Spin Physics with STAR at RHIC

STAR

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

1

Spin structure of nucleon

• Spin sum rule (longitudinal case) :

1 2



1 2

  

G

 

L q

,

g

 

Quark spin , (~30%)-DIS Gluon spin , Poorly known Helicity distribution:

q(x,Q 2 )



q



(x,Q 2 )



q



(x,Q 2 )

  • Little known in the transverse case:

1



2 1 2

   

L

q, g

 Transversity: 

q(x,Q 2 )



q



(x,Q 2 )



q



(x,Q 2 )

Orbital Angular Momenta Little known

Proton spin

  

Proton spin

    2

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

x

RHIC- the first polarized pp collider in the world Absolute Polarimeter (H  jet) RHIC pC Polarimeters

PHOBOS BRAHMS

Siberian Snakes Siberian Snakes

PHENIX STAR

Spin Rotators (longitudinal polarization) Pol. H Source Solenoid Partial Siberian Snake

LINAC BOOSTER

200 MeV Polarimeter Rf Dipole

AGS

Spin flipper Spin Rotators (longitudinal polarization) Helical Partial Siberian Snake AGS Internal Polarimeter AGS pC Polarimeters

Strong Helical AGS Snake

4

RHIC- the first polarized pp collider in the world Absolute Polarimeter (H  jet) RHIC pC Polarimeters

PHOBOS BRAHMS

Siberian Snakes Siberian Snakes

PHENIX STAR

Spin Rotators (longitudinal polarization) Pol. H Source Solenoid Partial Siberian Snake

LINAC BOOSTER

200 MeV Polarimeter

pp Run Year < Polarization> % L max [ 10 30 s -1 cm -2 ] L int [pb -1 ] at STAR (Long./Transverse)

*first 500 GeV run

2002 15 2 0 / 0.3

Rf Dipole

2003 30 6 0.3 / 0.25

AGS 2004 40-45 6 0.4 / 0

Spin flipper Spin Rotators (longitudinal polarization) Helical Partial Siberian Snake AGS Internal Polarimeter AGS pC Polarimeters

Strong Helical AGS Snake 45-50 16 3.1 / 0.1 60 30 8.5 / 3.4 45 35 0 /3.1

2009(200/ 500 ) 55 / 35* 40 / 85* 22 / 10.5*

5

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 A N (SSA) on  0 ,  • QCD mechanisms (Sivers, Collins, high-twist) -- forward  +jet production on Sivers effects 6

MRPC ToF barrel 100% ready for run 10

STAR Detector (current)

EMC barrel FPD

BBC

TPC EMC End Cap FMS PMD

DAQ1000 FTPC

Complete Ongoing 7



g determination from DIS

• Recent measurements from DIS:  

q q

COMPASS, PLB676,31(2009) 8

Accessing ∆g(x) in pp collision

• Longitudinal spin asymmetry:

A LL

             

f 1



f 2

    9

pQCD works at RHIC energies-unpolarized cross sections

STAR

PRL 97, 252001

STAR

PRL 97, 152302 • Mid-rapidity jet cross section is consistent with NLO pQCD over 7 orders of magnitude • Forward rapidity π 0 cross section also consistent with NLO pQCD • Many other examples 10

STAR inclusive π

0

A

LL

at various rapidities

|



| < 0.95

1 <



< 2



= 3.2, 3.7

• During Run 6, STAR measured A LL rapidity regions for inclusive π 0 for three different • Mid-rapidity result excludes large gluon polarization scenarios • Larger rapidity correlates to stronger dominance of

qg

scattering with larger

x

• Expect A LL quarks and smaller

x

gluons to decrease as  increases 11

STAR inclusive π

0

A

LL

at various rapidities

|



| < 0.95

1 <



< 2



= 3.2, 3.7

|



| < 0.35

PHENIX, arXiv:0810.0694

• During Run 6, STAR measured A LL rapidity regions for inclusive π 0 for three different • Mid-rapidity result excludes large gluon polarization scenarios • Larger rapidity correlates to stronger dominance of

qg

scattering with larger

x

• Expect A LL quarks and smaller

x

gluons to decrease as  increases 12

Results on jet X-section and spin asymmetry Experimental cross section agrees with NLO pQCD

over 7 orders of magnitude

PRL 97, 252002 (2006)

PRL 97, 252001 (2006)

QuickTime™ and a decompressor are needed to see this picture.

13

Results on jet X-section and spin asymmetry Experimental cross section agrees with NLO pQCD

over 7 orders of magnitude

2005 PRL 100, 232003 (2008)

PRL 97, 252002 (2006)

2006

14

Impact of RHIC early results on  g(x) de Florian et al., PRL101(2008) RHIC constraints

STAR

• 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

15

Future inclusive jet measurements: Increasing Precision Projected sensitivities:

Run 9

&

500 GeV running

Projected improvement in x  g from

Run 9

x T =2p T /  s • Precision will be significantly improved in future runs.

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

16

Inclusive Jets: LO (W. Vogelsang) 10 20 p T /GeV 30

- Inclusive measurement cover integration of x-gluon.

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

- Need correlation measurements to constrain the shape of

Δg(x)

17 17

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 A LL ( π + ) 18

Probing



g(x) with di-jets production

• Upcoming Correlation Measurements :  access to partonic kinematics through di-jet production, direct photon+jet production 19

Sensitivity of di-jets measurements

• Projections with 50 pb -1 provide high sensitity to gluon polarization: 20



Direct Photon - Jet Correlations

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

x 2 x 1

Reconstruction of partonic kinematics --> x-dependence of  g !

A LL

 

g(x 1 )



g(x 1 )



q



e q

2

q

[



q(x 2 )

e q

2 [q(x 2 )

 

q (x 2 )]



q (x 2 )]



qg



q



a LL



(1



2)

21

Anti-quark helicity distribution

• From global fit with DIS data: D. de Florian et al, PRL101(2008) 22

Extrating



q(x) in Semi-inclusive DIS

PRD71,2005 23

Flavor separation of proton spin 

u,



d,



u ,



d

through W



production)

• Quark polarimetry with W-bosons:  

d ( x)u( x)

W-detection through high energy lepton   • Spin measurements:

A L W

               u( x

1

)d ( x

2 )

  d ( x

1

)u( x

2 )

u( x

1

)d ( x

2 )

 d ( x

1

)u( x

2 )



A W L

 

x

1

    d( x

1 )

d( x

1 )

 u ( x

1 )

u ( x

1 ) ,

y W

,

y W

  

0



0



e y

, x

2

 

e



y and

 

M w

2

/ s.    u( x

1 )

u( x

1 ) ,

y W

  d ( x

1 )

d ( x

1 ) ,

y W

 

0



0

24 

Sensitivity of W measurements

• Strong impact on constraining the sea quark polarizations with 300 pb -1 : 25



Strange quark polarization

•  S~ -0.08 from inclusive DIS under SU(3)_f symmetry 

S

 

s

  s , 

s

 

0 1

 s( x)

dx

• SDIS results at HERMES: D. de Florian et al, PRL101(2008)

x[



s(x)

 

s (x)]

PLB666(2008)  • • 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.

26

D LL -Longitudinal spin transfer at RHIC • Expectations at LO show sensitivity of D LL for anti-Lambda to 

s

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

p T s

 200 GeV  8 GeV

Pol. frag. func. models



s

models Typ. range at RHIC Q. X, E. Sichtermann, Z. Liang, PRD 73(2006)077503 - Promising measurements---effects potentially large enough to be observed.

- D LL of  is less sensitive to  s, due to larger u and d quark frag. contributions.

27

Spin transfer for Lambda hyperons • (anti-)Lambda reconstruction using TPC tracks: p 

V0_vertex

 

V0_DCA

r

  • D LL extraction: 

D LL

 

p



p

 

X



p



p

 

X

  

p



p

 

X



p



p

 

X

• First proof-of-principle measurement; ~10% precision with p T up to 4 GeV.

- not yet to discriminate pol. pdfs, - extend p T with specific trigger 28



Transverse spin program

• Single transverse-spin asymmetry

A N



N L N L

 

N R N R x F

 2p

// /

s

STAR, Phys. Rev. Lett.

92

(2004)171801  • Basic QCD calculations (leading twist, zero quark mass) predict A N ~0 ---A N ~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 fragmentation process (related to transversity )  correlation in Twist-3 correlation and the k  dependent distribution/fragmentation in intermediate p T generate the same physics.

Ji-Qiu-Vogelsang-Yuan,PRL97,2006 29

Recent results on SSA

STAR, PRL97,152302(2006) • X-section reproduced with pQCD • A N increase with x F, in agreement with pQCD model calculation.

30

Recent results on SSA

STAR, PRL97,152302(2006) • X-section reproduced with pQCD • A N increase with x F, in agreement with pQCD model calculation.

• pQCD based models predicted decreasing A N with p T , which Is not consistent with data. STAR, Phys. Rev. Lett.

101

(2008)222001 31

Run 6 inclusive



A

N

at large x

F

STAR 2006 PRELIMINARY

η ~ 3.66

• A N for the η mass region is much larger at high

x F >0.55

  = 0.36 +/- 0.06  = 0.08 +/- 0.02 32

Large SSA of different hadrons in different experiments



+

p T

~ 1 / 

-

s

 19.4

GeV

E704 Nucl. Phys. B 510 (1998) 3 200 GeV 62.4 GeV

BRAHMS,PRL101(2008) 33

Separating Sivers and Collins effect in pp collisions

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

S P k



,q p

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

S P p

p p Sensitive to orbital

angular momentum

Sensitive to

transversity S q k



, π

•

For hadron SSA, both Sivers and Collins effects can contribute. • Forward jets and photon may provide separation of them.

34

A N of jet production - Sivers effect • A N of mid-rapidity consistent with zero: STAR, PRL99,142003(2007) • Mid-rapidity jet A N ~0, different as the conventional calculations with Sivers function fitted from SDIS.

Sivers distribution, is process dependent (not universal), An example: Drell-Yan   Sivers | DIS repulsive color interaction attractive color interaction 35 

Probing Sivers effect with



+ mid-rapidity jet

Bacchetta et al., PRL 99, 212002  + jet

~

 Sivers | DIS  • • • 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 36

Forward jet reconstruction with FMS+FHC

MRPC ToF barrel

STAR Detector - future

100% ready for run 10 FMS

 =2.8

FPD

TPC

FHC HFT

FGT Ongoing

R&D

37

SSA with forward jets and photons

Jet energy profile from FHC+FMS: Projected precision of A N for p  +p  jet + X : •

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 38

Transverse spin transfer of hyperons and  q(x)  • Transverse spin transfer of hyperons transverse spin can provide access to transversity, via channel  ->n+  0 :

P T H



d



( p



p



H



X )

d



( p



p



H



X )

d



T



( p



p



H



X )

 

abcd

 

d



( p



p



H



X )

d



( p



p



H



X )



d



d T

  TIFF (LZW) decompressor are needed to see this picture.



dx a dx b dz



f a

(

x a

) f

b

(x

b

)



T D c H

(z)d



T



( a



b



c



d )

 transversity distribution :  f(x) = f  (x) - f  (x) Transversely polarized fragmentation function : pQCD Measurement at BELLE ?

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

39

Transverse hyperons polarization in unpolarized pp • Large polarization with unpolarized beam p + p    + X , observed in different experiments.

Still not fully understood.



p b

N :

N



p b



p

 /(| p

b



p



|)

produced



target production plane



How about at RHIC energy?

( = 2p L /  s) 40

Summary & Outlook - I

Longitudinal spin physics at STAR:  Determination of gluon polarization  G :  Currently inclusive probes with jets , are providing important constraints on  G. 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  G.

 Determination of sea quark polarization:  With 500 GeV collisions, W-production the anti-quark polarization.

provide unique tool to study  Spin transfer of hyperons provides sensitivity to strange quark polarization.

41

Summary & Outlook -II

Transverse spin physics at STAR:  STAR has observed large transverse single-spin asymmetries forward particle production.

for  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 42

FMS: expanding

STAR

’s forward acceptance

STAR

Forward Meson Spectrometer 2.5 < η < 4.0

STAR

• Expanded p T inclusive π 0 A N range for during Run 8 43

What is the FHC?

• Two identical 9x12 enclosures of E864 hadron calorimeter detectors ---100X100X117 cm 3 • Refurbished and used by PHOBOS collaboration as forward hadron multiplicity detectors for run 3 d+Au

Recycle

44

PHENIX, arXiv:0810.0694

45

46

World efforts for spin physics

Finished experiments: SLAC, EMC, SMC, HERMES • Current running – Lepton-nucleon scattering: COMPASS, JLAB – Polarized proton-proton scattering, RHIC SLAC E142-155 • Future facilities – EIC (BNL) – JPARC (Japan) – GSI-FAIR (Germany) RHIC@BNL pp@200&500GeV EMC@ CERN Jefferson Lab e-p@6,12GeV HERMES@ DESY e + -p @27GeV COMPASS@CERN  p@160GeV

All these experiments have their unique coverage on



q,



g, Lq,g, and they are complementary as well

47

48

Hyperon spin transfer at forward region

Forward hyperons,  reconstructed via n+  0 channel, and polarization can be determined through decay product, i.e, dN/dcos  * = N 0 (1+ a  P  cos  *) • Longitudinal spin transfer D LL : Provide access to pol.p.d.f. and fragmentation functions   

f a

(x 1 )



f b

(x 2 )

    

D



(z)

 Model evaluation shows D LL provide sensitivity to pol. parton distributions.



s



T (



) 200 GeV



2 GeV

 s(x) models 49

Jet Finding in STAR

 Jet reconstructed with TPC tracks and EMC energy deposits, using midpoint Cone Algorithm : 50

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

51

Transverse spin asymmetry

- spin structure of nucleon • Large single transverse-spin asymmetry observed at RHIC:

A N



N L N L

 

N R N R



x F

 2p

// /

s

STAR, Phys. Rev. Lett.

92

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

97

(2006)152302 • Basic QCD calculations (leading- twist, zero quark mass) predict A N ~0, while cross sections are found to be in agreement with pQCD evaluations. 52