Transcript Transverse Single Spin Asymmetries in Large-x Production at STAR p

Transverse Single Spin Asymmetries in
Large-xF p0 Production at STAR
A Review of Several Mysteries
L.C. Bland
Brookhaven National Laboratory
31 July 2007
Kinematics
large-pT physics in p+p collisions
pbeam
-pbeam
large
pT
p or jet or g or …
Largest pT reached by detecting produced particles at
 ~ 90 (midrapidity, h~0)
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Kinematics
large-xF (with sufficient pT) physics in p+p collisions
pbeam
large
pL
-pbeam
p or jet or g or …
Large pL (produced particle at large h) is required to
reach large Feynman-x,
xF = pL / pbeam= 2 pL / s
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RHIC Spin Goals
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
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Transverse Single-Spin Asymmetries (AN)
Probing for orbital motion within transversely polarized protons
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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
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A Brief and Incomplete History…
p  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).
p0 – E704, PLB261 (1991) 201.
p+/- - E704, PLB264 (1991) 462.
•
•
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Two of the Explanations for Large Transverse SSA
Spin-correlated kT
Collins/Hepplemann mechanism
requires transverse quark polarization
and spin-dependent fragmentation
Sivers mechanism
requires spin-correlated transverse
momentum in the proton (orbital motion).
SSA is present for jet or g
initial
state
final
state
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Require experimental separation of Collins and Sivers contributions
RHIC Polarized Collider
RHIC pC Polarimeters
BRAHMS & PP2PP
Absolute Polarimeter (H jet)
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
2006: 1 MHz collision rate;
Polarization=0.6
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Transverse Single-Spin Asymmetries
World-wide experimental and theoretical efforts
• Transverse single-spin asymmetries are observed in
semi-inclusive deep inelastic scattering with transversely
polarized proton targets
 HERMES (e-); COMPASS (m); and planned at JLab
• Transverse single spin asymmetries are observed in
hadron-pair production in e+e- collisions (BELLE)
• Intense theory activity underway
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RHIC Spin Probes
Polarized proton collisions / hard scattering probes of DG
quark
pion or jet
quark
gluon
c
d p    dxa  dxb  dzc f a ( xa ) f b ( xb ) Dcp ( 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
Do we understand forward p0 production in p + p?
√s=23.3GeV
At s < 200 GeV, not really…
√s=52.8GeV
Data-pQCD
difference at
pT=1.5GeV
2 NLO
collinear
calculations
with different
scale:
q6o
pT and pT/2
q10o
q15o
q53o
q22o
xF
Bourrely and Soffer
[Eur. Phys. J C36
(2004) 371], data
references therein
to ISR and fixed
target results
xF
data/pQCD appears to be function of q, √s in addition to pT
Collinear NLO pQCD underpredicts the data at s < 200 GeV
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Does pQCD describe particle production at RHIC?
Compare cross sections measured for p+pp0 +X at s=200 GeV
to next-to-leading order pQCD calculations
S.S. Adler et al.
(PHENIX), PRL 91
(2003) 241803
J. Adams et al. (STAR), PRL 92 (2004)
171801; and PRL 97 (2006) 152302
Cross sections agree with NLO pQCD down to pT~2 GeV/c over a wide
range, 0 < h < 3.8, of pseudorapidity (h = -ln tan q/2) at s = 200 GeV. 13
STAR-FPD
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:
d 3
C
E 3  xT-a 1 - xF  pT-n 
dp


s / 2 1 - xF  pT-n-a  B  n  a
a
C
 Require s dependence to disentangle pT and xT dependence
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Forward p0 production in hadron collider
p0
Ep
p
E
d N
qq
xqp
qp
xgp
EN
qg
2E p
s
s  2E N
Ep
z

q
Eq
h  -ln(tan( ))
2
p -h
xg  T e g
xq  xF / z
s
(collinear approx.)
Q 2 ~ pT2
p
Au
xF 
• Large rapidity p production hp>4 probes asymmetric partonic collisions

• Mostly high-x valence quark + low-x gluon
• 0.3 < xq< 0.7

p  p  p ,hp  3.8, s  200GeV
0
<z>

• 0.001< xg < 0.1
<xq> NLO pQCD
Jaeger,Stratmann,Vogelsang,Kretzer
• <z> nearly constant and high 0.7 ~ 0.8
<xg>
• Large-x quark polarization is known to be large from DIS
• Directly couple to gluons  probe of low x gluons
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Spin Effects in the Forward Direction
√s=200 GeV, <η> = 3.8
Spin effects
initially observed
in RHIC run 2
confirmed by
measurements
in runs 3,5.
STAR collaboration Phys. Rev. Lett. 92 (2004) 171801
D. Morozov, for STAR [hep-ex/0512013]
Can be described by several models available as predictions:
• Sivers: spin correlated k in the proton (orbital angular momentum)
• Collins/Heppelmann: spin and k correlation in quark  p fragmentation
• Qiu/Sterman (initial state) / Koike (final state): twist-3 pQCD  multi-parton correlations
 Transverse SSA persist at large xF at RHIC energies
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Overview of transverse spin runs at STAR
with forward calorimetry: 2001→2006
Run2
EEMC
detector and FPD
prototypes
sampled
Run3
Run5
Run6
6
matrices
of FPD
full FPD
(8 matrices)
East FPD
West FPD++
PBEAM , %
~15
~30
~45
~60
 Ldt , pb
0.15
0.25
0.1
6.8
3.8
±3.3/±4.0
±3.7/±4.0
-3.7/3.3
<h 
-1
FOM (P2L) in Run 6 is ~50 times larger than from all the
previous STAR runs, and ~ 725 times larger than for Run 2
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h= -1
RUN6 configuration
x
y
z
h=2
FPD
East-side
FPD++
West-side
Inclusive p0 in forward region: -4<h<-3 (FPD), 2.5<h<4 (FPD++)
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FPD++ Physics for Run6
We staged a large version of the FPD to prove our ability to detect
jet-like events, direct photons, etc. with the STAR FMS
Run-5 FPD
The center annulus of the run-6 FPD++ is similar to arrays used to measure
forward p0 SSA. The FPD++ annulus is surrounded by additional calorimetry
to increase the acceptance for jet-like events and direct g events.
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pT GeV/c
Acceptance of FPD
Inclusive p0
6
STAR
5
4
FPD
3
2
1
0
0
0.2
0.4
0.6
0.8
xF
Strong xF - pT correlation because of limited acceptance
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pT GeV/c
Acceptance of FPD and FPD++
6
Inclusive p0
STAR
5
FPD++
4
FPD
3
2
1
0
0
0.2
0.4
0.6
0.8
xF
Study of the pT dependence needs large acceptance
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p0 Identification and Spin Dependence
• Large rapidity measurements require careful calibration
• Left/right symmetric detectors cancels many sources of systematic errors
• Spin effect is visible in the raw spin-sorted yields
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π0 AN at √s=200 GeV – xF-dependence
• AN at positive xF grows with
increasing xF
• AN at negative xF is consistent
with zero
• Run 6 data at <η>=3.7 are
consistent with the existing
measurements
• Small errors of the data points
allow quantitative comparison
with theory predictions
STAR
hep-ex/0612030
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Estimates of Systematic Errors
Consistency of repeated measurements
<η>=3.3
<η>=3.7
Analyzing power is measured
by “cross-ratio” method with
two-arms (left-right) detector:
 N N - N N
L
R
R
L
AN 

PBeam  N   N   N   N 
L
R
R
L

1




ANrun - < AN 
ANrun
Run-by-run comparison with mean
is consistent with statistics, except
right near the threshold
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AN(pT) at xF > 0.4
Run3+Run5 data (hep-ex/0512013):
• Online calibration of CNI
polarimeter
• Hint of AN decrease with
increasing pT at pT~1-2 GeV/c
residual xF-dependence?
=> AN mapping in (xF,pT)
plane is required
Run6 data (hep-ex/0612030):
• more precise measurements
• consistent with the previous runs in the overlapping pT region
• complicated dependence on pT , but not in agreement with theoretical predictions
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STAR
AN(pT) in xF-bins
• Combined data from three runs at
<η>=3.3, 3.7 and 4.0
• In each xF bin, <xF> does not
significantly changes with pT
• Measured AN is not a smooth
decreasing function of pT as predicted
by multiple theoretical models
(hep-ex/0612030)
D’Alesio & Murgia PRD 70 (2004) 074009
Kouvaris, Qiu, Vogelsang, Yuan PRD 74 (2006) 114013
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Summary
• Firmly established that large transverse single spin asymmetries are observed at
s = 200 GeV, where generally cross sections agree with pQCD calculations.
• Large transverse single spin asymmetries are observed only at large xF;
midrapidity asymmetries are small.
• Large xF spin asymmetries show the same pattern for 20  s  200 GeV
• First observation of pT dependence at fixed-xF, enabled by the run-6
luminosity/performance
 Some aspects of the theory are still not understood
• Intense theory activity is underway to understand these spin effects. Most
theorists agree the Sivers mechanism is responsible for the dynamics
 evidence for partonic orbital angular momentum?
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Outlook
Forward Meson Spectrometer
Installation completed 2007
PHYSICS
OBJECTIVES
• Near full
EM coverage -1<h<4
1.
•
•
2.
•
•
3.
FMS for Run 7
NOW!!
A d-Au measurement of the
Pairs
of Forward
Pionsdensity
parton
model gluon
distributions
x g(x)
same
side correlations
in
gold
nuclei
0.001< x
(Fragmentation –for
Collins)
<0.1. For 0.01<x<.1, this
measurement tests the
Event
by eventof “x”
universality
the gluon
measurement
distribution. from two jets.
Characterization
of correlated
Opposite
side correlated
pions
pion
cross
sections
as
a
(dijets)
2
2
function of Q (pT ) to search
–for Sivers
effectof
the onset
–gluon
d-Ausaturation
(Gluon saturation
in
effects
Nuclei)
associated with macroscopic
gluon fields. (again d-Au)
FMS
construction
completed
installation and
commissioning
during Run 7
(NOW)
Other future objectives
with
–Measurements
Forward Lepton
pairs
transversely polarized
–protons
Charm that are expected to
resolve the origin of the
large transverse spin
asymmetries in reactions for
forward p0 production.
(polarized pp)
FMS ½ Wall Pb. Glass
FMS Wall
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