II-2. Nuclear Physics (Theory)

RIKEN Accel. Prog. Rep. 55 (2022)

RIKEN Accel. Prog. Rep. 55 (2022)

Proof-of-principle calculations in the ab initio no-core Monte Carloshell model†
T. Abe,∗1 P. Maris,∗2 T. Otsuka,∗3,∗1,∗4 N. Shimizu,∗5 Y. Utsuno,∗4,∗5 and J. P. Vary∗2-4

†
∗1
∗2
∗3
∗4
∗5Energy per nucleon (MeV)

Understanding of nuclear structure from ﬁrst principles is a major challenge in low-energy nuclear theory.
This challenge includes not only the conﬁrmation of existing experimental data but also the predictions where
the experimental information has not been available
yet. In addition, one hopes to extract a detailed understanding of a variety of complex nuclear phenomena,
described by successful nuclear models, from the underlying microscopic descriptions. Under these motivations, a number of ab initio investigations have been
actively done for more than a decade. This trend in
ab initio studies has been supported by the rapidly
growing computational power of supercomputers andcontinuing improvements of ab initio techniques fornuclear many-body calculations.
Here we report the proof-of-principle calculationsin the ab initio no-core Monte Carlo shell model(MCSM).1) This is the extension of the MCSM withan assumed inert core2–4) to the ab initio no-core shellmodel.3–5) Ground-state energies are obtained in thebasis spaces up to seven harmonic oscillator (HO) major shells (Nshell = 7) with several HO energies, h̄ω,
around the optimal h̄ω for the convergence of groundstate energies. These energy eigenvalues are extrapolated to obtain estimates of converged ground-stateenergies in each basis space using energy variances ofcomputed energy eigenvalues. We further extrapolatethese energy-variance-extrapolated energies obtainedin the ﬁnite basis spaces to inﬁnite basis-space results.
Figure 1 shows our results with two nonlocal N N
interactions6,7) in comparison with the experimental
data.8) In Fig. 1, the JISP166) and Daejeon167) results are shown by blue and red symbols, respectively,
with estimated error bars from our ﬁt to Nshell = 4–7
results at their respective optimal h̄ω values. FromFig. 1, the JISP16 results yield overbinding at 12 Cand beyond. An improved picture emerges using Daejeon16 with some overbinding still evident when compared with experiments. There are encouraging trends
with the Daejeon16 results compared with the JISP16results. However, this ﬁnding suggests the necessity offurther revisions of nonlocal N N interactions for theheavier-mass region beyond the p shell and/or the ex--6

4

He8JISP16
Daejeon16
AME2012Be12C16O

20

Ne-8-10-12Fig. 1. Comparison of the no-core MCSM results for theenergy per nucleon with the JISP166) and Daejeon167)
N N interactions to experimental data.8) The MCSM
results are shown for the basis-space extrapolations using the Nshell = 4–7 energy-variance-extrapolated results with the respective optimal h̄ω values.

plicit inclusion of a 3N interaction. Note that the nocore MCSM results are in reasonable agreement withthe other ab initio results with the same interactions
where those are available.
The current study oﬀers a foundation for pathways
to investigate nuclear structure from ﬁrst principles,for instance, α-cluster structure and dineutron correlations of valence neutrons on the p-shell nuclei. Also
the sd-shell nuclei and beyond continue to provide richinsights into emergent nuclear phenomena. With improved nuclear interactions and increasingly precise abinitio tools for nuclear many-body calculations such as
the no-core MCSM, we expect an opportunity to probethese emergent phenomena and, at the same time, toprobe the limits of our knowledge of the strong andelectroweak interactions.
References
1) T. Abe et al., Phys. Rev. C 104, 054151 (2021).
2) T. Otsuka et al., Prog. Part. Nucl. Phys. 47, 319 (2001).
3) N. Shimizu et al., Prog. Theor. Exp. Phys. 2012,01A205 (2012).
4) N. Shimizu et al., Phys. Scr. 92, 063001 (2017).
5) T. Abe et al., Phys. Rev. C 86, 054301 (2012).
6) A. M. Shirokov et al., Phys. Lett. B 644, 33 (2007).
7) A. M. Shirokov et al., Phys. Lett. B 761, 87 (2016).
8) G. Audi et al., Chin. Phys. C 36, 1603 (2012).

Condensed from the article in Phys. Rev. C 104, 054315(2021)RIKEN Nishina Center
Department of Physics and Astronomy, Iowa State University
Department of Physics, University of TokyoAdvanced Science Research Center, Japan Atomic Energy
AgencyCenter for Nuclear Study, University of Tokyo- 39 -1