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Physics with RHIC Spin Collider
Hideto En’yoa ba

RIKEN, Hirosawa 2-1, Wako, Saitama, 351-0198 Japanb

RIKEN BNL Research Center, BNL, Upton, NY, 11973 USA

The year 2002 has been very exciting and√ successful for the RHIC spin program, withthe ﬁrst collisions of polarized protons at s = 200 GeV, which is an order of magnitude
higher energy than any previous polarized proton collisions. The achievements in the ﬁrst
year of the polarized collider are reported.

1. INTRODUCTION
The ﬁrst polarized proton collider started its operation at RHIC in December 2001,
when RHIC became not only Relativistic Heavy Ion Collider to ﬁnd a new state of matter,
quark gluon plasma, but also the Spin Collider to solve the spin puzzle, how the spin of
the proton is made of its constituents.
It is worthwhile to mention that the US-Japan collaborations have played essential
roles toward this accomplishment. The collaboration between KEK and BNL made theKEK polarized proton source available as the ion source for RHIC, and the RIKEN-BNL
collaboration made the snake and spin-rotator magnets available in the RHIC main ring.
In this manuscript, we describe how the machine behaved in the ﬁrst year operation asa Spin Collider and why this machine built, i.e. the physics of the Spin program. Thenwe highlight some of the results from the ﬁrst year of running.
2. OPERATION OF POLARIZED PROTON COLLIDER
Figure 1 shows a schematic view of the RHIC Spin Collider complex. Polarized protons
are accelerated in a LINAC and then in a BOOSTER. Up to this point there are no
depolarization resonances. In the AGS, the partial Snake made from a solenoidal magnet
was newly introduced to preserve the proton polarization in the AGS. After the AGS, theprotons are transferred to RHIC to be accelerated to 100 GeV in the last run, or eventually
to 250GeV in future runs. Here some inventions and investments were required.
The key elements in the RHIC main ring, Siberian Snake, consist of four helical dipole
magnets. The helical dipole was the major invention needed to realize the Spin Collider,because it rotates the spin of a proton by 180 degrees with minimum excursion of the
proton’s orbit. Two snakes in each ring have the snake axis orthogonal to each other.
The snake axis is the vector around which spin is rotated. With this trick, the Larmor

2Figure 1. Layout of the RHIC Spin Collider

precession of the proton spin becomes π per turn independent from the energy of proton1
so that the polarization can be kept up to the top energy of 250 GeV.
Another important invention was the RHIC polarimeter, located near the 12 o’clockinteraction point. This polarimeter contains six silicon detectors in each ring, placed
azimuthally about the beam. At the center, a very thin carbon ﬁlament is moved into thebeam during the measurements. With this device, recoil carbons elastically scattered byprotons in the beam to 90 degrees are detected. The scattered carbons with energy of afew hundred keV are sensitive to the proton spin due to the Coulomb-nuclear interference(CNI). Analyzing power of this device was measured at the AGS 2 . It is like one percent
depending on t[1]. A wave form digitizer equipped with FPGA electronics selects thesecarbons and makes 0.5 MHz operation possible. So, in a minute, the amplitude and theorientation of the polarization vector are determined.
We had 5 weeks of polarized proton run and in the latter half the accelerator delivered
an average luminosity of 0.5 × 1030 cm−2 s−1 with the highest record of 1.8 × 1030 cm−2 s−1 .
During that period PHENIX got 150 nb−1 and STAR got 300 nb−1 .
Figure 2 shows the polarization of each ring( the Blue and the Yellow) over the run
period, as measured with the RHIC polarimeter. Good news was polarizations are there
in all the runs. So Snake worked well. Bad news was relatively low polarization like
15-20% while we have expected 70%.
The limitation of polarization was in the AGS. It was very unfortunate that, just before
the run, the power generator for the AGS malfunctioned and had to be replaced by a lesspowerful spare. With this supply, the ramp up speed was reduced by two, so the eﬀectof spin resonances was doubled. For the next year, the power generator has been ﬁxed
already and the ion source was improved from 70% to 80%. So, we expect about 50%
1

Without the snake magnets, the precession angle is 2πGγ per turn, where G is the abnormal magnetic
moment of proton. Depolarization resonance occurs when Gγ becomes integer (every 523MeV). The
snake magnets eliminate those resonaces by canceling the precession in each turn.
2
The energy dependence of the CNI analyzing power is expected to be small. A polarized proton gas jettarget is under development to calibrate it in an absolute scale.

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Figure 2. Polarization of the Yellow(left) and Blue(right) beams throughout the run,
January 2002.

of polarization in the coming run. In the future, a new AGS helical snake is underdevelopment. With the new AGS snake, we do not have any polarization losses in the
AGS. We will have 70-80% polarization in the RHIC ring.
Next year we will have more ions in a bunch, the number of bunches will be increased
from 55 to 110 as designed, the β ∗ tune was 3 m which will be 1 m as designed. So,
we anticipate almost a factor of ten increase in the luminosity for the next run with thepolarization of about 50%. It will take some time to get higher luminosity as all theaccelerators in history, and both the experiments and accelerator are on a great learning
curve.
3. PHYSICS WITH SPIN COLLIDER
There are 3 spin experiments at RHIC. STAR and PHENIX are the larger ones.
PP2PP2 is a smaller one 3 . Both STAR and PHENIX are equipped with spin rotators, almost same device as the snake magnet. Any type of the spin direction can bechosen at these collision points without any interference to other experiments. PP2PP is
to measure elastic scattering. They use silicon detectors in two Roman Pots in the bothsides of the collision point. To reach small t below 0.004 GeV/c2 , they require a diﬀerent
setting of beam, larger β ∗ like 10m. So they cannot always run together with others.
The RHIC spin collider was born to solve the spin puzzle. To obtain a complete pictureof the spin structure of the nucleon, it is essential to combine spin-dependent hadroninduced reactions with diﬀerent probes. In the lepton deep inelastic scattering, virtualphotons are only sensitive to electric charge. Therefore, it is diﬃcult either to separate
ﬂavors or to probe gluons.
The ﬁrst target with RHIC is ∆g, the gluon polarization in nucleon. With Spin Col3

Another experiment BRAHMS is planning to measure AN at forward xF over a broad pT range in the
coming runs.

4lider, we are also able to measure anti-quark polarization, and transversity, the structure
function in transversely polarized nucleon. As a kind of bi-products, we have a lot of new
tool to understand the strong interaction. The entire scope of RHIC spin physics is welldescribed in the reference[2].
The unpolarized gluon distribution has been studied using the cross sections for prompt
photon, jet, and heavy quark productions in hadron collisions, by both ﬁxed target andcollider experiments. In addition, Q2 evolution of the structure function F2 (x, Q2 ), obtained in lepton scattering experiments, has been used to constrain the gluon distribution
in the small-x region. These eﬀorts have led to a gluon distribution with a reasonableprecision. For the polarized case, by the Fermilab E704 experiments, helicity asymmetriesALL√for high-mass multi-γ pair production in polarized pp collisions has been measured
at s=19.4 GeV[3]. The data were compared with the model calculation of ALL andprovided a restriction on ∆g, but the statistical signiﬁcance was marginal.
In lepton scattering experiments, ∆g(x) has been determined through Q2 -evolution of
the spin dependent structure function g1 (x, Q2 ) but the uncertainty is still too large andleaves a room for any models. Recently, a proposal to use pairs of oppositely charged
hadrons to probe the photon-gluon fusion process to access gluons in lepton scatteringexperiments has been made[4]. The HERMES experiment at DESY has measured such an
asymmetry and gave the ﬁrst ﬁnite ∆g values but with large errors[5]. 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 production via longitudinal
double-spin asymmetry.
To visualize the importance of the RHIC gluon measurements, Figures 3 explain how
well the gluon polarization is determined by DIS experiments up to now,
√ and how it
becomes better after 100 days of direct photon measurement at RHIC at s=200 GeV
with the designed luminosity. One can ﬁnd that a great improvement is expected for thedetermination of the gluon polarization. It is worthwhile mentioning that this study evenassumes a shape of ∆g(x), which is still not known at all.

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. Thesegraphs are given from the Asymmetry Analysis Collaboration and are preliminary[6].

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At RHIC, the parity violating asymmetry in W productions will directly measure thepolarizationof ﬂavor-identiﬁed quark and antiquark. These measurements will be done√
at s=500 GeV by detecting high pT leptons. Similar quark identiﬁcation is not possible
in lepton scattering experiments without additional model assumptions, while the W production explicitly separates them. In addition to the helicity distributions of quarks andanti-quarks, RHIC Spin is also sensitive to the transversity distributions. One of the cleanest measurements will be provided by the Drell-Yan production of lepton pairs. Recent
theoretical work suggested another eﬀective measurement for transversity using two-pioncorrelation[7], which will be tested in RHIC and will possibly improve our sensitivity tolarge extent.
4. HIGHLIGHTS FROM THE FIRST YEAR DATA
As a consequence of the low polarization, the entire physics data taking was performed
with vertically polarized beams 4 . Thus the goal of the ﬁrst year was to commission thedetectors for polarized proton collisions, to determine the unpolarized pp cross section asa reference for spin and heavy ion physics, and most interestingly
to perform the ﬁrst√
measurements of single spin left-right asymmetries (AN ) at s = 200GeV.
PHENIX has measured the π 0 cross section from 1GeV/c to 13GeV/c of pT [8]. Over 108
of magnitude, the spectrum was nicely reproduced by a NLO-pQCD calculation within theuncertainties from the scale dependence[9]. Although the statistics were limited, PHENIX
has measured J/ψ’s[10] with a consistent rapidity distribution to PYTHIA and the total
cross section consistent to the color evaporation model.
The left- right asymmetry, AN , is expected be small in high xT due to the nature ofpQCD. Previous experimental studies showed so far zero-consistent results, which will berevisited by the new data from
√RHIC. In the large xF region, however, story is diﬀerent.
The data from FNAL E704 at s=20 GeV[11], taken about 10 years ago, showed beautiful
asymmetries for π + , π 0 , and π − . The measured asymmetry reaches a level of 10% in thexF region larger than 0.3. Recently a lot of theoretical work have been performed torelate this eﬀect to the transversity, spin eﬀect in the fragmentation function, KT eﬀect,
or orbital motion of quark[9]. Theoretical interpretation for this phenomena is, however,not settled yet5 . Nevertheless, there were enough reasons to believe this dependence isscale to xF , and the region to watch is not covered by the main detectors of STAR and
PHENIX so that some new detectors were added for such measurements. If the asymmetry
exists it will be a wonderful tool - called local polarimeter - to determine the polarization
at the interaction points.
Figure 6 illustrates the kinematical coverage of the spin experiments. STAR has prepared the Forward π 0 Detector (FPD) to cover the same xF region as E704. PHENIX has
developed new zero degree calorimeters. And pp2pp covers the elastic scattering region
by its nature.
STAR FPD has measured the L-R asymmetries of π 0 production at the pseudo-rapidity
4

Because the sensitivity of the double spin asymmetry, ALL , is diluted by 1/P 2 .
The essential but unknown ingredient is the spin eﬀect in the quark fragmentation process. Recently, ateam comprising PHENIX and STAR members joined the Belle collaboration at KEK to determine thespin dependent fragmentation function in two-quark jet events in the e+ + e− collider.

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Figure 4. The transverse momentum
spectrum of p + p → π 0 + X at mid ra√pidity, measured at s = 200 GeV by
the PHENIX experiment. The data arecompared to NLO pQCD calculations.

Figure√ 5. The production cross section of J/ψ
at s = 200 GeV is compared to previous
experiments. The curve is the prediction of
the color evaporation model.

region 3.4 ≤ η ≤ 4.1, As shown in Figure 7[12] the xF dependence looks similar to theE704 data as well as√the theoretical predictions[13]. This means such an eﬀect still existingat 10 times larger s.
Finally a surprise from 12 o’clock should be mentioned. There, developments of the
PHENIX local polarimeter has been performed. Before and after the collision point there
are dipole magnets to steer both beams into parallel. So, beyond these magnets only
neutral particles can be measured. There they placed a hadron-calorimeter type counterin one end and an EM calorimeter type in the other end. Both of them were segmented
to measure left-right asymmetries. The kinematical coverage is schematically shown in
Figure 6 as the detectors covered very small pT below 300 MeV/c in the xF region larger
than 0.3.
Figure 8 shows the measured asymmetry for forward neutrons. Large asymmetry in neutron (∼-10%) was consistently observed in the both calorimeters. The γ events have small
asymmetry (∼ -1%), and π 0 ’s showed no signiﬁcant asymmetry. They have conﬁrmed thatthe neutron asymmetry as shown in Figure 9 has the expected angular dependence fora true left-right asymmetry. Such neutron asymmetry has never been observed before.
Its existence and its amplitude are both unexpected. It is a bit ironical that the device

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Figure 6. The kinematical coverage of the
RHIC spin experiments (very schematic).

Figure 7. The analyzing power of π 0 production measured by STAR FPD, at the pseudorapidity region 3.4 ≤ η ≤ 4.1. The curves are
the theoretical predictions[13].

was developed aiming to see the π 0 asymmetry, but the nature is apparently beyond ourimagination. We hope to have a consistent picture for all these AN measurements, which
is now under consideration by many theorists.
In the low xF and the high xT region where our 100M$ detectors are situated, STAR
reported the analyzing power of charged hadrons up to pT =5 GeV/c[14]. The results are
consistent with zero as expected from pQCD. It should be stressed that measurements inthis region become very important in the next year with longitudinally polarized beams.
Both jet measurements in STAR and leading hadron measurements in PHENIX will access∆g directly.
5. SUMMARY AND OUTLOOKThe RHIC spin program has begun. We have a series of programs over 5 years to
go. In Year 2002, the ﬁrst polarized proton collision was successfully carried out withtransversely polarized beams, producing even unexpected spin phenomena which may
relate to unknown behavior of QCD (transversity, etc.). With spin-state averaged beams,
the validity of pQCD calculation is being conﬁrmed over a broad range of pT .
In Year 2003, the ﬁrst measurements of helicity asymmetry will soon be performed at
200 GeV. Sizable ∆g would generate surprises in the data.
In Year 2004 or later, statistics will be much improved at 200 GeV, ensuring the determination of ∆g. The spin puzzle will hopefully be solved by then. Also weak bosons
will be produced at RHIC at 500 GeV, the new ﬂavor sensitive tool for anti-quark/quark
polarizations and unpolarized structure functions of anti-quark measurements.

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Figure 8. The asymmetry observed for neutrons in the local polarimeter development
for PHENIX (preliminary).

Figure 9. Angular dependence of the neutron asymmetry (preliminary). The abscissa is angle with respect to the spin direction of the incoming beam.

ACKNOWLEDGEMENTS
The author is grateful to the RHIC spin collaboration, which gathers coherent eﬀorts
among the RHIC accelerator, STAR, PHENIX, PP2PP and the theory community. This
project emerged in 1990 at Penn State, ignited for the construction in 1995 by Prof. M.
Ishihara, the former chief scientist of RIKEN, and has ﬁnally launched to discovery.
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