strengths in 106Sn and 108Sn.

The reduced transition probabilities, B(E2; 0(gs)+ -->2(1)+), have been measured in the radioactive isotopes (108,106)Sn using subbarrier Coulomb excitation at the REX-ISOLDE facility at CERN. Deexcitation gamma rays were detected by the highly segmented MINIBALL Ge-detector array. The results, B(E2;0(gs)+ -->2(1)+)=0.222(19)e2b2 for 108Sn and B(E2; 0(gs)+-->2(1)+)=0.195(39)e2b2 for 106Sn were determined relative to a stable 58Ni target. The resulting B(E2) values are approximately 30% larger than shell-model predictions and deviate from the generalized seniority model. This experimental result may point towards a weakening of the N=Z=50 shell closure.

Z = 50 shell gap near 100Sn from intermediate-energy Coulomb excitations in even-mass 106-112Sn isotopes.

Rare isotope beams of neutron-deficient 106,108,110Sn from the fragmentation of 124Xe were employed in an intermediate-energy Coulomb excitation experiment. The measured B(E2,0(1)(+)-->2(1)(+)) values for 108Sn and 110Sn and the results obtained for the 106Sn show that the transition strengths for these nuclei are larger than predicted by current state-of-the-art shell-model calculations. This discrepancy might be explained by contributions of the protons from within the Z = 50 shell to the structure of low-energy excited states in this region.

Ab-initio coupled-cluster study of 16O.

We report converged results for the ground and excited states and matter density of 16O using realistic two-body nucleon-nucleon interactions and coupled-cluster methods and algorithms developed in quantum chemistry. Most of the binding is obtained with the coupled-cluster singles and doubles approach. Additional binding due to three-body clusters (triples) is minimal. The coupled-cluster method with singles and doubles provides a good description of the matter density, charge radius, charge form factor, and excited states of a one-particle, one-hole nature, but it cannot describe the first-excited 0(+) state. Incorporation of triples has no effect on the latter finding.

Coupled cluster calculations of ground and excited states of nuclei.

The standard and renormalized coupled cluster methods with singles, doubles, and noniterative triples and their generalizations to excited states, based on the equation of motion coupled cluster approach, are applied to the 4He and 16O nuclei. A comparison of coupled cluster results with the results of the exact diagonalization of the Hamiltonian in the same model space shows that the quantum chemistry inspired coupled cluster approximations provide an excellent description of ground and excited states of nuclei. The bulk of the correlation effects is obtained at the coupled cluster singles and doubles level. Triples, treated noniteratively, provide the virtually exact description.

N=82 shell quenching of the classical r-process "waiting-point" nucleus 130Cd.

First beta- and gamma-spectroscopic decay studies of the N=82 r-process "waiting-point" nuclide 130Cd have been performed at CERN/ISOLDE using the highest achievable isotopic selectivity. Several nuclear-physics surprises have been discovered. The first one is the unanticipatedly high energy of 2.12 MeV for the [pi g(9/2) multiply sign in circle nu g(7/2)] 1(+) level in 130In, which is fed by the main Gamow-Teller transition. The second surprise is the rather high Q(beta) value of 8.34 MeV, which is in agreement only with recent mass models that include the phenomenon of N=82 shell quenching. Possible implications of these new results on the formation of the A approximately 130 r-process abundance peak are presented.

Phase Transitions in Neutron Stars and Maximum Masses.

Using the most recent realistic effective interactions for nuclear matter with a smooth extrapolation to high densities including causality, we constrain the equation of state and calculate maximum masses of rotating neutron stars. First- and second-order phase transitions to, e.g., quark matter at high densities are included. If neutron star masses of approximately 2.3 M middle dot in circle from quasi-periodic oscillations in low-mass X-ray binaries are confirmed, a soft equation of state as well as strong phase transitions can be excluded in neutron star cores.