G. Bunce Moriond QCD, March 2008

The Gluon’s spin contribution to the proton’s spin ---as seen at RHIC

I would like to thank Les Bland, Werner Vogelsang, Abhay Deshpande, Sasha Bazilevsky, Matthias Grosse Perdekamp, for their advice and many plots.

a history of the strong interaction: 1964: “quarks” …to understand the zoo of strongly interacting particles; “color” quantum number …to describe the Ω-

( sss, S=3/2)

1967: quarks are real! …from hard inelastic scattering of electrons from protons at SLAC 1973: the theory of QCD …quarks and “gluons” and color; perturbative QCD 1980s to present: e-p and pbar p colliders …beautiful precision tests of pQCD,

unpolarized

………………………………………………………………….

1970s:

polarized

beams and targets 1988: the spin of the proton is not carried by its quarks!

1990s to present: confirmed in “DIS” fixed target experiments using electrons and muons to probe the spin structure of the proton 2001 to present: probe the spin structure of the proton using quarks and gluons

(strongly interacting probes see both the gluons and quarks in the proton):

RHIC

EMC at CERN: J. Ashman et al., NPB 328, 1 (1989): polarized muons probing polarized protons   

u

 

d

 

s

 12  9 ( stat )  14 ( syst )% “proton spin crisis”

•

What else carries the proton spin ?

 

How are gluons polarized ?

How large are parton orbital angular mom. ?

DIS pp

high p

T

Probing



G in pp Collisions

pp



hX

A LL



d



d

      

d



d

     

a



b

, 

f a



a

 ,

b



f b f a

 

d

 ˆ

f b f a f b



fX



d

 ˆ 

a

ˆ

f a LL f b



fX f a f b



fX



D f h



D h f

Double longitudinal spin asymmetry A LL sensitive to  G is

RHIC Polarized Collider

Absolute Polarimeter (H



jet)

PHOBOS

RHIC pC Polarimeters

BRAHMS & PP2PP

Siberian Snakes

PHENIX

Siberian Snakes

STAR

Spin Rotators (longitudinal polarization) Pol. H Source

LINAC BOOSTER

200 MeV Polarimeter Spin Rotators (longitudinal polarization)

Helical Partial Siberian Snake AGS Strong AGS Snake

AGS pC Polarimeter

2006: 1 MHz collision rate; P=0.6

Exquisite Control of Systematics

“Yellow” beam “ Blue” beam

A LL

A LL

                1 |

P B P Y N

  |

N

 

L

 

L

   

N

 

N

 

L

 

L

  ++ same helicity +  opposite helicity

(P) Polarization (L) Relative Luminosity (N) Number of pi0s

RHIC Spin Runs

P L

(pb^-1)

Results 2002 15% 0.15 first pol. pp collisions!

2003 2004 30% 40% 1.6

3.0

pi^0, photon cross section, A_LL(pi^0), 3 PRLs absolute beam polarization with polarized H jet 2005 2006 2008 50% 13 large gluon pol. ruled out (P^4 x L = 0.8) 60% 46 first long spin run (P^4 x L = 6) 2007 no spin running 50% (short) run in progress

RHIC Polarimetry

Jet Polarization

• PHOTO of Jet Pol



for proton-proton elastic scattering

Polarization Measurements 2006 Run

PHENIX and STAR

PHENIX: High rate capability High granularity Good mass resolution and PID Limited acceptance

STAR

STAR: Large acceptance with azimuthal symmetry Good tracking and PID Central and forward calorimetry

Cornerstones to the RHIC Spin program

Mid-rapidity: PHENIX pp

 

0 X Forward: STAR

To appear PR

D

Rapid, hep-ex-0704.3599

PRL

97

, 152302 (2006)

And

Jets and Direct

g

pp



jet X : STAR pp

 g

X : PHENIX

PRL 97, 252001 (2006) PRL 98, 012002 (2007)

10 20 pT(GeV)

A

LL

: jets

STAR Preliminary Run5 (



s=200 GeV) GRSV Models:

“  G = G”:  G(Q 2 =1GeV 2 )=1.9

“  G = -G”:  G(Q 2 =1GeV 2 )=-1.8

“  G = 0”:  G(Q 2 =1GeV 2 )=0.1

“  G = std”:  G(Q 2 =1GeV 2 )=0.4

Large gluon polarization scenario is not consistent with data Run3&4: PRL 97, 252001

A

LL

:



0

PHENIX Preliminary Run6 (



s=200 GeV)

5 10 pT(GeV)

GRSV model:

“  G = 0”:  G(Q 2 =1GeV 2 )=0.1

“  G = std”:  G(Q 2 =1GeV 2 )=0.4

Stat. uncertainties are on level to distinguish “std” and “0” scenarios? … Run3,4,5: PRL 93, 202002; PRD 73, 091102; hep-ex-0704.3599

From A

LL

to



G (with GRSV)

Calc. by W.Vogelsang and M.Stratmann



“3 sigma”

“std” scenario,  2 (std)   2 min >9  G(Q 2 =1GeV 2 )=0.4, is excluded by data on >3 sigma level: Only exp. stat. uncertainties are included (the effect of syst. uncertainties is expected to be small in the final results)  Theoretical uncertainties are not included

Extending x range is crucial!

Gehrmann-Stirling models  G(x gluon = 0  1) = 1 

G(x gluon = 0.02



0.3) ~ 0

GRSV-0:  G(x gluon = 0  1) = 0 

G(x gluon = 0.02



0.3) ~ 0

GRSV-std:  G(x gluon = 0  1) = 0.4



G(x gluon = 0.02



0.3) ~ 0.25

Current data is sensitive to  G for x gluon = 0.02

 0.3

 

q-



q at RHIC via W production

u unpol.



d



u



W

 

u



d



W

 

d



u



W

 

u



d



W



Expected start: 2009

Transverse spin: pion A_N - very large forward asymmetries

A N (  ) at 62 GeV

STAR Kyoto Spin2006

RHIC Spin Outline

The key points for RHIC Spin are:

• • • • •

Spin structure of proton Strongly interacting probes ---------- P=60%, L=2x10^31, root(s)=200 GeV in 2006 Polarized atomic H jet: absolute P, pp elastic physics --------- Cross sections for pi^0, jet, direct photon described by pQCD

• • • •

Helicity asymmetries: sensitivity to gluon spin contribution to proton --------- W boson parity violating production: ubar and dbar polarizations in proton --------- Very large transverse spin asymmetries in pQCD region --------- Future: transverse spin Drell Yan

A Fundamental Test of Universality:

Transverse Spin Drell Yan at RHIC vs Sivers Asymmetry in Deep Inelastic Scattering

• Important test at RHIC of recent fundamental QCD predictions for the Sivers effect, demonstrating…

attractive vs repulsive c o l o r charge forces ----------------

• Possible access to quark

orbital angular momentum

• Requires very high luminosity (RHIC II) • Both STAR and PHENIX can make important, exciting, measurements • Discussion available at http://spin.riken.bnl.gov/rsc/

Attractive vs Repulsive “Sivers” Effects

Unique Prediction of Gauge Theory !

DIS: attractive Drell-Yan: repulsive Sivers = Dennis Sivers (predicted orbital angular momentum origin of transverse asymmetries)

Experiment SIDIS vs Drell Yan:

Sivers| DIS = − Sivers| DY *** Probes Q C D attraction and Q C D repulsion ***

HERMES Sivers Results RHIC II Drell Yan Projections

0 0

Markus Diefenthaler DIS Workshop Munich, April 2007

0.1 0.2 0.3 x

Concluding Remarks

• • • • •

High luminosity and high polarization achieved!

------------- Delta G: direct photon; global fits with RHIC, DIS; new vertex and forward detectors ------------- W boson parity violating production: ubar and dbar ------------- Very strong theoretical support ------------- Transverse spin renaissance



Drell Yan crucial test of our understanding of the underlying physics!

Spin is one of the most fundamental concepts in physics, deeply rooted in Poincare invariance and hence in the structure of space-time itself. All elementary particles we know today carry spin , among them the particles that are subject to the strong interactions, the spin ½ quarks and the spin 1 gluons . Spin, therefore, plays a central role also in our theory of the strong interactions, QCD , and to understand spin phenomena in QCD will help to understand QCD itself.

To contribute to this understanding is the primary goal of the spin physics program at RHIC.