Transverse Spin Asymmetries at RHICAccess to transverse momentum dependent distributions

L.C. BlandBrookhaven National Laboratory11 June 2007

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Relativistic Heavy Ion Collider

3 Spin Experiments PHENIX STAR BRAHMS PHOBOS (heavy-ion)

Characteristics•2 counter-circulating rings3.8 km in circumference•Top Energies (each beam):

•100GeV / Au-Au•250GeV / p-p•Mixed Species (d+Au)

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RHIC Polarized ColliderBRAHMS & PP2PP

STAR

PHENIX

AGS

LINAC BOOSTERPol. 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 Asymmetry (SSA)Definitions: how and what to measure in an experiment

• Definition:

• dσ↑(↓) – differential cross section of when incoming proton has spin up(down)

One way to measure:• Single arm detector:

R – relative luminosity

PBeam – beam polarization

ddddAN

LLR

RNNRNN

PA

BeamN

1

• positive AN: more 0 from spin up( than from spin down(

• Accuracy, AN NN½ many events required for good accuracy

Left

p p

pL,xFp

pT,

, xF>0 xF<0

In general, AN is a function of both xF and pT

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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

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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).

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Two of the Explanations for Large Transverse SSA

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

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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

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Hard ScatteringHard scattering hadroproduction

cabccbb

cbaaacba dzDxfxfdzdxdxd

ˆ)()()( ,,

Factorization theorems state that the inclusive cross section for p+p +X can be computed in perturbative QCD using universal PDF and fragmentation functions, and perturbatively calculated hard-scattering cross sections, , for partonic process a+bc. All such processes are summed over to yield the inclusive production cross section.

)(zDc

cabd̂

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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

Ed3

dp3 [

b/G

e V3 ]

Data-pQCD difference at pT=1.5GeV

xF

E

d3 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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Cross sections at Cross sections at forward rapidity y=2.95forward rapidity y=2.95 are consistent with NLO pQCD.NLO pQCD.

pp collision at s=200GeV (3)

pT GeV/c

hep-ex/0701041Accepted for publication in Phys. Rev. Lett.

K+ p p

0 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5

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STAR-FPD STAR-FPD Cross SectionsCross Sections

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

65

13

3

BC

pxdpdE B

TC

F

Expect QCD scaling of form:

anBpxspxxdpdE an

TC

F

anT

CF

aT 12/13

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 p

Au

q

g

Q2 ~ pT2

s 2EN

ln(tan(2

))

xq xF / zENxqp xgp

xF 2E

s

z E

Eq

xg pT

se g

EN

(collinear approx.)

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First Transverse SSA at RHICprior to RHIC run 6

S.S. Adler et al. Phys. Rev Lett. 95 (2005) 202001

Particle production at ~90 (midrapidity) relative to the colliding beams have zero tranverse single spin asymmetry.

p+p collisions , s = 200 GeV

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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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p,K identification < 30 GeV/c pbar > 17 GeV/c with efficiency ~ 97%

p

e

K

Particle Identification using RICH

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SSAs at 2.3 and 4 deg. at s = 200 GeV

• SSA () survive.• SSA(+): positive

SSA(): negative 4-6% in 0.15 <xF< 0.3.

+

2.3 4

0.1

0.1

0

SSA

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SSAs at 2.3 deg. at √s = 200 GeV

• SSA(K+), SSA(K-): positive 2-5% for 0.15 <xF <0.3. • SSA(pbar), SSA(K-) > 0: Contribution from sea-quarks. • SSA(p) ~ 0: Significant fraction of proton can be mostly from

polarized beam proton, but only ones showing SSA~0.

pK

0.1

0.1

0

SSA

p

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Overview of transverse spin runs at STAR with forward calorimetry: 2001→2006

Run2 Run3 Run5 Run6

detectorEEMC

and FPDprototypes

6 matrices of FPD

full FPD(8 matrices)

East FPDWest 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

Polarization Measurements2006 Run

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RHIC Luminosity Run-6 vs. Run-5100 x 100 Gev pp RUN05-06, PHENIX Integrated Luminosity

(final delivered)

0

5

10

15

20

25

30

35

40

45

0 7 14 21 28 35 42 49 56 63 70 77 84 91

PHENIX Days in Physics mode

Inte

grat

ed L

umin

osity

(pb

-1)

RUN05

RUN06

Plot by Phil Pile

An extraordinary Run-6!

Average Polarization 60%!

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FPD

=2

= 1

FPD++East-side West-side

• Di-jet results: 1<<2 (Barrel EMC, Endcap EMC, 2)

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

RUN6 configurationx

zy

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Di-jets at STARp+p, s=200 GeV

STARSTAR

Large acceptance of STAR ideal for di-jet detection.

arXiv:0705.4629v1 [hep-ex]

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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

Boer & Vogelsang, PRD 69 (2004) 094025

jet

jet

AN pbeam (kT ST)

pbeam into page

STAR Results vs. Di-Jet Pseudorapidity SumSTAR Results vs. Di-Jet Pseudorapidity SumRun-6 ResultRun-6 Result

STARSTARarXiv:0705.4629v1, submitted to PRL

Emphasizes (50%+ ) quark Sivers

AN consistent with zero

~order of magnitude smaller in pp di-jets than in semi-inclusive DIS quark Sivers asymmetry!

VY 1, VY 2 are calculations by Vogelsang & Yuan, PRD 72 (2005) 054028

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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 and FPD++

STARSTAR

pp+p+p00XX

s = 200 GeV

• Single <> limited acceptance strong xF and pT correlation

• Study of pT dependence needs large acceptance.

FPD++

FPD

xF0 0.2 0.4 0.6 0.8

4

5

0

2

3

1

6p T G

eV/c

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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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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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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

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PHENIX Muon Piston Calorimeter

SOUTH

• 192 PbWO4 crystals with APD readout

• Better than 80% of the acceptance is okay

2.22.2 18 cm3

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PHENIX Goes ForwardFirst results with muon piston calorimeter from run 6

p+p+X, s = 62 GeV

Transverse SSA persists with similar characteristics over a broad range of collision energy (20 < s < 200 GeV)

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SummaryFrom RHIC run 6 (“Renaissance Run”)

• 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, 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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OutlookStill More RHIC Run 6 Results to Come

• Is the single spin asymmetry observed for also present for the jet the comes from?

• Answer discriminates between Sivers and Collins contributions

• Trigger on energy in small cells, reconstruct and measure the energy in the entire FPD++

• Average over the Collins angle and define a new xF for the event, then measure analyzing power versus xF

Expect that jet-like events are Expect that jet-like events are ~~15% of 15% of events events

(hep-ex/0602012)

• N>3 requirement should allow analysis

• (upper left) for each event, examine PYTHIA record for final-state hard scattered partons event selection chooses jet-like events.

• (upper right) event-averaged correlation between photon energy and distance in space from thrust axis events are expected to exhibit similar jet characteristics as found at 0

• (middle) multi-photon final states enable reconstruction of parent parton kinematics via momentum sum of observed photons.

• (bottom) projected statistical accuracy for data sample having 5 pb-1 and 50% beam polarization.

Azimuthal symmetry of FPD++ around thrust axis, selected by Etrig condition, enables

• integration over the Collins angle isolating the Sivers effect, or

• dependence on the Collins angle isolating the Collins/Heppelmann effect

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OutlookRHIC Run 8, polarized p+p collisions at s=200 GeV

Project 95 / pb of Integrated Luminosity

http://spin.riken.bnl.gov/rsc/report/RHIC_spin_LRP07.pdf

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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

Forward Meson SpectrometerInstallation 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)