RHIC SPIN PROJECT – OPENING A NEW ERA OF HADRON PHYSICS –HIDETO EN’YO
RIKEN (The Institute of Physical and Chemical Research), Wako, Saitama 351-0198, Japan
andRIKEN- BNL Research Center, Brookhaven National Laboratory, Upton, NY 11973, USA
E-mail:enyo@riken.go.jp

Polarized pp collisions at 200 GeV of the center of mass energy will take place for the firsttime in December 2001, which opens a unique opportunity to study the spin structure of thenucleon and the symmetries in nature. This new field of physics is opened under the treaty
between RIKEN and Brookhaven National Laboratory by adding necessary equipments in
RHIC, Relativistic Heavy Ion Collider. The physics program with polarized beams at RHIC isoverviewed

1.

Introduction

Although we are still remembering the sudden termination of SSC, there wasanother collider called ISABELL abandoned in 1983 at Brookhaven National
Laboratory leaving a tunnel with a circumference of 3.85 km. The project wasreborn after more than 15 years of sleep as the first heavy ion collider and the firstpolarized proton collider. The new collider RHIC consists of two independent rings
and is able to collide any nuclei on any other nuclei with the nucleon-nucleon centerof mass energies S of 48 - 500 GeV in proton-proton collisions and 22 - 200 GeV
in gold-gold collisions. Luminosities are expected to reach 2 x 1032 in p-p and 2 x1026 in Au-Au collisions.
In year 2000, the first collision was achieved for Au on Au at S of 130 GeV,
and commissioning of polarized proton acceleration was successfully done. In year
2001 at the time of this symposium we are already taking the data at S = 200 GeV
for Au-Au collisions, at the time of writing this manuscript the machine study ofpolarized proton acceleration is taking place, and we expect the first collision to beachieved in the middle of December of year 2001. This manuscript describes the
RHIC spin project with physics highlights expected from this new Spin Collider.
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2.

Physics of RHIC spin project

Nucleon is a bound state composed of 3 valence quarks glued together withgluon. It has spin 1/2 so that the simplest spin picture of nucleon is that the threequarks are in the s state with spin directions of (1/2,1/2, -1/2) configuration. This
picture was very naïve but quite successful in describing the magnetic moments ofmany baryons. In this picture the fraction ∆Σ of proton spin carried by quarks(∆u, ∆d, ∆s) is unity,∆Σ = 1 = ∆u + ∆d + ∆s .
Sehgal already pointed out theoretically the fragileness of this simple picture in
1974[1]. He took the Bjorken sum rule[2] and neglected s-quark polarization, andconcluded that ∆Σ=0.6. To convince this puzzle experimentally the spin structure of
the nucleon has been studied for over two decades using deep-inelastic scattering of
longitudinally polarized leptons off a longitudinally polarized nucleon target. Those
measurements have reached the conclusive results around 1989[3,4],∆Σ = 0.1 ~ 0.3 ,
namely this puzzle is even a crisis of our understanding of the nucleon spin
structure. Apparently there is a significant spin carrier other than quarks, and weneed to go back to the most basic spin sum rule;1 1= ∆Σ + ∆g + Lz ,2 2

where ∆g is the spin carried by gluon and Lz is the angular momentum of quarks andgluons. More generally, we need to understand the proton in term of the polarized
parton distribution function;

1
1
1
1= ³ dx[ ∆u ( x) + ∆d ( x) + ∆s ( x) + ∆g ( x)] + Lz ,22
22
to obtain a complete picture of the nucleon spin structure. Another important
implication from the deep-inelastic scattering experiments was that ∆s ≠ 0 , i.e. thestrange quark polarization is not zero although the total strange content in the
nucleon is zero. This also indicates the anti-quarks (sea quarks) are possibly bepolarized as well. Thus the important questions remain are; 1) is the gluon
polarized? 2) is the sea quark polarized? and 3) is there angular momentum exist
in the nucleon?
The RHIC spin project is planned to solve most of the issues described above.
Especially direct measurements of gluons and anti-quarks are the highlights over the
deep-inelastic scattering, since virtual photons from an incident lepton can neither
probe the neutral partons (gluons ) nor identify antiquarks from quarks.
The other new category, to be measured at RHIC Spin Collider, is the transversity
distribution, which is the parton structure function, to appear in the collision of atransversely polarized proton on a transversely polarized proton. Although thisstructure function is known to exist in the leading order, no experimental
measurements are done so far. The measurements will become available in RHIC
and complete Table 1, i.e. to complete our picture of the nucleon spin structure.

Table1: parton distributions of the nucleon in the leading order. Horizontal and verticalarrows denote spin direction of parton in helicity and transverse direction, respectively.

Quark

Gluon

Helicity distribution q( x) = q + q = q ↑ + q ↓ ∆q ( x) = q − q g ( x) = g + g ∆g ( x) = g − gTransversity distribution

δq ( x ) = q ↑ − q ↓Not applicable

Spin average distribution

Once these spin structure functions are measured to a reasonable precision, thepolarized proton beams can be regarded as quark and gluon beams with known
luminosities and energies, which can be used to explore searches of new physics.
Especially parity violating effects can be extracted by using longitudinally polarized
quark beams, which can potentially reveal the substructure of a quark[5].
3.

The first polarized collider: RHIC

The layout of the RHIC polarized collider is shown in Figure 1. The major
machine parameters are listed in Table 2. There are five experiments, Brahms, Star,
Phenix, Phobos, and pp2pp. Amongst those, Star, Phenix, and pp2pp are to performspin physics measurements, and the potential of spin physics with Brahms andPhobos is also being studied. It is worth mentioned that the energy region covered
with this program is more than a factor of 10 higher comparing to previous fixed
target experiments.
To realize polarized proton collisions at such energy, many aspects are changed
and added to the original RHIC design. The optically pumped polarized ion source
was supplied from KEK, modified in TRIUMPH and moved into the BNL Linac. A
solenoidal magnet and an RF dipole were installed in AGS (Alternate GradientSynchrotron) to keep the polarization up to 24GeV of the injection energy to the
RHIC main ring.
As the most important equipment for the spin collider, technology to fabricate asuper-conducting helical dipole magnet was developed. This device is essential torealize the Siberian Snake magnets and the Spin Rotators in RHIC. The Snakemagnets are used to maintain polarization in accelerating and storing beams by
flipping spin direction by 180o around the beam in every half or full revolution tocancel the accumulated spin precession. The Spin Rotators are used to change thespin direction at collision points as needed for the experiments and to return it backto the original direction. Those Spin Rotators are to be installed for the two large
experiments, Phenix and Star.

Figure 1: Outline of RHIC polarized collider
RHIC pC Polarimeters

PHOBOS

Absolute Polarimeter

RHIC
PHENIXSTAR
Siberian Snakes

Spin Rotators

Pol. Proton Source
500 µA, 300 µs

2 × 1011 Pol. Protons / Bunch
ε = 20 π mm mrad

AG S
AGS Internal Polarimeter

200 MeV Polarimeter

RfDipole

Table 2: RHIC accelerator specification.

Configuration
Two Concentric Super-conducting magnet Rings
Circumference
3.8 kmInteraction point
6
Number of Bunches 60; 120 in enhanced mode
Ion Species
Ranges from proton to Gold, p+A /A’+A are possible
p+p
Au+Au
Injection &Linac-Booster-AGS
Linac-Booster-AGS
Ecm(Maximum)
500 GeV
200 GeV
Luminosity
2x1032/cm2/s
2x1026/cm2/s
Polarization
70%
In year 2000, following a successful physics run with gold ion beams, wecommissioned one RHIC ring with a polarized proton beam. In the commissioning,
the polarimeter is the most critical device to tune the machine for the maximum
polarization. We have developed the polarimeter basing on proton-Carbon elastic
scattering in the Coulomb-Nuclear Interference (CNI) region and installed in the
RHIC ring. The analyzing power AN is expected to be as large as 4% at t = 3x10-3(GeV/c)2. The recoiled Carbon was detected with Si-strip detectors which areplaced 15 cm from a carbon target surrounding azimuthally around the beam line at
φ =45o, 135o, 225o and 315o. This configuration allows us to measure the directionof the polarization vector.
The proton beam with vertical polarization was injected to the RHIC ring at
24.3GeV (Gγ=46.5) 1 with the Siberian Snake off. In this case, the stable spin
direction is vertical, and the measured asymmetry clearly showed verticalpolarization. The polarization is estimated to be 19 ± 1% (see Figure 2). Then weturned the Snake on adiabatically to rotate the spin to the horizontal direction.
The beam was accelerated to 25.1GeV (Gγ=48), where we expected the spin
direction to be rotated by 180o, and we observed the spin vector as expected. It isalso demonstrated that no polarization was maintained with the snake off. Throughthis test the principle of the snake magnet operation was proven.

Figure 2: Measured asymmetries during the spin commissioning. Measured polarization vectors areshown in (top left) Gγ=46.5 with the Snake off, and (top right) with the Snake on, (bottom left) Gγ=48.0
with the Snake on and (bottom right) with snake off.

1

Gγγ=46.5 Injection Energy
Snake OFF: Vertical polarization

Gγγ=46.5 Injection energy
Snake ON: Horizontal polarization

Gγγ= 48 Acceleration with Snake ON
Horizontal polarization

Gγγ= 48 Acceleration with Snake OFF
Polarization lost

G is anomalous magnetic moment of the proton and γ is a Lorentz factor. Gγ isoften used rather than beam energy, because it represents the number of spin
precession per turn.

4.

The highlights of physics outputs for the first few years

Here we review the physics highlights assuming the integrated luminosities of
800pb-1 for s =500GeV and 320pb-1 for s =200GeV, which correspond to 100
days of running with 50% efficiency with full luminosities. We are expecting toobtain such integral luminosities in the first few years of the RHIC operation.
Measurements in the RHIC spin program
To obtain a complete picture of the spin structure of the nucleon, it is essential to
combine spin-dependent hadron-induced reactions with different probes. In the
lepton deep inelastic scattering, virtual photons are only sensitive to electric charge;
therefore it is difficult either to separate flavors or to probe gluons. On the other
hand in hadron collision the Drell-Yan process producing lepton pairs guarantees
the main contribution come from quark-antiquark annihilation. Prompt photon
production is dominated by gluon Compton scatterings, which measures gluon-quark
collisions. The gluon-fusion process dominates open heavy flavor productions. Suchselectivity is fascinating merit of hadron-induced interaction.
Spin asymmetries for various reaction channels in polarized p+p collisions arelisted with the primary goals of their measurement in Table 3.
Table 3: Spin asymmetries for various pp reaction channels with major goals of their measurements.
References shown do not necessarily represent the initial work.

Channel

AL

ATT

AN

-

0 [7]

Twist-3 [8]∆g × ( ∆g+ ∆Σ)[9]Sub quark [10]

0 [7]

-

pp → QQ∆g × ∆g[11]Z, Higgs
doublet[12]--pp → W + ⋅ X∆u × ∆d [13]∆u × ∆d [13]~ 0[14]-

pp → W − ⋅ X∆u × ∆d [13]∆u × ∆d [13]~ 0[14]-

pp → γ * ⋅ X∆q × ∆q [15]γ*/Z mixing[16]δq × δq [17]Twist-3[18]

pp → γX
pp → jet ⋅ X

ALL∆g × A1 [6]P

Gluon Helicity Distribution
The unpolarized gluon distribution has been studied using the cross sections forprompt photon, jet and heavy quark productions in hadron collisions, both in fixed
target and collider experiments. In addition Q2-evolution of the structure function

F2(x, Q2), obtained in lepton scattering experiments, has been used to constrain thegluon distribution in the small-x region. These efforts have led to a gluon
distribution with a reasonable precision.
For the polarized case, by the Fermilab E704 experiments, helicity asymmetries
ALL for high-mass multi-γ pair production in polarized pp collisions has beenmeasured at S =19.4 GeV[19]. The data were compared with the model
calculation of ALL and provided a restriction on ∆g, but the statistical significancewas marginal. In lepton scattering experiments, ∆g(x) has been determined throughQ2-evolution of the spin dependent structure function g1(x, Q2) but the uncertainty is
still too large and leaves a room for any models[20].
Recently a proposal to use pairs of oppositely charged hadrons to probe thephoton-gluon fusion process to access gluons in lepton scattering experiments has
been made [21]. The HERMES experiment at DESY has measured the asymmetry
and gave the first finite ∆g values with large errors[22]. The interpretation, however,is still controversial due to a difficulty of theoretical treatments.
At RHIC, the best ∆g measurement will be done with prompt photon productions.
Its longitudinal double-spin asymmetry is expressed as;
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ALL =



2
σ (++) − σ (+−) ∆g ( x1 ) Σ q eq [∆q( x2 ) + ∆q ( x2 )] ≈aLL (qg → γq) ,σ (++) + σ (+−) g ( x1 ) Σ q eq 2 [∆q( x2 ) + ∆q ( x2 )]where a LL is the partonic level asymmetry and is calculable in perturbative QCD,
and the quark part is well knows as A1P from lepton deep-inelastic scatterings (DIS).
The only unknown, ∆g(x)/g(x), is thus deduced from the measurement.
Figure 3: Left) Present precision of the gluon helicity distribution restricted from all theavailable DIS data. Right) Precision to be achieved after 1-year data in RHIC. These graphs
are given from the Asymmetry Analysis Collaboration (AAC: M. Hirai et al.) and are
preliminary .

To visualize the importance of the RHIC gluon measurements, Figure 3 illustrates
the present situation of ∆g(x)/g(x) determination (left) and the one after 1 year of
RHIC measurements (right). The left figure shows the ∆g(x)/g(x) uncertainty
constrained by all the available data up to now. One can find a great improvement is
expected for the determination of the gluon polarization. It is worth mentioned that
this analysis even assumes a shape of ∆g(x), which is still not known at all.

Flavor Decomposition of Quark and Anti-Quark, Transversity
Figure 5:Polarization of u, d , u , d quarks as functions of x, modeled by Bourreley-Soffer,and Gehrmann-Stirling. Sensitivities of the RHIC Spin W measurements are shown.

At RHIC, the parity violating asymmetry in W productions will directly measure
the polarization of identified quark and antiquark, namely;∆u ( x1 )d ( x2 ) − ∆d ( x1 )u ( x2 )andu ( x1 )d ( x2 ) + d ( x1 )u ( x2∆d ( x1 )u ( x2 ) − ∆u ( x1 )d ( x2 ).
AL (W − ) =u ( x1 )d ( x2 ) + d ( x1 )u ( x2

AL (W + ) =These measurements will be done at S =500GeV by detecting high pT leptons.
The sensitivity with Phenix Muon Arms is shown in Figure 5. Similar quark
identification is not possible in lepton scattering experiments without additionalmodel assumptions, while the W production explicitly separates them.
In addition to the helicity distributions of quarks and anti-quarks, RHIC Spin isalso sensitive to the transversity distributions. One of the cleanest measurements
will be provided by the Drell-Yan production of lepton pairs. One year sensitivity inPhenix Muon Arm at s =200 GeV is shown in Figure 6. Although the sensitivity
seems marginal in this plot, the measurement will serve as the first and cleanest
measurement and will become powerful when the proposed upgrade in luminosityand energy are achieved. Recent theoretical work suggested another effective
measurement for transversity using two-pion correlation[23], which will be tested in
RHIC and will possibly improve our sensitivity to large extent.
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Figure 6: Double transverse-spin asymmetry for Drell -Yan dimuon production at
S NN =200GeV. Phenix Muon Arm sensitivity of 1-year measurements is shown.

5.

SUMMARY

We have reviewed the physics highlights with the first polarized proton collider,
RHIC, and its status towards the first spin physics runs in 2001. Further descriptions
can be fond in the reference[24]. With the RHIC spin program we will see the deep
insight of nucleon that has been unknown over 20 years, and with the original planof heavy ion collision we will create the new state of matter, Quark Gluon Plasma.
The both programs together are opening a new era of hadron physics in the
beginning of 21st Century.
Acknowledgments
The idea of RHIC spin collider emerged in 1990. Since then so many people
contributed to realized the project for over 10 years. The author is very muchgrateful to those who have supported the project from various aspects. We are
finally taking off. It should be noted again that without RIKEN this project has
never been in life. The author would like to thank the organizers of this symposium
and whishes to have more Italian friends participating in RHIC spin physics
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