Ⅵ. ORGANIZATION AND ACTIVITIES OF RNC

RIKEN Accel. Prog. Rep. 56 (2023)

Subnuclear System Research Division
RIKEN BNL Research Center
Experimental Group
1. Abstract

RIKEN BNL Research Center (RBRC) Experimental Group studies the strong interactions (QCD) using RHIC accelerator at
Brookhaven National Laboratory, the world ﬁrst heavy ion collider and polarized p + p collider. We have three major activities: Spin
Physics at RHIC, Heavy ion physics at RHIC, and detector upgrades of PHENIX experiment at RHIC.
We study the spin structure of the proton using the polarized proton-proton collisions at RHIC. This program has been promoted
by RIKEN’s leadership. The ﬁrst focus of the research is to measure the gluon spin contribution to the proton spin. Results fromPHENIX π0 measurement and STAR jet measurement has shown that gluons in the proton carry about 30% of the proton spin. This isa major milestone of the RHIC spin program. The second goal of the spin program is to measure the polarization of anti-quarks in the
proton using W → e and W → µ decays. The results of W → e measurement was published in 2016. The ﬁnal results of W → µ was
published in 2018. The focus of the RHIC spin program is moved to study of transverse spin measurement.
The aim of Heavy ion physics at RHIC is to re-create Quark Gluon Plasma (QGP), the state of Universe just after the Big Bang.
Two important discoveries, jet quenching eﬀect and strong elliptic ﬂows, have established that new state of dense matter is indeed
produced in heavy ion collisions at RHIC. We are now studying the property of the QGP. We measured direct photons in Au + Au
collisions for 1 < pT < 3 GeV/c, where thermal radiation from hot QGP is expected to dominate. The comparison between the data
and theory calculations indicates that the initial temperature of 300 MeV to 600 MeV is achieved. These values are well above thetransition temperature to QGP, which is calculated to be approximately 160 MeV by lattice QCD calculations.
We had major roles in detector upgrades of PHENIX experiment, namely, the silicon vertex tracker (VTX) and muon trigger
upgrades. The VTX is the main device to measure heavy quark (charm and bottom) production and the muon trigger is essential
for W → µ measurement. The results from the ﬁrst run with VTX detector in 2011 was published. The results show that electrons
from bottom quark decay is strongly suppressed at high pT , but the suppression is weaker than that of charm decay electron for
3 < pT < 4 GeV/c. PHENIX recorded 10 times as much Au + Au collisions data in each of the 2014 run and 2016 run. A paper on the
suppression of electrons from charm and bottom decays in the 2014 run was submitted for publication. The data shows clear diﬀerentof the suppression of b → e and c → e.
PHENIX completed its data taking in 2016, and construction of a new detector, sPHENIX, as upgrade of PHENIX was started.
sPHENIX will measure jets, photons, and Upsilon particles and will complete the scientiﬁc mission of RHIC. We constructed aintermediate-silicon tracker INTT for sPHENIX. INTT was completed in 2022 and it was installed in sPHENIX in March 2023. The
sPHENIX will start taking data in 2023.

2. Major Research Subjects(1) Experimental Studies of the Spin Structure of the Nucleon(2) Study of Quark-Gluon Plasma at RHIC
(3) sPHENIX INTT detector

3. Summary of Research Activity

We study the strong interactions (QCD) using the RHIC accelerator at Brookhaven National Laboratory, the world ﬁrst heavy ion
collider and polarized p + p collider. We have three major activities: Spin Physics at RHIC, Heavy ion physics at RHIC, and detector
upgrades of PHENIX experiment. Y. Akiba (Experimental Group Leader) is the Spokesperson of PHENIX experiment since 2016.
(1) Experimental study of spin structure of proton using RHIC polarized proton collider
How is the spin of proton formed with 3 quarks and gluons? This is a very fundamental question in Quantum Chromodynamics(QCD), the theory of the strong nuclear forces. The RHIC Spin Project has been established as an international collaboration between
RIKEN and Brookhaven National Laboratory (BNL) to solve this problem by colliding two polarized protons for the ﬁrst time inhistory. This project also has extended the physics capabilities of RHIC.
The ﬁrst goal of the Spin Physics program at RHIC is to determine the gluon contribution to proton spin. It is known that the spinof quark accounts for only 25% of proton spin. The remaining 75% should be carried either by the spin of gluons or the orbital angularmomentum of quarks and gluons. One of the main goals of the RHIC spin program has been to determine the gluon spin contribution.
Before the start of RHIC, there was little experimental constraint on the gluon polarization, ∆G.
PHENIX measures the double helicity asymmetry (ALL) of π0 production to determine the gluon polarization. Our most recent
publication of π0 ALL measurement at 510 GeV shows non-zero value of ALL , indicating that gluons in the proton is polarized. Global
analysis shows that approximately 30% of proton spin is carried by gluon spin. PHENIX measured the parity-violating single spin
asymmetry AL of the W boson production in p + p in wide rapidity range. The results of the W boson measurements were publishedin 2016 and 2018, and these results give constraints on the anti-quark polarization in the proton. The focus of the spin physics is now
moved to the measurements of the single transverse spin asymmetry AN .
PHENIX measured AN of single electrons from heavy ﬂavored hadrons at mid rapidity. Production of heavy quarks (charm andbottom) is dominated by gluon-gluon fusion, and the measurement is sensitive to the gluon correlators that are related to the orbital
angular momentum of gluons via theoretical models. Figure 1 shows the AN of single electrons from heavy ﬂavor decays. The data

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RIKEN Accel. Prog. Rep. 56 (2023)

Fig. 1. The single spin asymmetry AN of heavy ﬂavor decay electrons (left) and the constraint on the model parameter from the data (right). Published in Phys.
Rev. D 107, 052012 (2023).

are compared with a theoretical model by Z. Kang and J. Qiu, and the two parameters λ f and λd of the model are constrained from thedata. The right panel of Fig. 1 shows the constraint on the model parameters. The parameters are determined as λ f = −0.01±0.03 GeV
and λd = 0.11 ± 0.09 GeV. The size of these parameters are expected to be O(0.1)GeV. This is the ﬁrst constraints on these model
parameters. These results are published in Physical Review D.
(2) Experimental study of Quark-Gluon Plasma using RHIC heavy-ion collider
The goal of high energy heavy ion physics at RHIC is study of QCD in extreme conditions i.e. at very high temperature and atvery high energy density. Experimental results from RHIC have established that dense partonic matter is formed in Au + Au collisions
at RHIC. The matter is very dense and opaque, and it has almost no viscosity and behaves like a perfect ﬂuid. These conclusions areprimarily based on the following two discoveries:• Strong suppression of high transverse momentum hadrons in central Au + Au collisions (jet quenching);• Strong elliptic ﬂow.
These results are summarized in PHENIX White paper, which has more than 3400 citations to date. The focus of the research inheavy ion physics at RHIC is now to investigate the properties of the matter. RBRC have played the leading roles in some of the mostimportant results from PHENIX in the study of the matter properties. These include (1) measurements of heavy quark production fromthe single electrons from heavy ﬂavor decay (2) measurements of J/ψ production (3) measurements of di-electron continuum and (4)measurements of direct photons.
Our most important result is the measurement of direct photons for 1 < pT < 5 GeV/c in p + p and Au + Au through their internalconversion to e + e− pairs. If the dense partonic matter formed at RHIC is thermalized, it should emit thermal photons. Observation ofthermal photon is direct evidence of early thermalization, and we can determine the initial temperature of the matter. It is predicted that
thermal photons from QGP phase is the dominant source of direct photons for 1 < pT < 3 GeV/c at the RHIC energy. We measured
the direct photon in this pT region from measurements of quasi-real virtual photons that decays into low-mass e + e− pairs. Strong
enhancement of direct photon yield in Au + Au over the scaled p + p data has been observed. Several hydrodynamical models canreproduce the central Au + A data within a factor of two. These models assume formation of a hot system with initial temperature of
T init = 300 MeV to 600 MeV. This is the ﬁrst measurement of initial temperature of quark gluon plasma formed at RHIC. Y. Akiba
received 2011 Nishina memorial Prize mainly based on this work.
PHENIX experiment recently measured the ﬂow in small collision systems (p + Au, d + Au, and 3 He + Au), and observed strong
ﬂow in all of these systems. Theoretical models that assume formation of small QGP droplets best describe the data. These results are
published in Nature Physics in 2019.
We constructed VTX detector of PHENIX. VTX is a 4-layer silicon tracker and it is the main device for measurement of charmand bottom quark production in PHENIX. VTX took data from the 2011 to 2016 when PHENIX completed data taking. PHENIX
recorded high statistics Au + Au data, approximately 20 billion events with VTX in each of the 2014 run and the 2016 run. A paper
reporting the results of 2014 run was submitted to Physical Review C.
PHENIX measured the nuclear modiﬁcation factor of J/ψ and ψ(2S ) in the forward and the backward direction in p + Au and√d + Au collisions at sNN = 200 GeV. The results are shown in Fig. 2.
In the forward rapidity region, the suppression of J/ψ and ψ(2S ) is similar. This indicates that initial state eﬀects such as
shadowing dominates in this region. In the backward rapidity region, the stronger suppression of ψ(2S ) than J/ψ is observed. This
result suggests presence of ﬁnal state eﬀect in p + Au collisions. The paper reporting these results was published in Physical Review
C and it was selected as Editor’s suggestion.
√
PHENIX measured low pT direct photons in Au + Au collisions at sNN = 39 and 62 GeV. The yield of these photons exceeds
what is expected from p + p collisions at the same energy and it is consistent with large contribution from thermal photons from a
hot quark-gluon plasma. Compared with the data of other collision energies, the data suggest that the bulk of thermal photons are- 362 -

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RIKEN Accel. Prog. Rep. 56 (2023)

Fig. 2. The nuclear modiﬁcation factor of J/ψ and ψ(2S ) in small collision systems as function of rapidity. Published in Phys. Rev. C 105, 064912 (2022).

Fig. 3. Eﬀective temperature, T eﬀ , from the low pT direct photon spectra as function of the collision energy. Published in Phys. Rev. C 107, 024914 (2023).

emitted near the transition of the quark-gluon plasma. The paper reporting these results was published in Physical Review C, and it
was selected as Editor’s suggestion.
(3) sPHENIX INTT detector
The group had major roles in several PHENIX detector upgrades, namely, the silicon vertex tracker (VTX) and muon trigger
upgrades. VTX is a high precision charged particle tracker made of 4 layers of silicon detectors. It is jointly funded by RIKEN andthe US DOE. The inner two layers are silicon pixel detectors and the outer two layers are silicon strip detectors. Y. Akiba is the project
manager. The VTX detector was completed in November 2010 and subsequently installed in PHENIX. The detector started taking
data in the 2011 run. With the new detector, we measure heavy quark (charm and bottom) production in p + p, A + A collisions
to study the properties of quark-gluon plasma. The ﬁnal result of the 2011 run was published. The result show that single electrons
from bottom quark decay is suppressed, but not as strong as that from charm decay in low pT region (3 < pT < 4 GeV/c). This is
the ﬁrst measurement of suppression of bottom decay electrons at RHIC and the ﬁrst observation that bottom suppression is smaller
than charm. We have recorded 10 times as much Au + Au collisions data in each of the 2014 run and 2016 run. The results ofbottom/charm ratios in p + p collisions at 200 GeV from the 2015 run was published (Phys. Rev. D 99, 092003 (2019)). A paper
reporting measurements of the nuclear suppression factor RAA of charm and bottom in Au + Au collisions from the 2014 data wassubmitted for publication to Physical Review C.
PHENIX completed its data taking in 2016. We constructed intermediate silicon tracker INTT for sPHENIX, a new experiment at
RHIC that will start taking data in 2023. INTT was completed in fall 2022 and it was installed in sPHENIX in March 2023. sPHENIX
will start taking data in June 2023.

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RIKEN Accel. Prog. Rep. 56 (2023)

Fig. 4. Left: Completed two halves of INTT detector with members of INTT teams. Right: INTT detector and INTT team just after installation to sPHENIX.

Members
Group Leader
Yasuyuki AKIBARBRC Researchers
Jan C. BERNAUER
Genki NUKAZUKA
Yuji GOTO

Itaru NAKAGAWA
Ralf SEIDL
Atsushi TAKETANISpecial Postdoctoral Researcher
Minho KIM
Junior Research Associate
Takuya KUMAOKA
Special Temporary Research Scientists
Takashi ICHIHARA

Yasushi WATANABE

Senior Visiting Scientist
Rachid NOUICER (BNL)
Visiting Scientists
Stefan BATHE (City Univ. of New York)Takashi HACHIYA (Nara Women’s Univ.)
Gaku MITSUKA (KEK)

Takao SAKAGUCHI (BNL)
Takashi SAKO (Univ. of Tokyo)
Milan STOJANOVIC (Purdue Univ.)

Visiting Researcher
Shima SHIMIZU (JSPS)
Student Trainees
Joseph T. BERTAUX (Purdue Univ.)
Kai-Yu CHENG (Nat’l Central Univ.)
Jaein HWANG (Korea Univ.)
Seunghwan LEE (Sejong Univ.)

Han-Sheng LI (Purdue Univ.)
Cheng Wei SHIH (Nat’l Central Univ.)
Wei-Che TANG (Nat’l Central Univ.)
Maki WAKATA (Rikkyo Univ.)

Research Part-time Worker
Hikaru IMAI

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RIKEN Accel. Prog. Rep. 56 (2023)

List of Publications
Publications[Original Papers]
N. J. Abdulameer et al., “Improving constraints oon gluon
spin-momentum correlations in transversely polarized protons vias midrapidity√
open-heavy ﬂavor electrons in p + p collisions at s = 200 GeV,” Phys. Rev. D 107, 052012 (2023).
√
N. J. Abdulameer et al., “Low pT direct-photon production in Au + Au collisions at sNN = 39 and 62.4 GeV,” Phys. Rev. C 107, 024914(2023).
N. J. Abdulameer et al., “Measurement of second-harmonic Fourier coeﬃcients from azimuthal anisotropies in p + p, p + Au, d + Au,√
and 3 He + Au at sNN = 200 GeV,” Phys. Rev. C 107, 024907 (2023).
√√
N. J. Abdulameer et al., “Measurement of ψ-meson production in Cu + Au at sNN = 200 GeV and U + U collisions at sNN = 193 GeV,”
Phys. Rev. C 107, 014907 (2022).
√
U. A. Acharya et al., “ψ meson production in p + Al, p + Au, d + Au, 3 He + Au collisions at sNN = 200 GeV,” Phys. Rev. C 106, 014908(2022).
U. A. Acharya et al., “Measurement of ψ(2S ) nuclear modiﬁcation at backward and forward rapidity in p+ p, p+ Al, and p +Au collisions√
at sNN = 200 GeV,” Phys. Rev. C 105, 064912 (2022).
√
U. A. Acharya et al., “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).

Award
Y. Akiba, “Research of High temperature and high density matter through relativistic heavy ion collisions,” Commendation for Scienceand Technology by the Minister of Education, Culture, Sports, Science, and Technology for 2023.

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