RIKEN Accel. Prog. Rep. 50 (2017)

Research Activities 1991–2016

Pursuit of Spin Physics Program at RHIC
M. Ishihara ∗1,∗2by G. Bunce and T. Roser. Thus, these two groups
from US and Japan were destined to take the leading role in promoting the RHIC Spin Physics Program. To help such practice G. Bunce was invited
to serve as the Deputy Leader of the Experimental
Group of RBRC while I served as the Leader. Aswas always the case for the RHIC project, this program was conducted under a framework of international collaboration. Between the two major collaborations formed for the RHIC project, we have chosen thePHENIX Collaboration, rather than the STAR Collaboration, since we anticipated close cooperation with alarge number of Japanese researchers who had been
already registered in the former Collaboration. TheRadiation Laboratory group in the early stage consisted of N. Saito, Y. Goto, N. Hayashi, K. Kurita,
M. Okamura, S. Yokkaichi, T. Ichihara, Y. Watanabe,
A. Taketani, and J. Murata besides myself. The group
was further reinforced by acquiring several fellows atRBRC, including M. G. Perdekamp, A. Deshpande,and A. Bazilevsky. They soon manifested themselves
to become leading ﬁgures in the Spin Physics Program.
Throughout the 1990s, our research activities weremainly devoted to the construction of the PHENIX
detector assembly and of accelerator devices requiredfor polarized protons. Speciﬁcally, the RIKEN group
was committed to the construction of one unit of muonarm, where the magnet assembly was manufactured by
Mitsubishi Electric Co. under the direction of T. Ichihara while A. Taketani and K. Kurita took major roles
in completing detector arrays for muon ID and J. Murata for muon tracker. The development of a laserdriven ion source was an important R&D issue to realize an intense polarized proton beam, and M. Okamura made a central contribution to its accomplishment. Another important agenda was to develop thepolarimeter, which is crucial to determine the degreeof spin polarization of the accelerated protons. TheRIKEN group led by N. Saito and K. Kurita wassuccessful in establishing the relevant methodology.2)
Meanwhile, Y. Goto worked on the EM calorimeter inthe central arm of the PHENIX assembly to make it
useful for the Spin Physics Program. M. G. Perdekamp
and A. Bazilevsky also joined in this activity. In fact
the π 0 detection as described later was facilitated withthis device.
At Wako a computer facility called the Computing
Center in Japan for RHIC physics (CC-J) was built 3)
under the leadership of T. Ichihara. Y. Watanabe andS. Yokkaichi also made great contributions to the development and operation of the CC-J. With a computation speed equivalent to 160 high-performance CPUs
and a disk capacity of about 135 TB supplemented byThroughout 1990s, the Radiation Laboratory directed by M. Ishihara was engaged in two major programs: ﬁrstly the RIB Physics Program using RIPS
at RARF, RIKEN and secondly the Spin Physics Program using the Relativistic Heavy-ion Collider (RHIC)
at Brookhaven National Laboratory (BNL), USA. This
article describes the latter engagement by emphasizing
the aspects of historical evolution of the program.
Our engagement in the international collaboration
program on spin physics was started in 1995, aiming
at clariﬁcation of the anomalous phenomenon of proton spin crisis,1) which casted a fundamental question
about the origin of nucleon spin. As a matter of fact,
our intensive commitment served to implement thisprogram as a ﬂagship objective of the RHIC project,
being alternate with the Heavy Ion Physics Program as
originally founded. The RIKEN BNL Research Center(RBRC) was created in 1997 to support our activities
on this program. The spin program has been successful
in revealing the important roles of gluons and parton
orbital angular momenta, and further extension of theprogram has motivated the proposal of the US nextgeneration project of Electron Ion Collider (EIC).
The Spin Physics Program is one of the two principal programs at RHIC, BNL, aiming at the determination of the polarized parton distribution functionof nucleons. Our initiative to commit to this program followed a recommendation of the headquarters
of RIKEN, which strongly encouraged us to set up any
cutting-edge program on nuclear physics to be conducted in international collaboration. The RHIC Spin
Physics Program turned to reality when the Radiation
Laboratory gave its commitment to this program bybringing a sizable grant from the Science and Technology Agency (STA) to build a full set of Siberian snakes
and spin rotators. Indeed, these devices were essential
to facilitate the acceleration and manipulation of spinpolarized protons through the rings of RHIC. A grant
was also obtained to build a single unit of muon arm
for the PHENIX detector assembly. Moreover, RBRC
was created in the BNL campus to support our research activities at the site of the RHIC facility.
Since we were quite new to the ﬁeld of relativistic
QCD physics, our major eﬀorts in the initial stage weredevoted to call for relevant international collaborators.
The Japanese spin physics team was soon formed, consisting of groups from Kyoto University and Tokyo Institute of Technology as well as the RIKEN Radiation
Laboratory. We also managed to establish a strong tie
with the powerful spin physics group around BNL led∗1∗2

Chief Scientist of Radiation Laboratory (1984–2000)
Group Leader of Experiment Division, RBRC (1998–2001)

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Research Activities 1991–2016

RIKEN Accel. Prog. Rep. 50 (2017)

gluon behaviors in the small x region. A. Deshpande atthe Experimental Group of RBRC has been a central
force to work out this proposal.
Meanwhile, the Theory Group of RBRC has developed in its own way. Since T. D. Lee took the Directorship of RBRC, he extended the center objective toserve as a world center for nursing brilliant young scientists in theoretical physics related to QCD. To enhance this activity, a novel system of joint appointment between any of the US universities and RBRC
was introduced. This initiative, strongly pursued bythe Director himself, resulted in great success, producing many leading nuclear theorists worldwide, such as
D. E. Kharzeev, who now directs the Theory Group ofRBRC.

tape storage capacity of 1400 TB, the CC-J immediately gained the status of the primary center for data
analysis for the Spin Physics Program. It also served
as a principal Asian local center for the PHENIX Collaboration by allowing Japanese and Asian colleagues
to analyze the data from the Heavy-Ion Program aswell. The CC-J has been in sound operation since its
commissioning in 2001, being reinforced by a series ofsuccessive upgrades. It has thus far contributed to thepublication of 42 papers and awarding of 42 Ph.D. degrees.
The Spin Physics Program at RHIC was originallyinspired by the so-called spin puzzle that the quarkcontribution alone is not suﬃcient to account for thenucleon spin of 1/2. Hence, the extent of gluon contribution was inquired to pin down the anomalous phenomenon. In this respect, spin-polarized proton beams
at RHIC were indispensable since collisions with protons well reveal various gluon-induced processes, which
in turn may exhibit the polarized gluon distribution of
the nucleon. Many simulation works were then performed to identify the observables most sensitive tothe spin-polarized gluons. Thus, the measurementof√
the double-helicity asymmetry of π 0 at s = 200 GeV
was chosen to be the ﬁrst experiment to be performed.
RHIC was commissioned in the year 2000, while theleadership of this program has been taken over since2001 by H. En’yo, the newly assigned Chief Scientist of the Radiation Laboratory. The experiment on
the double-helicity asymmetry of π 0 was realized in2003 4) and several succeeding years.5) In later years
the measurement of double-helicity asymmetry was extended
to other cases, such as at diﬀerent energies of√s = 62.4 6) and 510 GeV 7) and with diﬀerent probeslike the mid-rapidity jet.8) These data were combined
to be subjected to global QCD analysis, revealing thebehavior of the polarized gluon distribution over therange of Bjorken x, 0.05 < x < 0.2. The gluon spinpolarization integrated over the observed x region is
reduced to 0.15 ± 0.06 9) or 0.10 + 0.06/ − 0.07 in theunit of Planck constant,10) depending on the methodof global analysis. These values are roughly comparable with the known quark contribution of 20–30% of
the total spin of 1/2. Consequently the combined contributions of gluons and quarks so far observed is still
insuﬃcient to account for the total spin.
Thus, missing polarized spin should be sought forover the gluons in the domain of very small x and/orin another degree of freedom, i.e., the orbital angularmomenta carried by partons. In fact, several experiments performed at RHIC revealed a non-vanishinganalyzing power,11) indicating the signiﬁcant roles ofparton orbital angular momenta. The study of transverse components of parton spins is called for. As amatter of fact, the EIC project for the next-generation
parton physics aims at the investigation of generalized(3-dimensional) parton distributions as well as of theReferences
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2) J. Tojo et al., Phys. Rev. Lett. 89, 052302 (2002).
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Lett. 93, 202002 (2004).
5) e.g., A. Adare et al. (PHENIX Collaboration), Phys.
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6) A. Adare et al. (PHENIX Collaboration), Phys. Rev.
D 79, 012003 (2009).
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