Research Group for Advanced Theoretical Physics

Group Leader : HOSAKA Atsushi
Members : UTSUNO Yutaka, MARUYAMA Toshiki, Philipp GUBLER, UCHINO Shun,YOSHIDA Kazuki, SUZUKI Kei, YABUNAKA Shunsuke, YAMAGUCHI Yasuhiro, Guanjuan
WANG, MORI Michiyasu, IEDA Jun'ichi, ONISHI Hiroaki, NAKATA Koki, YAMAMOTOKei, ARAKI Yasufumi, KOURA Hiroyuki, OKA Makoto, HIYAMA Emiko, OTSUKA
Takaharu, MIYASHITA Seiji, MYO Takayuki, YASUTAKE Nobutoshi

Wide activities and step-up seminars for quantum manybody systems
The Advanced Theoretical Physics group conducts researchon fundamental physics to elucidate the dynamics of quantum
many-body systems. The research has its value not only for its
own sake but also for many applications. The members’
activities spread widely; quarks for hadrons, protons andneutrons for atomic nuclei, atoms for molecules and cold atomicsystems, electrons and phonons in condensed matter systems,
and so on. While members work on their own subjects, we
make an effort to share and exchange ideas and methods. An
attempt is the series of step-up seminars, inviting speakers todiscuss the various fields above. In the fiscal year of 2022, weinvited seven speakers covering open quantum mechanics,
resonances and the Casimir effects in condensed matter physics,
entropy in field theoretical systems, weakly bound nuclearsystems, topological magnetic textures such as skyrmions, etc.
Some of the ideas originating from particle and nuclear physics
are now recognized in condensed matter systems to havepotential applications to material sciences and engineering, andalso vice versa. As a result, we have seen several interdisciplinary works among young members.
Reimei project “Unveiling nuclear shells and correlations in
exotic nuclei through knockout reactions”
As a part of nuclear-physics activities, we have started a new
Reimei project whose PI is Prof. Alexandre Obertelli at
Technischen Universität Darmstadt, aiming at clarifying theproperties of exotic nuclei by using the knockout reaction.
Exotic nuclei stand for those having neutron to proton ratios
much larger than those of naturally occurring stable isotopes,
thus having very short half-lives. Some of the basic properties of
exotic nuclei, such as shell structure, are known to be quite
different from those of stable isotopes, which attracts muchinterest in the nuclear-physics community. The PI of this projectis responsible for the experiment, and our group is in charge ofthe theory.
In this fiscal year, we organized a workshop “Unveiling
nuclear shells and correlations in exotic nuclei through knockout
reactions” at Technischen Universität Darmstadt during October
10-12, 2022. The purpose of this workshop is to promote mutual
understanding of the activities of the core members and todiscuss how to proceed with our collaborations. One of therepresentative results in this fiscal year is to find out whatcauses the extraordinarily large nuclear radii of the calcium
isotopes observed for N > 28 (N: neutron number): by analyzing
the momentum distributions of the reaction residues of the52Ca(p,pn)51Ca knockout reactions, we have unambiguously
demonstrated that the enlarged nuclear radii are due to theextended neutron 2p3/2 orbital that is filled for N > 28, not due tothe onset of deformation or bulk expansion of those isotopes [1].

Reimei project “Collaborative research to evaluate QCD
vacuum properties at high density from ϕ meson decay
inside the nucleus”
In collaboration with researchers of the ASRC Hadron Physics
group and Prof. S.H. Lee of Yonsei University as PI, we haveinitiated an international Reimei research project with the goalof bringing together theoretical and experimental researchers, tostudy the ϕ meson properties in nuclear matter. The ultimate
goal of this research is to clarify the origin of hadronic masses,
which compose the majority of visible matter in our universe.
As part of the project, regular discussion sessions were heldamong the core members, which eventually led to thepublication of a first collaborative paper [2]. In this paper, theauthors discussed the angular distributions of the ϕ meson
decay into electron and kaon pairs (see Fig. 1, for the kaon
decay result) and how these distributions can be used toexperimentally disentangle the different ϕ meson polarization
modes at finite density. Furthermore, the international Reimei
workshop “Polarization phenomena and Lorentz symmetry
violation in dense matter” was held at Yonsei University inOctober 2022 with approximately 40 on-site and online
participants mostly from East Asian countries. Another Miniworkshop “Exotic Hadrons in Vacuum & Matter” was held on
the occasion of the visit of Prof. S.H. Lee to ASRC in March2023.

Fig. 1 Angular distribution of the outgoing K+ decaying
from a ϕ meson in the center-of-mass frame, fortransverse (T) and longitudinal (L) polarizations. θ isthe polar angle of the K+ with respect to the direction of
motion of the ϕ meson in the lab frame.
References[1] M. Enciu, H.N. Liu, A. Obertelli, P. Doornenbal, F.
Nowacki, K. Ogata, A. Poves, K. Yoshida et al., Phys. Rev. Lett.
129, 262501 (2022).
[2] I.W. Park, S. Sako, K. Aoki, P. Gubler and S.H. Lee, Phys.
Rev. D 107, 074033 (2023).