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.
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