Future Perspectives on Transverse Single
Spin Asymmetries at RHIC
Disclaimer: the realities and perspectives to be presented are my own,
although may be shared by others.
Forewarning: my perspective is that spin is best probed in a polarized p+p
collider in the forward direction. (Pause before starting so you can decide…)
L.C. Bland
Brookhaven National Laboratory
INT Workshop on 3D parton
structure of the nucleon
Seattle, September 2009
Conclusions and Summary
from Overview of Transverse Single Spin Asymmetry Measurements at RHIC
Transverse spin asymmetries are present at RHIC
Transverse spin asymmetries are present at large h
Particle production cross sections and correlations are
consistent with pQCD expectations at large h where
transverse spin effects are observed
Essential to go beyond inclusive (meson) production to
disentangle dynamical origins
3D parton structure, INT
Going Beyond Inclusive Meson Production
Future Transverse Single Spin Asymmetry Measurements at RHIC
(pending additional forward instrumentation+run time)
Increasing impact
Direct photon production at large h (+ away-side jet)
L polarization observables at large x
Large h jet production (+ p0 correlation in forward jet)
3D parton structure, INT
Increasing experimental accessibility
Drell-Yan/virtual photon production at large y
Comments About Present
3D parton structure, INT
RHIC is the First (Only) Polarized Proton Collider
RHIC pC Polarimeters
Absolute Polarimeter (H jet)
Siberian Snakes
Siberian Snakes
Spin Rotators
(longitudinal polarization)
Spin Rotators
(longitudinal polarization)
Pol. H Source
Helical Partial Siberian Snake
200 MeV Polarimeter
AGS pC Polarimeter
Strong AGS Snake
reference: RHIC Spin Plan (2008)
Determination of polarized gluon distribution (DG) using multiple probes
Determination of flavor identified anti-quark polarization using parity violating production of W
Transverse spin: connections to partonic orbital angular momentum (Ly) and transversity (dS)
3D parton structure, INT
RHIC is a Unique Collider…
…capable of colliding essentially all positive ions over a broad range of s
…with a broad and diverse physics program aimed at important questions
o What is quark-gluon plasma?  heavy-ion collisions
o How does the proton get its spin?  polarized proton collisions
o Does the gluon density saturate in a heavy nucleus?  d+Au/p+Au collisions
3D parton structure, INT
Plans for future runs at RHIC have been written…
Likely Beam Species
Science Goal
Au+Au at 200, 62.4 GeV,
assorted lower energy
low-mass dilepton spectrum; early collision temp.;
improved jet quenching studies; begin energy scan
for critical point
Subinjection Au+Au;
500 GeV p+p;
short 200 GeV U+U
continue critical pt. search; gluon pol’n at low x +
antiquark pol’n from W production; 1st
characterization of deformation effects in U+U
centrality distributions
Au+Au at 200 GeV;
500 GeV p+p
RHIC-II heavy ion goals: heavy flavor, g-jet,
quarkonium, multi-particle correlations; antiquark
polarization in proton
200 GeV p+p; further heavy
ion running to complement
earlier runs
continue RHIC-II heavy ion goals; transverse spin
asymmetries for g+jet; pp reference data for new
200 GeV Au+Au; low-E
Au+Au dictated by
Run10+11 results
continue pursuit of g+jet; energy scale and identified
heavy flavor
This is the only transverse spin science goal written in the plan for the next 5 years
3D parton structure, INT
Run-9 performance
Source: RHIC Collider Projections, W. Fischer et al. (2009)
• Challenges remain to be overcome to realize the best-case scenarios
• Luminosity increases at s=500 GeV relative to s=200 GeV were realized
3D parton
• Depolarizing resonances in RHIC
new tunes to reduce their impact8
Future Projections
Source: RHIC Collider Projections, W. Fischer et al. (2009)
• Luminosity projections for s = 500 GeV are sufficient for transverse-spin DY
• Improved polarization is important to achieve sufficient accuracy
3D parton structure, INT
STAR Detector
Forward Meson Spectrometer
commissioned/operated in
RHIC run 8.
Cluster-pair triggered readout
of Forward Time Projection
Chamber in RHIC run 9.
(Spatial resolution and pileup
suppression adequate?)
FTPC will be removed before
RHIC run 11.
STAR and PHENIX are primarily instrumented near mid-rapidity
Forward direction can be viewed at STAR, but present instrumentation is limited
and not completely compatible with high luminosity polarized p+p collisions
All experiments at RHIC are challenging, even with existing apparatus
3D parton structure, INT
Influence of STAR Solenoid
Impact on charged particles produced in the forward direction
Charges see
radial fringe
field as pT
D P α ( P z  B r - P r  B z )
• Radial and Azimuthal fields impart impulses in the Φ direction
• These impulses are small and in opposite directions (they partially
cancel each other)
 Field effects on forward charged particles are small
 Determining charge sign will require additional instrumentation
3D parton structure, INT
Forward p+p J/ψ – 2-Cluster Analysis
RHIC Run-8 Result
Reconstructed 2-cluster invariant mass
/ (~ 6 pb-1 Sampled Luminosity)
C.Perkins, QM09 arXiv:0907.4396
Fit with Gaussian + Offset
Gaussian Fit Parameters:
– μ = 3.080 ± 0.020 GeV/c2
– σ = 0.082 ± 0.026 GeV/c2
– χ2/d.o.f. = 20.83/26
– Significance from fit
4.5 σ
Cuts Applied:
– E_pair > 60.0 GeV
– Zγγ < 0.7
– Isolation Radius:
– 0.4 Dh-Df
– pT_cluster > 1.0 GeV/c
• high-xF J/ may have implications for intrinsic charm at large Bjorken-x in proton
3D parton structure, INT
• use to benchmark simulations for future transverse-spin Drell-Yan experiment
Forward p+p J/ψ – 3-Cluster Analysis
RHIC Run 8 Result
• Reconstructed invariant
mass of candidate χC → J/ψ + γ
Peak Counts = 8.40 ± 2.88
• 2.9 σ Significance
μ = 2.97 ± 0.025 GeV
σ = 0.070 ± 0.025 GeV
χ2/d.o.f. = 0.7 with 14 points fit.
• Significance depends
on background model
• 2.9 σ significance with
currently estimated
3D parton structure, INT
C.Perkins, QM09 arXiv:0907.4396
Attempts at realizing future
transverse single-spin asymmetry
A bottoms-up approach
3D parton structure, INT
Future Physics Goal (I)
p+pp0+X, s = 500 GeV
Motivations for measurement:
• Strong evidence that large-xF AN persists over a broad range of √s 
exploit existing capabilities to establish if this continues to √s = 500 GeV
• There are prospects for a transverse spin DY measurement at RHIC.
Likely best done at √s = 500 GeV. Persistence of pion AN to √s = 500
GeV is one physics requirement for transverse-spin DY
• Capabilities to robustly identify p0 production to >100 GeV. Existing
shower maximum detector in east FPD enables this identification (see
• Best estimates based on xF,pT scaling of sp, and limits on xT scaling,
suggest precision comparable to largest pT measurements at √s=200
GeV can be achieved with Lint=7 pb-1 with Pbeam=55%
3D parton structure, INT
Future Physics Goal (II)
p+p jet + X, s = 500 and 200 GeV
Motivations for measurement:
• Expectation that jets, with their p0 fragments, will enable separation of
contributions from Collins+Sivers(+other?), by analogy to semi-inclusive DIS
• Published calculations suggest strong interest as a test of present
• Measurement of jet energy (see below)  addition of Forward Hadron
Calorimeter behind existing FMS at STAR
• Addition of hadronic+electromagnetic energy at trigger level to eliminate bias
• Anticipate need for modest Lint, Pbeam concurrently achieved with other goals
3D parton structure, INT
Forward Upgrade (I)
Proposed Forward Hadron Calorimeter
3D parton structure, INT
Forward Jets with FMS + FHC
Importance of hadronic and EM jet fragments
Detectable hadrons and photons within acceptance of FMS+FHC are used
for summed-energy trigger and for cone-based jet reconstruction
Fraction of energy of reconstructed jet is a nearly uniform distribution
3D parton structure, INT
Forward Jets with FMS + FHC
Measuring the Jet Energy
Detectable hadrons and photons within acceptance of FMS+FHC are used for summedenergy trigger and for cone-based jet reconstruction. Results also checked via “trigger on
scattered parton into finite solid angle”
Photon-only jets do not measure the scattered parton energy.
Combining hadronic + EM energy does measure the scattered parton energy, limited mostly
by fragmentation effects.
Many jets are not particularly “jetty”, meaning only few hadrons are within the acceptance.
Jets with few hadrons do not give a good measure of the scattered parton energy. Invariant
mass from the FMS+FHC can discriminate “jetty” versus “non-jetty” fragmentations.
3D parton structure, INT
Future Physics Goal (III)
p+p g (+ jet) + X, s = 200 GeV
Motivations for measurement:
• Test predictions that AN for forward photon production will be negative
• DOE milestone as an experimental test of theoretical understanding
• Without a SMD, must be done at √s = 200 GeV to ensure single g/diphoton
separation at large xF.
• Requires robust performance from FMS, to ensure sufficient acceptance to
suppress backgrounds from p0, h, … decays
• Correlated g+jet will require development of trigger, to handle the rates
• Lint=30 pb-1 with Pbeam=65% at √s = 200 GeV
3D parton structure, INT
Forward Direct Photons
References: RHIC spin plan / STAR run-10 beam-use request
Suppress contributions from p0,h decays by requiring candidate direct photon in yellow-shaded
annulus, and by requiring effective isolation of the candidate using the remainder of the FMS
as an effective veto. This could be implemented at the trigger level via masks.
Primary background remains fragmentation photons.
For inclusive direct photons, it is expected that isolation can be improved with FHC behind the
FMS. The need to separate the FMS complicates spin-dependent correlation measurements.
3D parton structure, INT
Future Physics Goal (IV)
p+p L + X, s = 500
Motivations for measurement:
• Lambda reveals its polarization through the weak interaction
• Induced polarization measurement would be ~10x higher in s than from ISR
• DNN is sensitive to transversity without transverse-momentum dependent
• Addition of FHC behind FMS
• Trigger on hadronic cluster, tagged as neutral by BBC match
• Ability to detect soft photons from Lp0nggn
3D parton structure, INT
Can L be reconstructed via p0n?
Reconstructed versus simulated vertex
for events
with Lp0n
With the vertex, Mggn can be reconstructed.
Backgrounds mostly from Lg final states.
Forward Lp0n reconstruction appears feasible with FHC + FMS
Yields are model dependent, and may require elimination of hadronic showering in FMS
3D parton structure, INT
Future Physics Goal (V)
p+p e+e- + X, s = 500
Motivations for measurement:
• The world is waiting to see if there is a sign change relative to SIDIS…
• Most robust test of present understanding
• Charge-sign determination for DY daughters  restoration of tracking in
interval spanned by FTPC + FMS shower-maximum detector
• Robust understanding of forward dilepton spectrum at √s=500 GeV
• Establish that transverse spin effects persist to √s=500 GeV
• Lint ~ 250 pb-1 with Pbeam > 50% at √s=500 GeV
3D parton structure, INT
Rapidity and Collision Energy
Transverse Spin Asymmetries for the DY Process
Light mass DY,
Mg*> 4 GeV/c2
Rapidity distributions
for different s
Large rapidity acceptance required to probe valence quark Sivers function, also
3D parton structure, INT
where p+pp+X transverse spin asymmetries are found to be large at RHIC.
Forward Upgrade (II)
Shower Maximum Detector (SMD) for FMS
FMS-SMD is required for direct photon
physics at large xF for s=500 GeV p+p
collisions. Scope can be limited to
annular acceptance.
DY requires good space point at FMS and
track near vertex to get charge sign.
Feasibility of DY needs to be established,
and run-9 multi-cluster triggered slow
events can help. If feasible, restoration of
tracking coverage of FTPC is required.
Larger area coverage of FMS would then
be required by FMS-SMD.
Fiber/scintillator-strip factories are mostly gone, and would need to be
restored to build FMS-SMD.
Scope of FMS-SMD must be established before proceeding.
3D parton structure, INT
p0, Jet, photon, DY Lint requirements
s dependence
p+pg(+jet)+X 200
A N sign change
AN sign change
Lint (pb-1)
RHIC spin plan involved mix of longitudinal/transverse polarization
FHC addition enables jet measurements, and could be done at s=500 GeV
in run 11 during time to measure p+pp0+X with east FPD.
Feasibility tests of p+pL+X needed to establish Lint, Pbeam requirements
3D parton structure, INT
Measurements of transverse single spin asymmetries beyond inclusive
meson production in the forward direction will require additional
instrumentation + run time
The experiment with the greatest impact is transverse spin DY. Realizing
such an experiment will require demonstrated accelerator performance,
additional instrumentation and run time.
3D parton structure, INT
3D parton structure, INT
Why does high-xF intrinsic heavy
flavor matter?
• Diffractive Higgs production at the LHC via QQ in proton
– May provide a clear signal for Higgs production due to
Phys.Rev. D73 (2006) 113005
small background
• How can high-xF intrinsic heavy flavor happen?
– Not from Gluon Splitting (extrinsic heavy flavor)
– Heavy quarks are expected to be multi-connected to the
valence quarks within a proton and appear at large x via…
s gg  gg α
s gg  gg α
• Can intrinsic heavy flavor expectations be tested
3D parton structure, INT
East FPD Events from run 9
Existing east FPD layout…
Shower Maximum Detector
7x7 matrix of
lead glass cells
event requirements
• >1 cluster
• Egg > 50 GeV
Example of event identified as a
diphoton by the matrix and only a
single photon by the SMD
(7 Pb-glass cells)
• single g/diphoton separation for matrix
shown by GSTAR analysis to be robust to
E~55 GeV
• SMD response enables single g/diphoton
separation to E>100 GeV
• Plan is to add this performance to FMS in
the future for √s=500 GeV operation
3D parton structure, INT
Status/Plan of Large-xF DY
• Large-xF J/ production has been observed from bare large-y calorimeter response
in RHIC run 8.
• Cluster-pair trigger is operational for acquiring large-y tracking data in RHIC run-9.
Pending analysis, requirements for future DY can be established (e.g., fast-tracking
inside solenoid, space points in front of FMS).
• Sufficient luminosity for p+p s=500 GeV collisions has been established; further
development of polarization is required, as is measurement of AN(xF) for p+pp0+X
at s=500 GeV and measurement of large-xF J/ and U production at s=500 GeV,
to bracket light-mass DY region.
• Technical solutions exist for fast tracking inside solenoid (GEM trackers) and space
points in front of FMS (forward meson preshower). Construction to span 2.5<h<4
region is required, and could be completed in ~2 years, pending approval.
• RHIC schedule is oversubscribed  DY would be after RHIC run 11 (>2011).
• Run-10 will be Au+Au energy scan for deconfinement critical point search, and
Au+Au at sNN=200 GeV.
• Run-11 is expected to be polarized p+p, with unknown mix of s=200,500 GeV
and longitudinal/transverse polarization.
3D parton structure, INT

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