Liang Group

05 February 2024 (Mon)

Nuclear scientists expect to describe nuclear structure and decay properties within a unified model and under a unified nuclear force. I will briefly introduce the Configuration-Interaction Shell Model (CISM) in this presentation. Then, its applications in exotic nuclei are generally reviewed from light to heavy ones. Preliminary findings regarding the CISM employing a unified nuclear force with varying strengths in proton-proton and neutron-neutron interactions will be presented.

26 April 2023 (Wed)

Wobbling and chirality are unique characteristics of triaxial nuclei. Chirality in nuclei is now well established, both theoretically and experimentally. The other essential characteristic of triaxial nuclei is wobbling, first observed in the \( A \sim 160 \) region almost contemporaneously with experimental observation of nuclear chirality. A few years ago, wobbling was observed in the nucleus \( ^{135} \mathrm{Pr} \), opening a new region for detailed investigation of this phenomenon. Since then, both "transverse" and "longitudinal" wobbling have been observed. In our work at Gammasphere, we have obtained the first solid evidence for longitudinal wobbling motion in nuclei, in \( ^{187} \mathrm{Au} \). Furthermore, in \( ^{135} \mathrm{Pr} \), we have observed both chirality and wobbling in the same nucleus—a chiral wobbler.

20 March 2023 (Wed)

We propose a systematic approach to study the nucleon-nucleon interaction by applying time-ordered perturbation theory (TOPT) to covariant chiral effective field theory. Diagrammatic rules of TOPT, for the first time, are worked out for particles with non-zero spin and interactions involving time derivatives. They can be applied to derive chiral potentials at any chiral order. The effective potential, as a sum of two-nucleon irreducible time-ordered diagrams, and the scattering equation (i.e. Kadyshevsky equation) are obtained within the same framework.

According to the Weinberg power counting, at leading order, we find that NN potential is perturbatively renormalizable, and the corresponding integral equation has unique solutions in all partial waves. Through evaluating the two-pion exchange contribution at the one-loop level, we formulate the NN interaction up to next-to-next-to-leading order (NNLO). A good description of phase shifts is achieved by treating the full NNLO potential non-perturbatively.

References
[1] V. Baru, E. Epelbaum, J. Gegelia, Xiu-Lei Ren, Phys. Lett. B 798 (2019) 134987
[2] Xiu-Lei Ren, E. Epelbaum, J. Gegelia, Phys. Rev. C 106 (2022) 034001
[3] Xiu-Lei Ren, E. Epelbaum, J. Gegelia, in preparation

12 January 2022 (Wed)

In this seminar I will outline how in analogy to the Thouless criterion for the critical temperature of pairing, we can calculate the critical temperature for quartet and specially alpha particle condensation in infinite nuclear matter. We then also solve the full quartet order parameter equation at zero temperature. A key point will be the discussion why for quartets no condensation for positive chemical potentials is possible which is contrary to the pairing case. Therefore for quartets only BEC is possible and no BCS like phase exists. I will then switch to finite nuclei and for instance explain the fameous Hoyle state in 12C as an extended nuclear state at 7.65 MeV where the three alpha particles are, with their c.o.m. condensed in the same 0S-state. We will show almost perfect agreement of our results with the experimentally measured e.m. inelastic form factor from ground to Hoyle state. From this success it is not difficult to imagine that in heavier selfconjugate nuclei also alpha condensed states around the n-alpha threshold are present. We, for instance, will dwell on the example of four alphas in \({}^{16} \mathrm{O} \).