The Gluon’s spin contribution to the proton’s spin

---as seen at RHIC

G. BunceMoriond QCD, March 2008

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

)%syst(14)stat(912 ±±=Δ+Δ+Δ=ΔΣ sdu “proton spin crisis”

• What else carries the proton spin ?

How are gluons polarized ? How large are parton orbital angular mom. ?

DIS

pp

high pThigh pT

Probing ΔG in pp Collisions pp hX

hf

fXff

baba

hf

fXffLL

fXff

baba

LL Ddff

Dadff

dd

ddA

ba

baba

⊗⊗⊗

⊗⋅⊗Δ⊗Δ=

+−

= →

→→

−+++

−+++

∑∑

σ

σ

σσσσ

ˆ

ˆˆ

,

,

Double longitudinal spin asymmetry ALL is sensitive to ΔG

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; P=0.6

Exquisite Control of Systematics

ALL

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

−+−+++++

−+−+++++

−+++

−+++

+−

=+−

=LNLN

LNLN

PPA

YBLL ||

1

σσ

σσ

++ same helicity+ opposite helicity

“Yellow” beam “Blue” beam

RHIC Spin Runs

P L(pb^-1) Results

2002 15% 0.15 first pol. pp collisions!

2003 30% 1.6 pi^0, photon cross section,

A_LL(pi^0), 3 PRLs

2004 40% 3.0 absolute beam polarization

with polarized H jet

2005 50% 13 large gluon pol. ruled out

(P^4 x L = 0.8)

2006 60% 46 first long spin run

(P^4 x L = 6)

2007 no spin running

2008 50% (short) run in progress

RHIC Polarimetry

• PHOTO of Jet Pol

Jet Polarization

for proton-proton elastic scattering

Polarization Measurements2006 Run

PHENIX and STAR

STAR

STAR:Large acceptance with azimuthal symmetryGood tracking and PIDCentral and forward calorimetry

PHENIX:High rate capabilityHigh granularityGood mass resolution and PIDLimited acceptance

pp X

Cornerstones to the RHIC Spin program

To appear PRD Rapid, hep-ex-0704.3599

0Mid-rapidity: PHENIX

Forward: STAR

PRL 97, 152302 (2006)

And Jets and Direct pp X : PHENIX

PRL 98, 012002 (2007)

pp jet X : STAR

PRL 97, 252001 (2006)

ALL: jets

Run3&4: PRL 97, 252001

10 20 pT(GeV)

GRSV Models:“ΔG = G”: ΔG(Q2=1GeV2)=1.9“ΔG = -G”: ΔG(Q2=1GeV2)=-1.8“ΔG = 0”: ΔG(Q2=1GeV2)=0.1“ΔG = std”: ΔG(Q2=1GeV2)=0.4

STAR Preliminary Run5 (s=200 GeV)

Large gluon polarization scenario is not consistent with data

ALL: 0

pT(GeV)

Run3,4,5: PRL 93, 202002; PRD 73, 091102; hep-ex-0704.3599

5 10

GRSV model:“ΔG = 0”: ΔG(Q2=1GeV2)=0.1“ΔG = std”: ΔG(Q2=1GeV2)=0.4

Stat. uncertainties are on level to distinguish “std” and “0” scenarios? …

PHENIX Preliminary Run6 (s=200 GeV)

From ALL to ΔG (with GRSV)

Calc. by W.Vogelsang and M.Stratmann

“std” scenario, ΔG(Q2=1GeV2)=0.4, is excluded by data on >3 sigma level: 2(std)2

min>9 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

“3 sigma”

Extending x range is crucial!

Gehrmann-Stirling models

GSC: ΔG(xgluon= 01) = 1 ΔG(xgluon= 0.020.3) ~ 0

GRSV-0: ΔG(xgluon= 01) = 0 ΔG(xgluon= 0.020.3) ~ 0

GRSV-std: ΔG(xgluon= 01) = 0.4 ΔG(xgluon= 0.020.3) ~ 0.25

GSC: ΔG(xgluon= 01) = 1

GRSV-0: ΔG(xgluon= 01) = 0

GRSV-std: ΔG(xgluon= 01) = 0.4

Current data is sensitive to ΔG for xgluon= 0.020.3

unpol.

u

€

Δd + u →W −

Δu + d →W −

Δd + u → W +

Δu + d → W +

Expected start: 2009

Δq-Δq at RHIC via W production

Transverse spin: pion A_N--very large forward asymmetries

AN() at 62 GeV

Kyoto Spin2006

STAR

RHIC Spin Outline

• 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

The key points for RHIC Spin are:

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 color 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/

DIS: attractive Drell-Yan: repulsive

Attractive vs Repulsive “Sivers” EffectsUnique Prediction of Gauge Theory !

Sivers = Dennis Sivers (predicted orbital angular momentum origin of transverse asymmetries)

0.1 0.2 0.3 x

Siv

ers

Am

plitu

de

0

0

Experiment SIDIS vs Drell Yan: Sivers|DIS= − Sivers|DY

*** Probes QCD attraction and QCD repulsion ***

HERMES Sivers Results RHIC II Drell Yan Projections

Markus DiefenthalerDIS WorkshopMunich, April 2007

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 renaissanceDrell 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.