Overview of Transverse Single Spin
Asymmetry Measurements at RHIC
Outline
Review of Findings on Transverse SSA at RHIC
•
Motivations/goals and methods
•
Findings from the first polarized proton collisions at RHIC
(medieval times)
•
Findings from the renaissance
L.C. Bland
Brookhaven National Laboratory
•
First findings from the modern age
•
Possible paths forward (more on
3D parton structure, INT
INT Workshop on 3D parton
structure of the nucleon
Friday)
Seattle, September 2009
1
RHIC Spin Goals - I
How is the proton built from its known quark and gluon constituents?
As with atomic and nuclear structure, this is an evolving understanding
In QCD: proton is not
just 3 quarks !
Recall:
simple quark model
Rich structure of quarks
anti-quarks, gluons
3D parton structure, INT
2
RHIC Spin Goals - II
Understanding the Origin of Proton Spin
Spin Sum Rules
Longitudinal Spin
Transverse Spin
PRD 70 (2004) 114001
Understanding the origin of proton spin helps to understand its structure
3D parton structure, INT
3
RHIC Spin Goals - III
Objectives
•
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
4
RHIC Spin Probes - I
Polarized proton collisions / hard scattering probes of DG
quark
pion or jet
quark
gluon
c
d     dxa  dxb  dzc f a ( xa ) f b ( xb ) Dc ( zc )dˆ ab
a ,b , c
Describe p+p particle production at RHIC energies (s  62 GeV)
using perturbative QCD at Next to Leading Order,
relying on universal parton distribution functions and fragmentation functions
RHIC Spin Probes - II
Unpolarized cross sections as benchmarks and heavy-ion references
0 +cross
Large
,K,p
sections
for p+p,
s=200 GeV
p + rapidity
pp

s
=GeV
200
GeV
+ p, sX,
= 200
PRD 76 (2007) 051106
PRL 97 (2006) 252001
jets
PRL 98 (2007) 252001
PRL 92 (2004) 171801
direct g
PRL 98 (2007) 012002
Good agreement between
experiment
and theory
3D parton structure,
INT
 calibrated hard scattering probes of proton spin
6
Longitudinal Two-Spin (ALL)
Status of probing for gluon polarization via
measurements of ALLfor midrapidity jet,0 production
3D parton structure, INT
7
ALL: 0
PRL 103 (2009) 012003
[ 0.02, 0.3]
0.0
DGGRSV
 0.2  0.1(stat)  0.1(sys) -0.4
(shape)  0.1(scale)
3D parton structure, INT
8
Inclusive ALLjets Results
STAR
arXiv:0805.3004
 Data are compared to predictions within the
GRSV framework with several input values of DG.
B. Jager et.al, Phys.Rev.D70, 034010
GRSV-std
The inclusive measurements give
sensitivity to gluon polarization over a
broad momentum range
3D parton structure, INT
9
Global Analysis
Determining Dg from Existing World Data
PRL 101 (2008) 072001
Dg(x,Q2) is small in the accessible range of momentum fraction
[presently measured]
3D parton structure, INT
10
RHIC as a Polarized Proton Collider
RHIC pC Polarimeters
Absolute Polarimeter (H jet)
BRAHMS & PP2PP
PHOBOS
Siberian Snakes
Siberian Snakes
PHENIX
STAR
Spin Rotators
(longitudinal polarization)
Spin Rotators
(longitudinal polarization)
-
Pol. H Source
LINAC
BOOSTER
Helical Partial Siberian Snake
200 MeV Polarimeter
AGS
AGS pC Polarimeter
Strong AGS Snake
3D parton structure, INT
11
0
3D parton structure, INT
AN measurements initially motivated by search for local polarimeter
12
Transverse Single-Spin Asymmetries (AN)
Analyzing power is a tool to measure polarization and is one example of
transverse single spin asymmetries (SSA) with origins yet to be fully understood
Probing for
(1) orbital motion within transversely polarized protons;
(2) Evidence of transversely polarized quarks in polarized protons.
3D parton structure, INT
13
Expectations from Theory
What would we see from this gedanken experiment?
F0 as mq0 in vector gauge theories, so AN ~ mq/pT
or,AN ~ 0.001 for pT ~ 2 GeV/c
Kane, Pumplin and Repko PRL 41 (1978) 1689
3D parton structure, INT
14
A Brief History…
p  p    X
s=20 GeV, pT=0.5-2.0 GeV/c
• QCD theory expects very small
(AN~10-3) transverse SSA for particles
produced by hard scattering.
• The FermiLab E-704 experiment
found strikingly large transverse singlespin effects in p+p fixed-target
collisions with 200 GeV polarized
proton beam (s = 20 GeV).
0 – E704, PLB261 (1991) 201.
3D parton structure, INT
+/- - E704, PLB264 (1991) 462.
•
•
15
STAR
•
Large acceptance near midrapidity
•
Windows to large rapidity
3D parton structure, INT
16
PHENIX Detector
EMCal
0/g/hdetection
• Electromagnetic Calorimeter (PbSc/PbGl):
• High pT photon trigger to collect 0's,
h’s, g’s
• Acceptance: |h|<0.35,f2x /2
• High granularity (~10*10mrad2)
/• Drift Chamber (DC) for Charged Tracks
• Ring Imaging Cherenkov Detector (RICH)
• High pT charged pions (pT>4.7 GeV).
Relative Luminosity
• Beam Beam Counter (BBC)
ZDC
• Acceptance: 3.0< h<3.9
BBC
ZDC
• Zero Degree Calorimeter (ZDC)
• Acceptance: ±2 mrad
Local Polarimetry
• ZDC
• Shower Maximum Detector (SMD)
3D parton structure, INT
17
BRAHMS
3D parton structure, INT
18
Brahms
•Transvers beam pol
•Particle ID
BRAHMS measured AN s=62.4 GeV and 200 GeV
•Large xF dependent SSAs seen for pions and kaons
•Collinear factorization and (NLO) pQCD describe unpolarized
cross-section at RHIC in wide kinematic region
3D parton structure, INT
19
Medieval Times
First polarized p+p collisions at RHIC
3D parton structure, INT
20
Transverse Spin Asymmetries at Midrapidity
p+p  0/h± + X, s = 200 GeV
PRL 95 (2005) 202001
Transverse single spin asymmetries are consistent with zero at midrapidity
3D parton structure, INT
21
Measuring AN: Inclusive 0 Production
RHIC Runs 2-3 with Forward Pion Detector (FPD)
PRL 92, 171801 (2004)
PRL 97, 152302 (2006)
STAR
Cross-section is consistent
with NLO pQCD calculations
p+p0+X, √s=200 GeV, <η> = 3.8
Transverse spin asymmetries found at
lower √s persist to √s=200 GeV
3D parton structure, INT
22
STAR-Forward
Cross Sections
Similar to ISR analysis
J. Singh, et al Nucl. Phys.
B140 (1978) 189.
d 3
C
-B
E 3  1 - xF  pT
dp
C 5
B6
Expect QCD scaling of form:


a
d 3
C -n
C
-a
E 3  xT 1 - xF  pT  s / 2 1 - xF  pT-n-a  B  n  a
dp
 Require s dependence (e.g., measure 0 cross sections
at s = 500 GeV) to disentangle pT and xT dependence
3D parton structure, INT
23
The Renaissance
3D parton structure, INT
24
STAR Results vs. Di-Jet Pseudorapidity Sum
Run-6 Result
VY 1, VY 2 are calculations by
Vogelsang & Yuan, PRD 72 (2005) 054028
AN pbeam
 (kT(50%+
 S)T)
Emphasizes
jet
quark Sivers
Boer & Vogelsang, PRD 69
(2004) 094025
pbeam
into page
jet
Idea: directly measure kT by observing momentum imbalance
of a pair of jets produced in p+p collision and attempt to
measure if kT is correlated with incoming proton spin
AN consistent with zero
~order of magnitude smaller in pp  di-jets than in semi-inclusive DIS
quark Sivers asymmetry!
3D parton structure, INT
STAR
PRL 99 (2007) 142003
25
xF Dependence of Inclusive 0 AN
RHIC Run 6 with FPD++
STAR
PRL 101, 222001 (2008)
arXiv:0801.2990v1 [hep-ex]
Fits to SIDIS
(HERMES) is
consistent with
data
AN at positive xF
grows with
increasing xF
U. D’Alesio, F. Murgia
Phys. Rev. D 70, 074009 (2004)
arXiv:hep-ph/0712.4240
C. Kouvaris, J. Qiu, W. Vogelsang, F. Yuan,
Phys. Rev. D 74, 114013 (2006).
3D parton structure, INT
26
pT Dependence of Inclusive 0 AN
RHIC Runs 3,5,6 with FPD
STAR
B.I. Abelev et al. (STAR) PRL 101 (2008) 222001
• xF dependence is
consistent with Sivers
model
• Rising pT dependence is
not explained
STAR, PRL 101 (2008) 222001
3D parton structure, INT
6/1/2009
Chris Perkins
27
27
xF and pT dependence of AN for
p+p±+X, s=62 GeV
I. Arsene, et al. PRL101 (2008) 042001
• AN(+) ~ -AN(-), consistent with results at lower s and u,d valence differences
• At fixed xF, evidence that AN grows with pT
3D parton structure, INT
28
Transverse Spin Effects for Kaons
p+pK±+X, s=62 GeV
I. Arsene, et al. PRL101 (2008) 042001
• Large transverse single spin asymmetries are observed for kaons
3D parton structure, INT
29
PHENIX Muon Piston Calorimeter
2.22.2 18 cm3
• 192 PbWO4 crystals
with APD readout
• Better than 80% of the
acceptance is okay
SOUTH
3D parton structure, INT
30
PHENIX Goes Forward
First results with muon piston calorimeter from run 6
p+p0+X, s = 62 GeV
Transverse SSA persists with similar characteristics over
a broad range of collision energy (20 < s < 200 GeV)
3D parton structure, INT
31
Heavier mesons also accessible at high XF
p  p  M X
M g  g
s  200 GeV
STAR 2006 PRELIMINARY
Di-photons in FPD with
E(pair)>40 GeV
No “center cut” (requirement that
two-photon system point at middle
of an FPD module)
With center cut and Zgg<0.85
Average Yellow Beam
Polarization=56%
3D parton structure, INT
arXiv:0905.2840 (S. Heppelmann, PANIC 2008)
32
Towards Modern Times
• To separate Sivers and Collins effects need to
move beyond inclusive production
• To isolate Sivers effect, need to either avoid fragmentation
or integrate azimuthally
•
Full Jets, Di-Jets (away side), Direct photons, Drell-Yan
• To isolate Collins effect, need to look azimuthally within a jet.
3D parton structure, INT
33
Guzey, Strikman and Vogelsang
Phys. Lett. B603 (2004) 173
• constrain x value of gluon probed by high-x quark
by detection of second hadron serving as jet
surrogate.
• span broad pseudorapidity range (-1<h<+4) for
second hadron  span broad range of xgluon
• provide sensitivity to higher pT for forward 0 
reduce 23 (inelastic) parton process
contributions thereby reducing uncorrelated
background in Df correlation.
3D parton structure, INT
PYTHIA Simulation
34
STAR Forward Calorimeter Projects
F.Bieser2, L.Bland1, E. Braidot7, R.Brown1, H.Crawford2, A.Derevshchikov4,
J.Drachenberg6, J.Engelage2, L.Eun3, M.Evans3, D.Fein3, C.Gagliardi6, A.
Gordon1, S.Hepplemann3, E.Judd2, V.Kravtsov4, J. Langdon5, Yu.Matulenko4,
A.Meschanin4, C.Miller5, N. Mineav4, A. Mischke7, D.Morozov4, M.Ng2,
L.Nogach4, S.Nurushev4, A.Ogawa1, H. Okada1, J. Palmatier3, T.Peitzmann7, S.
Perez5, C.Perkins2, M.Planinic8, N.Poljak8, G.Rakness1,3, J. Tatarowicz3,
A.Vasiliev4, N.Zachariou5
1Brookhaven
National Laboratory
of California- Berkeley
3Pennsylvania State University
4IHEP, Protvino
5Stony Brook University
6Texas A&M University
7Utrecht, the Netherlands
2University
8Zagreb
University
These people built the Forward Meson Spectrometer (FMS) and/or its components
STAR Forward Meson Spectrometer
• 50 larger acceptance
than the run-3 forward
pion detector (FPD).
• 2 azimuth for 2.5<h<4.0
• Discriminate single g from
0gg up to ~60 GeV
PRL 101 (2008) 222001
North half of FMS
before closing
Runs
3-6
FPD
Run
8
FMS
3D parton structure, INT
36
Run-8 Results from
STAR Forward Meson Spectrometer
(FMS)
Full azimuth spanned with nearly contiguous
electromagnetic calorimetry from -1<h<4
 approaching full acceptance detector
3D parton structure, INT
37
Run 8 FMS Inclusive 0 Results
y
Octant
subdivision
of FMS for
inclusive 0
spin sorting.
x
f
P
• Azimuthal dependence as expected
• AN comparable to prior
measurements
arXiv:0901.2828
Nikola Poljak – SPIN08
3D parton structure, INT
38
pT Dependence
RHIC Run 8 with East FPD/FMS
Negative xF
Positive xF
Negative xF consistent with zero
Indication of Positive AN persists up to pT ~5 GeV
Needs more transverse spin running
3D parton structure, INT
arXiv:0901.2763 (J. Drachenberg–
SPIN08)
Akio Ogawa – CIPANP 09
39
First Look at “Jet-like” Events in the FMS
“Jet-shape” distribution of energy within jetlike objects in the FMS as a function of distance
from the jet axis.
Event selection:
•
•
•
•
>15 detectors with energy > 0.4GeV in the event
(no single pions in the event)
cone radius = 0.5 (eta-phi space)
“Jet-like” pT > 1 GeV/c ; xF > 0.2
2 perimeter fiducial volume cut (small/large cells)
• “Jet shape” in data matches simulation well
• Reconstructed Mass doesn’t match as well
• High-Tower Trigger used in Run 8 biases Jets
3D parton structure, INT
arXiv:0901.2828 (Nikola Poljak – SPIN08)
40
High xF Vector Mesons
RHIC Run 8 with FMS
Triple Photons : w0g
3 photon events to look for w0g
BR8.9%
•PT(triplet)>2.5 GeV/c
•E(triplet)>30 GeV
•PT(photon cluster)>1.5 GeV/c
•PT(π0)>1 GeV/c
Background only MC
Run8 FMS data
Fit is gaussian + P3
μ=0.784±0.008 GeV
σ=0.087±0.009 GeV
Scale=1339±135 Events
Significant (10) w0g
signal seen in the data.
•Comparison to dAu
Next :
•Spin-1 meson AN
3D parton structure, INT
arXiv:0906.2332
A Gordon– Moriond09
41
STAR Detector
• Large rapidity coverage
for electromagnetic
calorimetry (-1<h<+4)
spanning full azimuth 
azimuthal correlations
• Run-8 was the first run for
the Forward Meson
Spectrometer (FMS)
3D parton structure, INT
42
Azimuthal Correlations with Large Dh
E. Braidot (for STAR), Quark Matter 2009
Uncorrected Coincidence
Probability (radian-1)
p+p0+h±+X, s=200 GeV
0 requirements:
pT,>2.5 GeV/c
2.8<h<3.8
h± requirements:
1.5<pT,h<pT,
|hh|<0.9
• clear back-to-back peak observed, as expected for partonic 22 processes
• fixed and large h trigger, with variable hh  map out Bjorken-x dependence
• of greatest interest for forward direct-g trigger
3D parton structure, INT
43
Forward 0 – Forward 0 Azimuthal Correlations
• Possible back-to-back di-jet/di-hadron Sivers measurement
• Possible near-side hadron correlation for Collins fragmentation
function/Interference fragmentation function + Transversity
• Low-x / gluon saturation study – accessing lowest xBjgluon
3D parton structure, INT
44
Akio Ogawa- CIPANP 09
Proposals for the Future
SSA beyond inclusive meson production
•
Forward jets
•
Forward photons
•
Forward virtual photons
(Just mentioned here… much more about future prospects on Friday)
3D parton structure, INT
45
Conclusions and Summary
•
Transverse spin asymmetries are present at RHIC
energies
•
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 production to
disentangle dynamical origins
3D parton structure, INT
46