Transverse Single Spin Asymmetries in Large-xF Production at STAR

A Review of Several Mysteries

L.C. BlandBrookhaven National Laboratory31 July 2007

2

Kinematicslarge-pT physics in p+p collisions

pbeam -pbeam

largepT

or jet or or …

Largest pT reached by detecting produced particles at ~ 90 (midrapidity, ~0)

3

Kinematicslarge-xF (with sufficient pT) physics in p+p collisions

pbeam -pbeamlargepL or jet or or …

Large pL (produced particle at large ) is required to reach large Feynman-x,xF = pL / pbeam= 2 pL / s

4

RHIC Spin GoalsUnderstanding the Origin of Proton Spin

Transverse Spin PRD 70 (2004)114001

Spin Sum Rules

Longitudinal Spin

Understanding the origin of proton spin helps to understand its structure

5

Transverse Single-Spin Asymmetries (AN)

Probing for orbital motion within transversely polarized protons

6

Expectations from Theory

What would we see from this gedanken experiment?

F0 as mq0 in vector gauge theories, so AN ~ mq/pT

or,AN ~ 0.001 for pT ~ 2 GeV/cKane, Pumplin and Repko PRL 41 (1978) 1689

7

s=20 GeV, pT=0.5-2.0 GeV/c

�0 – E704, PLB261 (1991) 201.�+/- - E704, PLB264 (1991) 462.

Xpp

• QCD theory expects very small (AN~10-3) transverse SSA for particles produced by hard scattering.

A Brief and Incomplete History…

• The FermiLab E-704 experiment found strikingly large transverse single-spin effects in p+p fixed-target collisions with 200 GeV polarized proton beam (s = 20 GeV).

8

Two of the Explanations for Large Transverse SSASpin-correlated kT

Require experimental separation of Collins and Sivers contributions

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

final state

initial state

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RHIC Polarized Collider

BRAHMS & PP2PP

STAR

PHENIX

AGS

LINACBOOSTER

Pol. H- Source

Spin Rotators(longitudinal polarization)

Siberian Snakes

200 MeV Polarimeter

RHIC pC PolarimetersAbsolute Polarimeter (H jet)

AGS pC PolarimeterStrong AGS Snake

Helical Partial Siberian Snake

PHOBOS

Spin Rotators(longitudinal polarization)

Siberian Snakes

2006: 1 MHz collision rate;Polarization=0.6

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Transverse Single-Spin AsymmetriesWorld-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 (); and planned at JLab

• Transverse single spin asymmetries are observed in hadron-pair production in e+e collisions (BELLE)

• Intense theory activity underway

gluon

quark pion or jet

quark

RHIC Spin ProbesPolarized proton collisions / hard scattering probes of G

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

cabccbb

cbaaacba dzDxfxfdzdxdxd

ˆ)()()( ,,

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√s=23.3GeV √s=52.8GeV

Do we understand forward production in p + p? At s < 200 GeV, not really…

2 NLO collinear

calculations with different

scale:

pT and pT/2

Bourrely and Soffer [Eur. Phys. J C36 (2004) 371], data references therein to ISR and fixed target results

data/pQCD appears to be function of , √s in addition to pT

Collinear NLO pQCD underpredicts the data at s < 200 GeV

xF

Ed

3 d

p3 [b

/GeV

3 ]

Data-pQCD difference at pT=1.5GeV

xF

Ed

3 d

p3 [b

/GeV

3 ]

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Does pQCD describe particle production at RHIC?Compare cross sections measured for p+p +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 < 3.8, of pseudorapidity ( = -ln tan /2) at s = 200 GeV.

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STAR-FPD STAR-FPD Cross SectionsCross Sections

Similar to ISR analysisJ. Singh, et al Nucl. Phys. B140 (1978) 189.

6

5

13

3

B

C

pxdp

dE B

TC

F

Expect QCD scaling of form:

anBpxspxxdp

dE an

TC

F

anT

CF

aT 12/1

3

3

Require s dependence to disentangle pT and xT dependence

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<z>

<xq>

<xg>

• Large rapidity production >4 probes asymmetric partonic collisions

• Mostly high-x valence quark + low-x gluon

• 0.3 < xq< 0.7

• 0.001< xg < 0.1

• <z> nearly constant and high 0.7 ~ 0.8

• Large-x quark polarization is known to be large from DIS

• Directly couple to gluons probe of low x gluons

NLO pQCDJaeger,Stratmann,Vogelsang,Kretzer

p p 0, 3.8, s 200GeV

Forward production in hadron collider

pd

pAu

q

g

Q2 ~ pT2

s 2EN

ln(tan(2

))

xq xF / zEN

xqpxgp

xF 2E

s

z E

Eq

xg pT

se g

EN

(collinear approx.)

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Spin Effects in the Forward Direction

STAR collaboration Phys. Rev. Lett. 92 (2004) 171801

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 fragmentation

• Qiu/Sterman (initial state) / Koike (final state): twist-3 pQCD multi-parton correlations

√s=200 GeV, <η> = 3.8

D. Morozov, for STAR [hep-ex/0512013]

Spin effects initially observed in RHIC run 2 confirmed by measurements in runs 3,5.

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 Run3 Run5 Run6

detector

EEMC

and FPD

prototypes

6 matrices

of FPD

full FPD

(8 matrices)

East FPD

West FPD++

~15 ~30 ~45 ~60

0.15 0.25 0.1 6.8

3.8 ±3.3/±4.0 ±3.7/±4.0 -3.7/3.3

1, pbLdt

%,BEAMPsampled

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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FPD

=2

= 1

FPD++

East-side West-side

Inclusive 0 in forward region: 4<<3 (FPD), 2.5<<4 (FPD++)

RUN6 configurationx

zy

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FPD++ Physics for Run6

Run-5 FPD

We staged a large version of the FPD to prove our ability to detect jet-like events, direct photons, etc. with the STAR FMS

The center annulus of the run-6 FPD++ is similar to arrays used to measure forward SSA. The FPD++ annulus is surrounded by additional calorimetry to increase the acceptance for jet-like events and direct events.

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Acceptance of FPD

STARSTAR

Inclusive Inclusive 00

Strong xF - pT correlation because of limited acceptance

FPD

xF0 0.2 0.4 0.6 0.8

4

5

0

2

3

1

6p T

GeV

/c

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Study of the pT dependence needs large acceptance

Acceptance of FPD and FPD++

STARSTAR

Inclusive Inclusive 00

FPD++

FPD

xF0 0.2 0.4 0.6 0.8

4

5

0

2

3

1

6p T

GeV

/c

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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

STARSTAR hep-ex/0612030

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Estimates of Systematic ErrorsConsistency of repeated measurements

runN

NrunN

A

AA

LRRL

LRRL

BeamN

NNNN

NNNN

PA

1

Analyzing power is measured by “cross-ratio” method with two-arms (left-right) detector:

Run-by-run comparison with meanis consistent with statistics, exceptright near the threshold

<η>=3.7<η>=3.3

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AN(pT) at xF > 0.4 Run3+Run5 data (hep-ex/0512013):

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

• 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

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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

STARSTAR

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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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FMS FMS construction construction completedcompletedinstallation and installation and commissioning commissioning during Run 7 during Run 7 (NOW)(NOW)

FMS ½ Wall Pb. Glass FMS ½ Wall Pb. Glass FMS WallFMS Wall

FMS for Run 7FMS for Run 7NOW!! NOW!!

• Near full EM coverage -1<<4

• Pairs of Forward Pions same side correlations (Fragmentation – Collins)

• Event by event “x” measurement from two jets.

• Opposite side correlated pions (dijets)

– Sivers effect– d-Au (Gluon saturation in

Nuclei)

• Other future objectives– Forward Lepton pairs– Charm

PHYSICS OBJECTIVESPHYSICS OBJECTIVES

OutlookForward Meson Spectrometer

Installation completed 2007

1. A d-Aud-Au measurement of the parton model gluon density distributions x g(x) in gold nucleigold nuclei for 0.001< x 0.001< x <0.1<0.1. For 0.01<x<.1, this measurement tests the universality of the gluon distribution.

2. Characterization of correlated pion cross sections as a function of Q2 (pT

2) to search for the onset of gluon saturation effects associated with macroscopic macroscopic gluon fields. gluon fields. (again d-Au)(again d-Au)

3. Measurements with transversely polarized transversely polarized protonsprotons that are expected to resolve the origin of the resolve the origin of the large transverse spin large transverse spin asymmetriesasymmetries in reactions for forward forward production. production. (polarized pp)(polarized pp)