VI. RNC ACTIVITIESRIKEN Accel. Prog. Rep. 55 (2022)

Subnuclear System Research Division
Radiation Laboratory
1. Abstract
When Yoshio Nishina passed away in 1951, the research activities in Nishina’s Laboratory in RIKEN were inherited by threelaboratories, and Radiation Laboratory is the one of them. The Radiation Laboratory, whose legacy has been kept for 64 years, leadby F. Yamazaki, T. Hamada, M. Ishihara and last 21 years by H. En’yo, is closed in March 2022. In the last 28 years, we haveproposed and built a polarized proton collider at RHIC, Relativistic Heavy Ion Collider at Brookhaven National Laboratory in theUSA, and discovered that gluons in the proton carry a sizable portion of the proton spin which is 1/2. We also identiﬁed W bosons
in the electron/positron decay channel and in the muon decay channel, with which we showed how much anti-quarks carry the proton
spin. We were successful at RHIC to create Quark Gluon Plasma, the state of Universe just after the Big Bang. Such creation was
conﬁrmed through the many interesting discoveries such as “Jet Quenching,” “Elliptic Particle Flows,” “Thermal Photon Radiation”
and so on. The RIKEN-CCJ, Linux-based Computer Cluster in Japan, was ﬁrstly built in Japan and is still operational in our laboratory
and lead many analyses towards those discoveries. We have discovered the ﬁrst hint of the restoration of Chiral Symmetry Breaking,the mechanism to create 99% of our weight, through the observation of phi meson mass-modiﬁcation in nuclear matter at the 12-GeV
Proton Synchrotron in KEK. This program is now acceded by the E16 experiment in J-PARC.
On the course of those experiments, we have conducted handful of technical developments, which include laser-induced novel
ion sources, helical snake magnets for RHIC and AGS, ﬁne-pitch silicon pixel detectors for PHENIX and sPHENIX experiments,
high-performance trigger electronics, gas-electron-multipliers for hadron-blind detectors and trackers. With some of those new developments, we are preparing and starting new experiments at J-PARC and Fermilab to study the nature of hadron and preparing for theelectron-ion collider (EIC) yet to be built at BNL in near future.
The legacy of our laboratory is taken over by RIKEN-BNL Research Center (RBRC) in BNL and by the new laboratory “RHIC
Physics Research Group” headed by Y. Akiba in RIKEN Nishina Center for Accelerator-Based Science.

2. Major Research Subjects(1) Spin physics with relativistic polarized-proton collisions at RHIC
(2) Study of nuclear matter at high temperature and/or at high density(3) Technical developments on radiation detectors and accelerators

3. Summary of Research Activity(1) Experimental study of spin structure of proton using RHIC polarized proton collider[See also RIKEN-BNL Research Center Experimental Group for the activities at BNL]
After the previously published central-rapidity neutral and charged pion double-spin asymmetries at the highest collision energies
at RHIC of 510 GeV now also the world’s ﬁrst direct photon results have been brought to publication. The direct photon probe
also restricts the initial, hard interaction to be predominantly between a quark and a gluon thus further increasing the sensitivity to
the gluon spin. It has therefore been considered the golden channel to access the gluon spin since the inception of the RHIC spin
program more than two decades ago. Because of the electromagnetic interaction involved in the direct photon process the statistics
are limited, compared to hadronic ﬁnal states. Therefore, it took the largest polarized data set to ﬁnally be able to extract these double
spin asymmetries. The results are found to be consistent with global ﬁts of the previous hadron and jet measurements from RHIC aswell as semi-inclusive DIS. All these results will be included in future global ﬁts of all the existing experimental data in the world andwill improve the sensitivity of quark and gluon spin contributions to the total spin of the nucleon.
While orbital angular momentum cannot be directly accessed at RHIC, several transverse spin phenomena have been observed
which relate to orbital angular momentum and the three-dimensional structure of the nucleon. These phenomena by themselves havebecome a major ﬁeld of research as the dynamics of the strong interaction can be studies with these functions. Various single spin
asymmetry measurements have been obtained for various rapidities. When moving to central rapidities, these left-right asymmetries
are known to be expectingly small for neutral pions. Since then, they have been conﬁrmed to be small also for eta mesons. A
substantially improved data set with signiﬁcantly reduced uncertainties has been recently published in PRD for both ﬁnal states. A
recently published measurement by PHENIX for charged pions at mid-rapidity shows also small asymmetries but at the same time alsoa hint of a charge separation. Such a charge separation could originate from a ﬂavor dependence and could also explain the smallnessof the neutral meson asymmetries where the eﬀects from up and down quark ﬂavors could cancel each other out.
For the ﬁrst time also direct photon single spin asymmetries have been extracted at RHIC. The direct photon asymmetries areagain important here as they are only sensitive to the transverse spin eﬀects in the initial state and not the fragmentation-related
eﬀects. Furthermore, it provides sensitivity to a gluon correlation function that is not accessible in other processes. The direct photon
results have been published in PRL.
In June of 2017, an electro-magnetic calorimeter, RHICf, was installed in the most forward area of the STAR experiment and took
polarized proton collision data for neutral particle production (neutron, photon, neutral pion). The cross-section measurement will give
us new inputs to develop high-energy particle-collision models which are essential to understand air-shower from ultra-high energy
cosmic rays. The ﬁrst photon cross section measurements at very forward rapidities have recently been submitted for publication.
At similar rapidities also neutron asymmetries have been observed in the past. While previously only their general magnitude wasobtained, using unfolding techniques it was possible to extract the ﬁrst asymmetries as a function of the neutron transverse moment- 307 -

VI. RNC ACTIVITIESRIKEN Accel. Prog. Rep. 55 (2022)

for proton-proton collisions. The RHICf results have followed up these PHENIX results and, after conﬁrming the consistency at
small transverse momenta, extended the transverse momentum range signiﬁcantly. These results are currently being prepared for
publication. Similar PHENIX neutron asymmetries in proton-nucleus collisions have been analyzed as a function of both transverseand longitudinal momenta, as well as in (anti)correlation with hadronic detector activity elsewhere. These results have been publishedin PRD and clearly show the interplay between hadronic interactions that predominantly produce negative asymmetries and ultraperipheral collisions that produce positive asymmetries and gain in relevance with heavier nuclei.
Some of us are participating in the Fermilab SeaQuest experiment as a pilot measurement of muon pairs from Drell-Yan processusing a 120-GeV unpolarized proton at Fermilab. After ﬁnishing unpolarized measurements in 2017 to study the quark spin-orbit
eﬀect, a new measurement with a polarized proton target will start in 2022 to study the sea-quark orbit eﬀect of the polarized protonin the target. The ﬁrst result from the SeaQuest experiment on the asymmetry of the antimatter in the proton has been published in
Nature.
For many jet related measurements fragmentation functions are necessary to gain spin and or ﬂavor sensitivity. Those are currently
extracted by some of us using the KEK-Belle data. In addition to using the fragmentation results with RHIC measurements, they will
also provide the basis for most of the key measurements to be performed at the electron-ion collider. In 2021, a new fragmentation
analysis of predominantly vector mesons and D mesons decaying into two or three light hadrons has been prepared and is in theprocess of being ﬁnalized. All these measurements are essential to nearly all nucleon structure measurements at RHIC, semi-inclusive
DIS and the EIC.
As the Electron-Ion Collider is becoming a reality, many of us are participating in the various community eﬀorts to deﬁne thephysics goals of the EIC and how they inform on the choices of collisions energies, luminosities, and detector components. While theaccelerator eﬀorts are naturally led by the two main nuclear physics laboratories in the US, BNL and JLAB, a large EIC user group of
more than 1200 members from all around the world is working on making the EIC a reality. During 2021, the ﬁrst detector proposals
were prepared with the selection of the ﬁrst EIC detector by an external committee. The ﬁrst detector is part of the DOE based EIC
project. Given the participation in the sPHENIX experiment, we participated heavily in the ECCE proposal that also revolves aroundreusing the BABAR/sPHENIX magnet. We were leading the inclusive and semi-inclusive physics studies that showed the feasibilityof the main EIC goals with such a detector. Others participated strongly in the detector considerations for an ECCE zero-degreecalorimeter. At the end of 2021 the ECCE proposal was unanimously selected as the ﬁrst EIC detector and is currently being prepared
to form an actual collaboration.
(2) Experimental study of quark-gluon plasma using RHIC heavy ion collider[See also RIKEN-BNL Research Center Experimental Group for the activities at BNL]
We have completed several key measurements in the study of quark-gluon plasma at RHIC. As the top of them, we lead theanalysis of the ﬁrst thermal photon measurement in heavy ion collisions. The measurement indicates that the initial temperaturereached in the central Au + Au collision at 200 GeV is about 350 MeV, far above the expected transition temperature T c ∼ 170 MeV,
from hadronic phase to quark-gluon plasma. This work was rewarded by Nishina Memorial Prize given to Y. Akiba in 2011. We alsomeasured direct photons in d + Au and direct photon ﬂow strength ν2 and ν3 in Au + Au.
We led measurement of heavy quark (charm and bottom) using VTX, a 4-layer silicon vertex tracker which we jointly constructedwith US DOE. The detector was installed in PHENIX in 2011. PHENIX recorded approximately 10 times more data of Au+Au
collisions in the 2014 run than the 2011 run. PHENIX recorded high statistics p + p and p + A data in 2015, and the doubled the Au+ Au in 2016. PHENIX concluded its data taking in the 2016 run.
The results of the 2011 run were published in Physical Review C (Phys. Rev. C 93, 034904 (2016)). This is the ﬁrst publicationfrom VTX. The result showed that the electrons from bottom quark decay is suppressed for pT > 4 GeV/c, but the suppression factor issmaller than that of charm decay electrons for 3 < pT < 4 GeV/c. This is the ﬁrst observation of bottom electron suppression in heavy
ion collisions, and the ﬁrst result that shows the bottom and charm suppression is diﬀerent. A paper reporting the results of b → e
and c → e measurement in the 2015 p + p run has been published in Phys. Rev. D 99, 092003 (2019). The centrality dependence ofthe suppression b → e and c → e from the 2014 Au+Au data has been submitted for publication. The results show clear quark massdependence of energy loss in the QGP produced at RHIC. The 2016 run is the ﬁnal data taking run of PHENIX, and this run doubled
the dataset for heavy-ﬂavor measurement with VTX. The production of nDSTs of the 2016 VTX data is completed and they are readyfor physics analysis.
PHENIX published measurements of ﬂow strength in p + Au, d + Au, and 3 He+Au (Nature Physics 15, 214 (2019)). The results
provide strong evidence for formation of small droplet of quark gluon plasma in collisions of small systems at RHIC.
In Wako we are operating a cluster computer system (CCJ) specialized to analyze huge data sets taken with the PHENIX detector.
It consists of 25 nodes (17 old nodes and 8 new nodes) each of which has two CPUs and 10 sets of local disks for data repository(old node: quad-core CPU, 1 TB disk, new node: six-core CPU, 2 TB disk). There are 264 CPU cores and 380 TB disks in total.
This conﬁguration ensures the fastest disk I/O when each job is assigned to the node where the required data sets are stored. It is also
important that this scheme does not require an expensive RAID system and network. Through this development we have establisheda fast and cost-eﬀective solution in analyzing massive data.
The data of 1.1 PByte obtained by the PHENIX experiment is stored in a hierarchical storage system which is a part of HOKUSAI
BigWaterfall/SailingShip supercomputer systems operated by the Head Oﬃce for Information Systems and Cybersecurity. In addition,
we operate a dedicated server for the RHICf group and two servers for the J-PARC E16 group, to keep their dedicated compilationand library environments, and some data.

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VI. RNC ACTIVITIESRIKEN Accel. Prog. Rep. 55 (2022)(3) Study of properties of mesons and exotic hadrons with domestic accelerators
Preparation of the experiment E16 at J-PARC Hadron experimental facility is underway with several Grant-in-Aids. This experiment aims to perform a systematic study of the spectral modiﬁcation of low-mass vector mesons in nuclei to explore the physics ofchiral symmetry breaking and restoration in dense nuclear matter, namely, the mechanism proposed by Nambu to generate most of
hadron masses.
The Gas Electron Multiplier (GEM) technology is adopted for the two key detectors, GEM Tracker (GTR) and Hadron-blind
Cherenkov detector (HBD). To improve electron-identiﬁcation performance, lead-glass calorimeters (LG) are used in combination
with HBD. We are in the production phase. Read-out electronics and trigger logic modules are also installed and tested. We have beena member of the CERN-RD51 collaboration to acquire the read-out technology for GEM. The MoU for RD51 was extended for theperiod of 2019–2023.
Due to the budgetary limitation, we aim to install a part of the detectors at the beginning of the experiment, eight modules of aset of GTR, HBD and LG, out of 26 modules in the full installation. J-PARC PAC (Program Advisory Committee) gave us a stage-2
approval in July 2017 to perform commissioning runs (Run 0). Although there is a signiﬁcant delay from the originally planned dateof March 2016, the construction of the beam line by KEK was completed ﬁnally in early 2020 to perform this experiment.
After the successful commissioning runs held in JFY 2020, we performed the second commissioning run in June 2021. Twoelectron trigger to accumulate the data of vector meson decays were tested. The micro beam structures were newly found which
deteriorates DAQ and trigger performances. Countermeasures for the structures were already discussed in cooperation with theaccelerator group and Beam line group in J-PARC, and will be adopted in the coming beam time in the ﬁrst quarter of 2023. On theother hand, detector performances were almost satisﬁed. In our plan, physics run will be performed in 2023–24, after the beam study.
Approval of physics run will be discussed in PAC held in Summer 2022.
This activity is moved to Meson Science Laboratory after the closing of Radiation Laboratory.
(4) Detector development for PHENIX/sPHENIX experiment
The PHENIX experiment proposes substantial detector upgrades to go along the expected accelerator improvements, including
the future electron-ion collider “EIC.” The present PHENIX detector is repurposed to the sPHENIX (super PHENIX) detector whichreuses the Babar solenoid magnet at SLAC and is covered by the hadronic calorimeter which was not available in the previous RHIC
experiments. The sPHENIX was approved for the Project Decision-2/3 (corresponds to DOE’s Critical Decision-2/3) in May 2019.
We RIKEN group have been developing the one of the tracking devices of sPHENIX detector, so called intermediate tracker (INTT)
since 2015. The INTT provides the best timing resolution among the sPHENIX tracking system, in conjunction with a time projection
chamber and a MAPS based vertex detectors. The INTT silicon ladders have been demonstrated the satisfactory performance as
designed through the three beam tests in 2018, 2019, and 2021 in Fermilab Test Beam Facility (FTBF) and Research Center forElectron Photon Science, Tohoku University. The production of silicon-ladder assembly was completed by Spring 2022 in the Taiwan
Silicon Detector Facility (TSiDF) and in BNL. And the INTT barrel assembly has been started since June 2022 in BNL. The INTT
barrel is scheduled to be installed to the beam line at the sPHENIX site in January 2023 followed by the sPHENIX commissioning in
February 2023.
As the further investigation of the neutral pion production asymmetry discovered in the RHICf experiment, we started preparation
for the next phase of the experiment, namely RHICf-II. Since physics data taking at RHIC in 2024, which is required for RHICf-II,
will not be implemented, we are considering other possibilities to achieve its physics goal. The highlight of the upgraded experiment is
to adopt a larger acceptance with higher position resolution for the zero-degree calorimeter (ZDC). The detector technology developed
for the FoCAL upgrade project of the ALICE experiment at LHC well satisﬁes the RHICf-II performance requirement. We thus
resumed the associated membership of the ALICE collaboration, and the RHICf-II detectors are to be developed together with the
ALICE FoCAL collaboration. This new detector technology development is also a part of an essential R&D programs for a ZDC
detector for EIC.

Members
Director
Hideto EN’YO
Senior Research Scientists
Yuji GOTO
Itaru NAKAGAWA

Ralf SEIDL
Satoshi YOKKAICHI

Contract Researcher
Tomonori TAKAHASHI
Special Postdoctoral Researcher
Koki KANNO- 309 -

VI. RNC ACTIVITIESRIKEN Accel. Prog. Rep. 55 (2022)

Research Associate
Wataru NAKAI
Junior Research Associate
Tomoki MURAKAMI
Special Temporary Research Scientist
Yasushi WATANABE
Senior Visiting Scientist
Toshiaki SHIBATA (Nihon Univ.)
Visiting Scientists
Kazuya AOKI (KEK)
Wen-Chen CHANG (Inst. of Phys., Academia Sinica)
Tatsuya CHUJO (Tsukuba Univ.)
Shoichi HASEGAWA (JAEA)
Motoi INABA (Tsukuba Univ. of Tech.)Shunzo KUMANO (KEK)
Tsutomu MIBE (KEK)
Yuhei MORINO (KEK)

Ken-ichi NAKANO (Tokyo Tech)
Megumi NARUKI
Kyoichiro OZAWA (KEK)
Susumu SATO (JAEA)
Kenta SHIGAKI (Hiroshima Univ.)
Maya SHIMOMURA (Nara Women’s Univ.)
Kiyoshi TANIDA (JAEA)

Visiting Technician
Che-Sheng LIN (Academia Sinica)
Student Trainees
Daichi ARIMIZU (Kyoto Univ.)
Shunsuke KAJIKAWA (Tohoku Univ.)
Natsuki KURODA (Nara Women’s Univ.)
Shono KYAN (Tsukuba Univ.)
Miu MORITA (Nara Women’s Univ.)
Tomoki MURAKAMI (Univ. of Tokyo)

Satomi NAKASUGA (Kyoto Univ.)
Yumika NAMIMOTO (Nara Women’s Univ.)
Naoki OGATA (Kyoto Univ.)
Mika SHIBATA (Nara Women’s Univ.)
Runa TAKAHAMA (Nara Women’s Univ.)
Po-Hung WANG (Nat’l Central Univ.)

Interns
Hikaru IMAI (Rikkyo Univ.)
Miu MORITA (Nara Women’s Univ.)
Yusuke NAKAMURA (Rikkyo Univ.)
Genta NAKANO (Rikkyo Univ.)

Yumika NAMIMOTO (Nara Women’s Univ.)
Mika SHIBATA (Nara Women’s Univ.)
Runa TAKAHAMA (Nara Women’s Univ.)

AssistantKeiko SUZUKI

List of Publications & Presentations
Publications[Original Papers]
U. A. Acharya et al. (PHENIX Collaboration), “Improving constraints on gluon√ spin-momentum correlations in transversely polarized
protons via midrapidity open-heavy-ﬂavor electrons in p ↑ +p collisions at s = 200 GeV,” e-Print: 2204.12899 [hep-ex], April 27,(2022).
U. A. Acharya et al. (PHENIX Collaboration), “Measurements of second-harmonic Fourier coeﬃcients from azimuthal anisotropies in√
p+p, p+Au d+Au, and 3 He + Au collisions at sNN = 200 GeV,” e-Print: 2203.09894 [nucl-ex], March 18, (2022).
√
U. A. Acharya et al. (PHENIX Collaboration), “Study of φ-meson production in p+Al, p+Au, d+Au, and 3 He+Au collisions at sNN =200 GeV,” Contribution to 2022 Snowmass Summer Study, e-Print:2203.06087 [nucl-ex], March 11, (2022).
√
U. A. Acharya et al. (PHENIX Collaboration), “Measurement of direct-photon cross section and double-helicity asymmetry at s =510 GeV in p + p collisions,” e-Print: 2202.08158 [hep-ex], February 16, (2022).
U. A. Acharya et al. (PHENIX Collaboration),“Measurement of ψ (2S) nuclear modiﬁcation at backward and forward rapidity in p+p,√
p+Al, and p+Au collisions at s = 200 GeV,” e-Print: 2202.03863 [nucl-ex], February 8, (2022).
U. A. Acharya et al. (PHENIX√ Collaboration), “Transverse-single-spin asymmetries of charged pions at midrapidity in transversely
polarized p+p collisions at s = 200 GeV,” Phys. Rev. D 105, 032003 (2022).
U. A. Acharya et al. (PHENIX Collaboration), “Systematic study of nuclear eﬀects in p+Al, p+Au, d+Au, and 3 He+Au collisions at√
sNN = 200 GeV using π0 production,” Phys. Rev. C 105, 064902 (2022).
U. A. Acharya et al. (PHENIX Collaboration), “Transverse single spin asymmetries of forward neutrons in p+p, p+Al and p+Au colli√
sions at sNN = 200 GeV as a function of transverse and longitudinal momenta,” Phys. Rev. D 105, 032004 (2022).

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VI. RNC ACTIVITIESRIKEN Accel. Prog. Rep. 55 (2022)√
U. A. Acharya et al. (PHENIX Collaboration), “Kinematic dependence of azimuthal anisotropies in p+Au, d+Au, and 3 He+Au at sNN =200 GeV, Phys. Rev. C 105, 024901 (2022).
U. A. Acharya et al. (PHENIX Collaboration), “Probing gluon√ spin-momentum correlations in transversely polarized protons throughmidrapidity isolated direct photons in p ↑ +p collisions at s = 200 GeV,” Phys. Rev. Lett. 127, 162001 (2021).
U. A. Acharya √et al. (PHENIX Collaboration), “Transverse single-spin asymmetries of midrapidity π0 and η mesons in polarized p+p
collisions at s = 200 GeV,” Phys. Rev. D 103, 052009 (2021).
U. A. Acharya et al. (PHENIX √Collaboration), “Transverse momentum dependent forward neutron single spin asymmetries in transversely
polarized p+p collisions at s = 200 GeV,” Phys. Rev. D 103, 032007 (2021).
O. Adriani et al. (RHICf Collaboration), “Performance of RHICf detector during operation in 2017,” J. Instrum. 16, P10027 (2021).
T. N. Takahashi et al., “Data acquisition system in the ﬁrst commissioning run of the J-PARC E16 experiment,” IEEE Trans. Nucl. Sci.
68, 1907 (2021).

Presentations[International Conferences/Workshops]
Y. Goto (invited), “Prospective physics study at EIC,” 8th Asian Triangle Heavy-Ion Conference (ATHIC2021), Incheon, South Korea,
Online, Novermber 9, 2021.
T. N. Murakami (poster), “Construction and operation of gas electron multiplier tracker for the J-PARC E16 experiment in run0,” 2021
Virtual IEEE Nuclear Science Symposium, Online, October 16–23, 2021.
K. Kanno (invited), “Study of spectral change of vector mesons in nuclear medium at J-PARC,” 2nd International Workshop on the
Extension Project for the J-PARC Hadron Experimental Facility, Online, February 16–18, 2022.
S. Yokkaichi (invited), “J-PARC E16 has started—spectral change of vector mesons in nuclei—,” Reimei Workshop, “Hadrons in dense
matter at J-PARC”, Tokai, Japan, Hybrid, February 21–23, 2022.
S. Nakasuga (poster), “Commissioning of the electron identiﬁcation system for dilepton measurement in pA collisions at J-PARC,”
VCI2022 (The 16th Vienna Conference on Instrumentation), Online, February 21–25, 2022.

[Domestic Conferences/Workshops]
後藤雄二（招待講演）, 「RHIC Spin Physics」, 日本物理学会 2021 年秋季大会, オンライン, 2021 年 9 月 17 日.
√
後藤雄二（口頭発表）, 「PHENIX 実験での sNN = 200 GeV 偏極陽子＋原子核衝突による超前方中性子のシングルスピン非対称度の横運動量依存性」, 日本物理学会 2021 年秋季大会, オンライン, 2021 年 9 月 16 日.
高橋智則 (招待講演), 「有限密度媒質中でのクォーク相関の解明に向けたレプトン対精密測定」, 第 6 回クラスター階層領域研究会, オンライン, 2021 年 6 月 14 日, 6 月 19 日.
高橋智則 (招待講演), 「J-PARC E16 実験の計測システムの現状と課題」, 計測システム研究会 2021, 九州大学 (福岡市), ハイブリッ
ド, 2021 年 10 月 28–29 日.
村上智紀, 「J-PARC E16 実験 Run0 における GEM 飛跡検出器実機の性能評価」, 日本物理学会 第 77 回 年会, オンライン, 2022 年
3 月 15–19 日.
村上智紀, 「J-PARC 高運動量ビームラインを用いた原子核密度中における φ 中間子質量スペクトルの測定」, J-PARC ハドロン研究会, オンライン, 2022 年 3 月 22–24 日.

[Seminars]
H. En’yo, “Forty academic years with physics of high density nuclear matter (in Japanese),” Heavy Ion Pub, Nagoya University, Online,
November 12, 2021.
H. En’yo, “My 40 years seeking for the new phase of nuclear matter (in Japanese),” ELPH Seminar, Research Center for Electron Photon
Science (ELPH), Tohoku University, Sendai, Japan, Hybrid, December 23, 2021.

Press Release“Direct Photons Oﬀer Glimpse of Gluons’ Dynamic Motion—PHENIX data validate approach for future studies of proton spinand structure—,” October 12, 2011. https://www.bnl.gov/newsroom/news.php?a=119077,https://www.riken.jp/press/
2021/20211015_1/index.html .

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