IV. OPERATION RECORDS

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Activity report of the second-term (2014–2021) RIBF Theory ForumT. Abe,∗1 Y. Aritomo,∗2 W. Horiuchi,∗3 M. Kimura,∗3 H. -Z. Liang,∗4 T. Matsumoto,∗5 F. Minato,∗6K. Nakazato,∗7 N. Nishimura,∗8 and K. Yoshida∗9Preface
Recent advancements in radioactive-isotope (RI)
beam technology have opened up the research ﬁeld of
the physics of exotic nuclei. Since the RIBF started operation in 2007, our experimental capabilities have beensigniﬁcantly expanded, including neutron- and protonrich nuclei as well as superheavies. With increasing
experimental data for a vast mass region of the nuclear
chart, nuclear structure and reaction theories are expected to play a more important role in interpretingthe data and in profoundly understanding atomic nuclei. A mission of the second-term Theory Forum (TF2)
is to discuss physics in the future of RIBF. Taking overthe discussions in the ﬁrst-term Theory Forum (TF1)(2006–2014), we have been working through various activities. Although most of the members were renewed,Dr. Y. Utsuno, Dr. K. Ogata, and Dr. S. Wanajo acted
as mediators to bridge the activities of TF1 and TF2.
We would like to express our gratitude to them. This report summarizes our activities over the last seven years
to facilitate handover to the third-term Theory Forum.
Because of the limited space in this progress report, details about our activities are provided on a web page.1)
Short reports from WGs
TF2 was divided into several working groups (WGs)
to discuss physics cases intensively. We also had intergroup discussions regularly. Here, we present shortsummaries of these activities, which are categorized into
three subjects: nuclear astrophysics, the physics of superheavy elements, and the physics of nuclear manybody problems. More details on each subject can befound on the website.
Nuclear astrophysics: r-process, EOS, and nuclear matterIn the ﬁeld of nuclear astrophysics, discussions werestarted with two WGs. The primary purpose of ther-process WG was to improve the current theoretical
calculations of nuclear properties, including the masses,
decay half-lives, neutron-capture reaction rates, and nuclear ﬁssion rates for the r-process nucleosynthesis cal∗1
∗2
∗3
∗4
∗5
∗6
∗7
∗8
∗9RIKEN Nishina CenterKindai University
Department of Physics, Hokkaido University
Department of Physics, the University of Tokyo
Department of Physics, Kyushu University
JAEA
Faculty of Arts and Science, Kyushu University
Astrophysical Big Bang Laboratory, RIKEN
Department of Physics, Kyoto University

culations. The possibility of updating these rates usedin the nucleosynthesis community was discussed, particularly to reﬂect the latest results from nuclear theory
groups in Japan. A novel mass model based on theBayesian neural-network method was proposed to assess the model uncertainty and to evaluate the impactof new measurements on the prediction. The signiﬁcance of direct reaction on the neutron capture rates
was investigated through a microscopic approach. Furthermore, we found a compelling diﬀerence between
the phenomenological ﬁssion distributions widely usedin the nucleosynthesis calculations and the results obtained using a dynamical model based on the Langevinmodel in the neutron-rich r-process region. This shows
the necessity of considering dynamical ﬁssion in thedata on systematic ﬁssion yields for better r-process
calculation.
The equation of state (EOS) and nuclear matter WG
aimed to determine the nuclear-physics constraints on
the nuclear EOS. The connection between the astrophysical observations and the nuclear properties revealed at RIBF has been explored. We found that we
can construct a model that can account for the indication of GW170817 if the symmetry energy at twice
the saturation density of nuclear matter is 40–60 MeV.
The symmetry energy in this density region can be constrained from the collision experiment of neutron-richheavy ions measured by the SAMURAI spectrometer
at RIBF.
In addition to these activities focusing speciﬁcallyon theoretical nuclear physics, we organized a series ofworkshops to bring together nuclear physicists and astrophysicists motivated by the discovery of an r-process
event (kilonova) associated with the gravitational-wave
detection by the Advanced LIGO–Advanced Virgo frommerging neutron stars. The workshops were held atRNC (June 2018), YITP Kyoto University (May 2019),
and NAOJ (October 2020).
Superheavy elements
The main purpose of the superheavy element andﬁssion WG (SHE/Fission WS) was to give theoretical suggestions and make proposals for experiments on
new superheavy elements conducted at RIBF. A primary interest in the physics of superheavy elements isto synthesize new elements exceeding Z = 118, and to
produce nuclei at the center of the island of stability
located around Z = 114 and N = 184. For the former,
heavier projectiles and stable actinide targets are being
promoted. For the latter, there is no available set ofstable projectiles and targets because the center of theisland of stability is located in the neutron-rich region.

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IV. OPERATION RECORDS

RIKEN Accel. Prog. Rep. 54 (2021)

It is thus desirable to estimate the possibility of newelement synthesis based on the theoretical calculationand to propose new experiments.
To synthesize superheavy elements, we proposed theoptimum incident energy and the combination of a target and projectile based on the calculated evaporationresidue cross section. We also calculated the propertiesof new superheavy elements: lifetime, deformation, nuclear structure, ﬁssion barrier height, decay proprieties,etc. We then suggested a new approach to the synthesisof superheavy nuclei.
We (co)organized the RIBF ULIC mini-WS in August 2016 as a kick-oﬀ meeting and subsequently held aWS entitled “New developments in superheavy element
research” at Kyushu University in July 2018.
Nuclear structure, collective motion, many-body correlations, and halo
The nuclear systems under extreme conditions areexpected to reveal new facets associated with the correlations among nucleons. A uniqueness of drip-line
nuclei was discussed in view of the nuclear interactionsand the strong correlations in a dilute system, such as
the multi-neutron halo and its condensation. A possibility of measuring the many-body correlations wasalso discussed through the nuclear responses and reactions. The connections between the nuclear responses
and the EOS of nuclear matter or the reaction (decay)
rates relevant to the r-process were investigated.
The true pleasure in nuclear physics is the diversityof research topics. Thus, we realized the importanceof oﬀering an opportunity to young colleagues, particularly students and post-docs, where we can frankly
discuss nascent plans of physics research. To provide
such an environment and to explore important physics
for future experiments at RIBF in a variety of researchsubjects covered in the WG, we decided to organize“Hodan-kai” meetings. In contrast to the style of an ordinary workshop, the Hodan-kai aims to facilitate theexchange of unmatured ideas among young researchers.
To concentrate on physics discussions, the venue was a
relatively isolated place, and the presentation ﬁles are
not open to the public.
The ﬁrst Hodan-kai was held from July 31 to August2, 2017 and focused mainly on the latest theoretical research on nuclear physics. The second and third meetings were held in February 2019 and 2020, respectively.
The topics discussed were extended in the second andthird meetings, as listed in Table 1. In the third meeting, in addition to talks given in the regular Hodan-kai
style, two “panel discussion” sections consisting of introductory talks by organizers and 5–7 short talks byinvited panelists were newly organized to facilitate interaction among the participants. The program andsummary of the Hodan-kai are shared on their respective websites.2–4)

2Table 1. Number of talks by theoretical and experimental
nuclear physicists and researchers from other fields, aswell as participants in the successive Hodan-kai meetings.

1st2nd3rdDateAug 2017
Feb 2019
Feb 2020

Theor.
22
66Exp.
1
77

Other
0
12
4

Participants
4942
43

Collaboration and Perspective
In addition to the collaborations among the TF2members, collaborative works with other researchers
have been conducted, and further collaborations are expected in the near future. We have already co-authored4 papers on the experiments performed at RIBF, 2 proposals to the RIBF PAC, 24 papers on the theoreticalanalysis of the RIBF data, and 30 papers on the physics
expected in the future RI beam facilities. Furthermore,
30+ invited talks on the physics of RI beams have beendelivered by the TF2 members during this term.
We have been discussing nuclear ﬁssion with experimentalists of the Japan Atomic Energy Agency(JAEA). The measurement of nuclear ﬁssion processes
and the formation of nuclei in the actinide region using the multinucleon transfer reaction have been intensively studied at the tandem facility of JAEA. Throughjoint research between JAEA and Kindai University, we
have published a number of co-authored papers thathave advanced the analysis of experimental data andthe interpretation of experimental data. Students ofKindai University attended the summer training schoolat JAEA. These young students are expected to leadthe physics of exotic nuclei in the future. This activity
will thus be a starting point for further collaborative
research in the physics of superheavy nuclei and ﬁssion.
The JSPS-NRF-NSFC A3 Foresight Program “Nuclear Physics in the 21st Century” started in 2019. A
microscopic understanding of the r-process is one of themain projects, and theoretical collaboration among theA3 countries is key to success. The activities in TF2
would set a foundation for the extended collaboration.
A plan for the construction of a microscopic nuclear
data library based on nuclear density-functional theory
is described in Ref. 5).
References

1) https://www.nishina.riken.jp/RIBF/TheoForum/index.
html .
2) 1st Hodan-kai, https://indico2.riken.jp/event/2509/ .
3) 2nd Hodan-kai, https://indico2.riken.jp/event/2864/ .
4) 3rd Hodan-kai, https://indico2.riken.jp/event/3157/ .
5) K. Yoshida, in this report.

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