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Spin Physics with STAR at RHIC

徐庆华 , 山东大学威海， 2009.8.11

• Introduction

• STAR longitudinal spin program: results and future

• STAR transverse spin program: results and future

• Summary

STARSTAR

2

€

1

2=

1

2ΔΣ + ΔG+ < Lq, g >

Quark spin, (~30%)-DIS

Gluon spin,Poorly known

Orbital Angular MomentaLittle known

• Spin sum rule (longitudinal case):

Spin structure of nucleon

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Δq(x,Q2) = q + (x,Q2) − q− (x,Q2)

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δq(x,Q2) = q↑ (x,Q2) + q↓ (x,Q2)

• Little known in the transverse case:

Proton spin

Proton spin

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1

2∝

1

2δΣ + < Lq, g >

Helicity distribution:

Transversity:

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Detailed knowledge on ∆q(x), ∆g(x) (before RHIC)

x

4

RHIC- the first polarized pp collider in the world

BRAHMS

PHENIX

AGS

BOOSTER

Spin Rotators(longitudinal polarization)

Solenoid Partial Siberian Snake

Siberian Snakes

200 MeV PolarimeterAGS Internal Polarimeter

Rf Dipole

RHIC pC PolarimetersAbsolute Polarimeter (H jet)

AGS pC Polarimeters

Strong Helical AGS Snake

Helical Partial Siberian Snake

Spin Rotators(longitudinal polarization)

Spin flipper

Siberian Snakes

STAR

PHOBOS

Pol. H- SourceLINAC

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RHIC- the first polarized pp collider in the world

pp Run Year 2002 2003 20042005

2006 2008 2009(200/500)

< Polarization> % 15 30 40-45 45-50 60 45 55 / 35*Lmax [ 1030 s-1cm-2 ] 2 6 6 16 30 35 40 / 85*

Lint [pb-1 ] at STAR

(Long./Transverse)0 / 0.3 0.3 / 0.25 0.4 / 0 3.1 / 0.1 8.5 / 3.4 0 /3.1 22 /10.5*

*first 500 GeV run

BRAHMS

PHENIX

AGS

BOOSTER

Spin Rotators(longitudinal polarization)

Solenoid Partial Siberian Snake

Siberian Snakes

200 MeV PolarimeterAGS Internal Polarimeter

Rf Dipole

RHIC pC PolarimetersAbsolute Polarimeter (H jet)

AGS pC Polarimeters

Strong Helical AGS Snake

Helical Partial Siberian Snake

Spin Rotators(longitudinal polarization)

Spin flipper

Siberian Snakes

STAR

PHOBOS

Pol. H- SourceLINAC

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The STAR spin program

Longitudinal spin program: determination of the helicity distributions:

• Gluon polarization ∆g(x) in the nucleon

-- results & status (inclusive jet, hadrons)

-- status & future plan (di-jets, +jet, heavy flavor)

• Flavor separation: quark & anti-quark polarization

-- RHIC 500 GeV program (W prodction)

-- (anti-)hyperons spin transfer

Transverse spin program:

• Single spin asymmetry AN (SSA) on 0,

• QCD mechanisms (Sivers, Collins, high-twist)

-- forward +jet production on Sivers effects

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MRPC ToF barrelMRPC ToF barrel100% ready for run 10100% ready for run 10

PMD

FPD

FMS

EMC barrel

EMC End Cap

DAQ1000DAQ1000

Complete

Ongoing

TPC

FTPC

BBC

STAR Detector (current)

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COMPASS, PLB676,31(2009)

Δg determination from DIS

• Recent measurements from DIS:

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q

€

q

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Δf2Δf1

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ALL =σ + + − σ +−

σ + + + σ +−

• Longitudinal spin asymmetry:

Accessing ∆g(x) in pp collision

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challenging pion background

€

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STARSTAR PRL 97, 252001 STARSTAR PRL 97, 152302

pQCD works at RHIC energies-unpolarized cross sections

• Mid-rapidity jet cross section is consistent with NLO pQCD over 7 orders of magnitude

• Forward rapidity π0 cross section also consistent with NLO pQCD

• Many other examples

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STAR inclusive π0 ALL at various rapidities

• During Run 6, STAR measured ALL for inclusive π0 for three different rapidity regions

• Mid-rapidity result excludes large gluon polarization scenarios• Larger rapidity correlates to stronger dominance of qg scattering

with larger x quarks and smaller x gluons

• Expect ALL to decrease as increases

|| < 0.95 1 < < 2 = 3.2, 3.7

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STAR inclusive π0 ALL at various rapidities

• During Run 6, STAR measured ALL for inclusive π0 for three different rapidity regions

• Mid-rapidity result excludes large gluon polarization scenarios• Larger rapidity correlates to stronger dominance of qg scattering

with larger x quarks and smaller x gluons

• Expect ALL to decrease as increases

|| < 0.95 1 < < 2 = 3.2, 3.7

PHENIX, arXiv:0810.0694|| < 0.35

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PRL 97, 252002 (2006)

Results on jet X-section and spin asymmetry

QuickTime™ and a decompressor

are needed to see this picture.

PRL 97, 252001 (2006)

Experimental cross section agrees withNLO pQCD over 7 orders of magnitude

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Experimental cross section agrees withNLO pQCD over 7 orders of magnitude

PRL 97, 252002 (2006)

PRL 100, 232003 (2008)

Results on jet X-section and spin asymmetry

2005

2006

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RHIC constraints

Impact of RHIC early results on Δg(x)

de Florian et al., PRL101(2008)

• Early RHIC data (2005, 2006) included in a global analysis along with DIS

and SIDIS data.• Evidence for a small gluon polarization over a limited region of momentum

fraction (0.05<x<0.2).

STAR

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Future inclusive jet measurements: Increasing Precision

• Precision will be significantly improved in future runs.

• 500 GeV data will reach low x-range for Δg with high statistics.

Projected improvement in xΔg from Run 9

Projected sensitivities:Run 9 & 500 GeV running

xT=2pT/s

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- Inclusive measurement cover integration of x-gluon.

- High pT measurement begin to separate large x, but still suffer from mixture of subprocesses.

- Need correlation measurements to constrain the shape of Δg(x)

10 20

frac

tio

n

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Inclusive Jets: LO (W. Vogelsang)

pT/GeV

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First correlation study: charged pions opposite jets

• Trigger and reconstruct a jet, then look for charged pion on the opposite side

• Correlation measurement significantly increases the sensitivity of ALL(π+)

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Probing Δg(x) with di-jets production

• Upcoming Correlation Measurements :

access to partonic kinematics

through di-jet production,

direct photon+jet production

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Sensitivity of di-jets measurements

• Projections with 50 pb-1 provide high sensitity to gluon polarization:

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Direct Photon - Jet Correlations

• Direct +jet dominated by qg-Compton process: 90% from qg

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ALL ≈Δg(x1 )

g(x1 )⋅

eq2[Δq(x2)

q∑ + Δq (x2)]

eq2[q(x2)

q∑ + q (x2)]

⋅ ) a LL

qg →qγ + (1 → 2)

Reconstruction of partonic kinematics --> x-dependence of Δg !

x2

x1

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Anti-quark helicity distribution

D. de Florian et al, PRL101(2008)

• From global fit with DIS data:

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PRD71,2005

Extrating Δq(x) in Semi-inclusive DIS

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Flavor separation of proton spin

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(Δu,Δd,Δu ,Δd through W ± production)

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ALW +

=σ + −σ −

σ + + σ −

=−Δu( x1 )d ( x2) + Δd (x1 )u(x2)

u( x1 )d (x2) + d ( x1 )u( x2)=

€

−Δu(x1 )

u( x1 ), y

W + >> 0

Δd ( x1 )

d ( x1 ), y

W + << 0

• Quark polarimetry with W-bosons:

• Spin measurements:

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−Δd(x1 )

d( x1 ), y

W − >> 0

Δu ( x1 )

u (x1 ), y

W − << 0

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ALW −

=

€

where x1 = τ ey , x2 = τ e−y and τ = Mw2 / s.

W-detection throughhigh energy lepton

-

€

∝Δd (x)u(x)

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Sensitivity of W measurements

• Strong impact on constraining the sea quark polarizations with

300 pb-1 :

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• Clear need to measure. • Can we do it with hyperons at RHIC?

- hyperons contain at least one strange quark and their polarization can be determined via their weak decay.

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ΔS = Δs + Δs , Δs = Δs( x)0

1

∫ dx

D. de Florian et al, PRL101(2008)• ΔS~ -0.08 from inclusive DIS under SU(3)_f symmetry

Strange quark polarization

• SDIS results at HERMES:

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x[Δs(x) + Δs (x)] PLB666(2008)

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GeV 8

GeV 200

>=

Tps

Q. X, E. Sichtermann, Z. Liang, PRD 73(2006)077503

models sΔ

• Expectations at LO show sensitivity of DLL for anti-Lambda to :

GRSV00-M.Gluck et al, Phys.Rev.D63(2001)094005

Typ. range at RHIC

DLL-Longitudinal spin transfer at RHIC

- DLL of is less sensitive to Δs, due to larger u and d quark frag. contributions.

Pol. frag. func. models

- Promising measurements---effects potentially large enough to be observed.

sΔ

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Spin transfer for Lambda hyperons

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DLLΛ ≡

σp + p →Λ+X

−σp + p →Λ−X

σp + p →Λ+X

+ σp + p →Λ−X

• (anti-)Lambda reconstruction using TPC tracks:

p

V0_vertex

V0_DCA

rr

−

• First proof-of-principle measurement;

~10% precision with pT up to 4 GeV.

- not yet to discriminate pol. pdfs,

- extend pT with specific trigger

• DLL extraction:

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Transverse spin program

• Single transverse-spin asymmetry

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

• Basic QCD calculations (leading-

twist, zero quark mass) predict AN~0

---AN~0.4 for + in pp at E704 (1991)

• Understanding transverse spin

effect: Qiu and Sterman (initial-state) / Koike (final-state) twist-3 pQCD calculations

Sivers: spin and k correlation in initial state (related to orbital angular momentum)

Collins: spin and k correlation in fragmentation process (related to transversity)

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AN =NL − NR

NL + NR

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xF = 2p// / sTwist-3 correlation and the k dependent

distribution/fragmentation in intermediate pT generate the same physics.

Ji-Qiu-Vogelsang-Yuan,PRL97,2006

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Recent results on SSA

• AN increase with xF, in agreement

with pQCD model calculation.

• X-section reproduced with pQCD

STAR, PRL97,152302(2006)

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• pQCD based models predicted

decreasing AN with pT , which

Is not consistent with data.

• AN increase with xF, in agreement

with pQCD model calculation.

Recent results on SSA

STAR, PRL97,152302(2006)

• X-section reproduced with pQCD

STAR, Phys. Rev. Lett. 101 (2008)222001

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• AN for the η mass region is much larger at high xF>0.55

STAR 2006 PRELIMINARY

η ~ 3.66

Run 6 inclusive AN at large xF

<AN> = 0.36 +/- 0.06 <A> = 0.08 +/- 0.02

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19.4s GeV=

~ 1 /Tp GeV c

E704 Nucl. Phys. B 510 (1998) 3

+

-

200 GeV

62.4 GeV

BRAHMS,PRL101(2008)

Large SSA of different hadrons in different experiments

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Separating Sivers and Collins effect in pp

collisions Collins effect:

spin and k correlation in fragmentation process (related to transversity)

• For hadron SSA, both Sivers and Collins effects can contribute.

• Forward jets and photon may provide separation of them.

SPk,q

p

p

Sivers effect:

spin and k correlation in initial state (related to orbital angular momentum)

SP

p

p

Sq k,πSensitive to transversity

Sensitive to orbital angular momentum

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• Mid-rapidity jet AN~0, different as the conventional calculations with

Sivers function fitted from SDIS.

Sivers distribution, is process dependent (not universal),

AAN N of jet production - Sivers effect of jet production - Sivers effect

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Sivers|Drell-Yan=−Sivers|DIS

STAR, PRL99,142003(2007)

• AN of mid-rapidity consistent with zero:

An example:

attractive color interactionrepulsive color interaction

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Probing Sivers effect with + mid-rapidity jet

Bacchetta et al., PRL 99, 212002

• Conventional calculations predict the asymmetry to have the same sign in SIDIS and γ+jet

• Calculations that account for the repulsive interactions between like color charges predict opposite sign

• Critical test of our basic theoretical understanding

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Sivers|+ jet~ −Sivers |DIS

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STAR Detector - futureMRPC ToF barrelMRPC ToF barrel

100% ready for run 10100% ready for run 10

FMS

FGT Ongoing

MTD

R&DHFT

TPC

FHC

Forward jet reconstruction with FMS+FHC

=2.8FPD

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SSA with forward jets and photons

Projected precision of AN for p+pjet + X :Jet energy profile from FHC+FMS:

• Collins effects(spin and k correlation in fragmentation process ):

Accessed via spin-dependent correlations of hadrons within forward jet

• Sivers effect(spin and k correlation in initial state):

Accessed by symmetric azimuthal integration of hadrons from forward jet

Accessed by forward direct photons

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Transverse spin transfer of hyperons and δq(x)

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PTH =

dσ ( p↑ p→ H↑ X ) − dσ ( p↑ p→ H↓ X )

dσ ( p↑ p→ H↑ X ) + dσ ( p↑ p→ H↓ X ) =dΔTσ

dσ

• Transverse spin transfer of hyperons transverse spin can provide

access to transversity, via channel ->n+ :

transversity distribution :

δf(x) = f (x) - f(x)pQCD Transversely polarized

fragmentation function :

Measurement at BELLE ?

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

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dΔTσ (r p ⊥ p→

r H ⊥X ) ∝ dxadxbdzδfa (∫

abcd

∑ xa ) fb (xb )ΔT DcH (z)dΔT

) σ (

r a ⊥b →

r c ⊥d )

- Transverse spin transfer can provide access to transversity, which is still poorly known so far.

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• Large polarization with unpolarized beam p + p + X , observed in different experiments.

Still not fully understood.

target

produced

production plane

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rN :

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rN =

r p b ×

r p Λ /(|

r p b ×

r p Λ |)

bpr

Transverse hyperons polarization in unpolarized pp

How about at RHIC energy?

( = 2pL/s)

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Longitudinal spin physics at STAR:

Determination of gluon polarization ΔG :

Currently inclusive probes with jets, are providing important constraints on ΔG. Early results have been included in global analysis.

Near future probes:

Increased statistics and higher energy for inclusive jets will provides additional constraints with better precision and wider x-range.

Correlation measurements (di-jet, photon-jet) with access to partonic kinematics will provide better resolution in x and direct probe to ΔG.

Determination of sea quark polarization:

With 500 GeV collisions, W-production provide unique tool to study the anti-quark polarization.

Spin transfer of hyperons provides sensitivity to strange quark polarization.

Summary & Outlook - I

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Transverse spin physics at STAR:

STAR has observed large transverse single-spin asymmetries for forward particle production.

Study Collins and Sivers effects in pp reaction with Single-spin asymmetry with forward jet .

STAR transverse γ+jet measurements will provide a direct illustration of attractive vs. repulsive color-charge interactions

Transverse hyperon polarization at forward region at STAR

Summary & Outlook -II

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FMS: expanding STAR’s forward acceptance

• Expanded pT range for inclusive π0 AN during Run 8

STAR Forward Meson Spectrometer2.5 < η < 4.0 STARSTAR

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What is the FHC?

• Two identical 9x12 enclosures of E864 hadron calorimeter detectors

---100X100X117 cm3

• Refurbished and used by PHOBOS collaboration as forward hadron multiplicity detectors for run-3 d+Au

Recycle

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PHENIX, arXiv:0810.0694

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World efforts for spin physics

• Current running– Lepton-nucleon

scattering: COMPASS, JLAB

– Polarized proton-proton scattering, RHIC

• Future facilities– EIC (BNL)

– JPARC (Japan)

– GSI-FAIR (Germany)

HERMES@ DESY e+-p @27GeV

COMPASS@CERN p@160GeV

Jefferson Labe-

p@6,12GeV

RHIC@BNLpp@200&500Ge

VSLAC

E142-155 EMC@CERN

Finished experiments: SLAC, EMC, SMC, HERMES

All these experiments have their unique coverage on Δq, Δg, Lq,g, and they are complementary as well

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Hyperon spin transfer at forward region

Forward hyperons, reconstructed via n+ channel, and polarization can

be determined through decay product, i.e, dN/dcos * = N0(1+Pcos *)

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Δσ ∝ Δfa (x1 ) ⋅∫ fb (x2) ⊗Δ√ σ ⊗ΔDΛ (z)

Provide access to pol.p.d.f. and fragmentation functions

• Longitudinal spin transfer DLL:

Model evaluation shows

DLL provide sensitivity to

pol. parton distributions.

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s = 200 GeV

p T(Λ) > 2 GeVΔs(x) models

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Jet Finding in STAR

Jet reconstructed with TPC tracks and EMC energy deposits, using midpoint Cone Algorithm:

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The STAR Detector

Magnet• 0.5 T Solenoid

Triggering & Luminosity Monitor• Beam-Beam Counters

– 3.4 < || < 5.0• Zero Degree Calorimeters

Central Tracking• Large-volume TPC

– || < 1.3

Calorimetry• Barrel EMC (Pb/Scintilator)

– || < 1.0– Shower-Maximum Detector

• Endcap EMC (Pb/Scintillator)– 1.0 < < 2.0

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Transverse spin asymmetry- spin structure of nucleon

• Large single transverse-spin asymmetry observed at RHIC:

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

€

AN =NL − NR

NL + NR

€

xF = 2p// / s

STAR, Phys. Rev. Lett. 97 (2006)152302

• Basic QCD calculations (leading- twist, zero quark mass) predict

AN~0,

while cross sections are found to be in agreement with pQCD

evaluations.