Spin Physics at STAR
Akio Ogawa
for the STAR collaboration
Brookhaven National Laboratory, Department of Physics
Upton, NY, 11973-5000, USA
Abstract. The question of how the spin degrees of freedom in the nucleon are organized has
still not been fully answered even after recent polarized deep inelastic scattering experiments.
Studying polarized proton-proton collisions will add new and unique information to improve our
understanding of the spin structure of the nucleon.
The Relativistic Heavy Ion Collider (RHIC) successfully accelerated and collided polarized
proton beams in the beginning of 2002. STAR is one of the two large detectors at RHIC. STAR
has been taking heavy ion collision data since 2000 and will have excellent capability for spin
physics as well.
In this paper, an overview of the STAR spin program is given, covering a wide range of physics
topics including determination of gluon polarization, flavor separation of quark polarizations, and
quark transversity. Some details about the STAR detector, including future upgrade plans, are
presented. Results from the 2002 run with transversely polarized protons are summarized.
INTRODUCTION
The spin of the nucleon is known to be 1/2 [1, 2, 3, 4]. QCD describes the dynamics of
the nucleon's constituent partons: spin-1/2 quarks and spin-1 gluons. Exactly how the
quarks are confined in the nucleon is not fully understood. One of the central mysteries
in QCD is the spin structure of the nucleon. Polarized charged lepton deep inelastic
scattering (DIS) experiments [5, 6, 7, 8] found that the spins of the quarks and antiquarks carry only about 1/4 of the nucleon spin, contrary to the expectation of 2/3 from
the constituent quark model or the Ellis-Jaffe Sum Rule [9]. The spin of the nucleon is
built up from the spin and angular momentum of quarks and gluons:
i = iAE + AG+Z|+LzG.
(1)
Recent experiments are trying to find the missing contributions to the nucleon spin in
either gluon polarization or angular momentum of quarks and gluons.
1
For the full author list and acknowledgements, see the appendix to the proceedings.
CP675, Spin 2002:15th Int'l. Spin Physics Symposium and Workshop on Polarized Electron
Sources and Polarimeters, edited by Y. L Makdisi, A. U. Luccio, and W. W. MacKay
© 2003 American Institute of Physics 0-7354-0136-5/03/$20.00
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STAR SPIN PHYSICS PROGRAM
The STAR spin physics program will study nucleon spin structure in experiments that
are sensitive to gluon polarization, the flavor decomposition of quark and anti-quark
spins, and quark transversity. One of the advantages of the RHIC spin program is that
it extends spin structure studies to higher Q2 and lower xBj than fixed target polarized
DIS experiments. Measurements at large Q2 are important to minimize uncertainties
associated with pQCD theory. Data from a wide Q2 range is needed for a reliable global
pQCD analysis.
Gluon Polarization
Measurements of the longitudinal double spin asymmetry ALL in polarized proton
collisions provide sensitivity to gluon polarization. The cleanest way to determine the
gluon polarization is to study ALL for direct photon and jet production. In leading-order
pQCD, ~ 90% of the direct photon production cross section is from the QCD Compton
sub-process, qg —)• qj. When the direct photon is detected in coincidence with the away
side jet, ALL can be approximated as:
A
!Tr+Jet = if -^-M^ -> <7r).
(2)
The quark polarization weighted by the squared electric charge, A? is measured well
in polarized DIS experiments. The double spin asymmetry for the QCD Compton subprocess, aLL, is calculable in pQCD. The wide acceptance of the STAR detector is ideal
for containing the jets, which is important to determine the initial state kinematics.
When the direct photon is detected at forward rapidity in the acceptance of the
STAR endcap electromagnetic calorimeter (EMC), presently under construction, the
quark polarization is larger and the QCD Compton sub-process double spin asymmetry
increases, providing greater sensitivity to gluon polarization. It also enables us to access
lower Bjorken x gluons, which is important to determine the integral of AG.
Determination of ALL for direct photons detected in coincidence with away side jets
is a long term goal of the STAR spin program, requiring that RHIC delivers the design
luminosity (~ 1032cm~2s~1) and beam polarizations (Pbeam ~ 70%) and that the STAR
barrel and endcap EMC are completed.
Sensitivity to gluon polarization is also provided by measurements of ALL for inclusive
jet and di-jet production. These processes have much higher cross sections. STAR should
be able to study these processes starting with the next RHIC run, as discussed in B.
Surrow's proceedings [10].
Quark and Anti-Quark Polarization with Flavor Decomposition
Understanding the polarization of quarks and anti-quarks of specific flavors is important for a complete description of how the nucleon spin is built from its constituents.
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Flavor dependence of unpolarized sea anti-quark distributions has been observed [11].
Models that aim to understand this flavor asymmetry make different predictions for u
and d polarization.
Anti-quark polarization measurements from semi-inclusive DIS are still limited [12].
W^ bosons produced in pp collisions select spin and flavor and are therefore a unique
and ideal tool for studying flavor decomposition of the quark spin at RHIC. W+(~^ is
produced in u-\- d (d + u) collisions and is detected by its decay to e+^~\ When the
electron(positron) is detected with the endcap EMC (1.0 < T] < 2.0) from a polarized
proton propagating away from (toward) the endcap, the purity of W~^ coming from a
u quark (d quark) in the polarized nucleon is ~ 98% (~ 75%).
Transverse Spin Physics
Physics with transverse spin has received a lot of interest recently. Transversity is the
last unmeasured quark distribution function at leading twist, and studying it will give
another hint about the missing spin. The RHIC spin program will measure azimuthal
asymmetries within a jet at mid-rapidity. Such measurements are expected to be sensitive to transversity, if e+e~ collider experiments find these chiral-odd fragmentation
functions to be non-zero [13, 14, 15, 16].
The transverse single spin asymmetry (AN) for the p^ + p —> 7r° + X reaction was
measured at FNAL-E704 [17] at ^fs = 20 GeV and found to be large. After more than
a decade, this asymmetry still remains a mystery. STAR will measure AN for particle
production over a range of XF and pT comparable to that explored by E704. This
asymmetry may be sensitive to transversity or may give hints about the orbital angular
momentum of quarks in the nucleon.
THE STAR DETECTOR
STAR (Fig. 1) is one of the two large detectors at RHIC. The STAR detector is designed to cover a relatively large acceptance at mid-rapidity (2n in 0, —1.4 < T] < 1.4)
with tracking detectors to measure thousands of charged particles coming from AuAu collisions. The STAR barrel and endcap EMCs will increase their acceptance to be
azimuthally complete for — 1 < T] < 2 in the next 3 years. At forward rapidity, a BeamBeam Counter (BBC) and a Forward TT° Detector (FPD) were installed prior to the first
polarized proton run. A new sealer system counts 131,072 channels corresponding to
the 217 input patterns from 17 different physics signals for every bunch crossing, or
every 107 nsec. This sealer system was used to monitor bunch-by-bunch relative luminosity and to perform spin dependent counting experiments. A detailed description of
the STAR experiment can be found in [18].
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100 cm
TF&
«*/?&
M£&
::::£x^:?4v.v.v
FIGURE 1. The top view of the STAR detector as of the January the 2002 proton-proton run
HIGHLIGHTS OF PHYSICS FROM 2002 RUN
In December 2001, the first polarized proton-proton collider began operation. RHIC
accelerated beams to provide polarized proton collisions at •>/? = 200 GeV. The average
polarization, measured by the Coulomb Nuclear Interference (CNI) polarimeters [19],
was about 0.2 at the injection. The top luminosity was around 2 x 1030cm~2s~1 and
STAR recorded ~ 300 nb"1 in 33 days of operation. During this run, STAR collected
16 million minimum bias events, 3.5 million FPD events with full readout of the midrapidity detectors, 11 million FPD standalone events, 0.8 million EMC triggered events,
and 8 billion sealer events with BBC coincidences.
At this symposium, G. Rakness reported [20] on AN for inclusive 7r° production at
forward rapidity. J. Balewski reported [21] on AN for leading charged particles at mid
rapidity. A preliminary measurement indicates that at large XF, AN is as large at •>/? = 200
GeV as at •>/? = 20 GeV and that AN is consistent with zero at small XF. J. Kiryluk
reported [22] on AN measurements with the BBC and showed that STAR can measure
relative luminosity with systematic errors of less than 10~3 in the collider environment.
PLANS FOR THE NEXT AND FUTURE RUNS
The polarization in the next RHIC run is expected to be increased by a factor of 2
(from 0.2 to 0.4) and the luminosity will be increased by a factor of 10 (from 1030
to 1031cm-2s-1). STAR will have an upgraded FPD and BBC, and the barrel EMC
will increase its acceptance to 2n in 0 and 0 < rj < 1. These will enable us to do
precise measurements of AN for processes that were measured in the last run and to
use these asymmetries for tuning the spin rotators which are being installed to produce
longitudinal polarization at the STAR interaction point [23]. Once the spin rotators are
tuned, we should be able to start studying double spin asymmetries for single jet and
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dijet production [10].
Beyond the next RHIC run, STAR will be completing the barrel and endcap EMC.
STAR will have an exciting spin physics program including direct photon, W, and
transversity measurements.
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