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RHIC Spin…to study the spin structure of the
proton
SLAC Summer Institute21 July 2006
G. Bunce
•a history—from quarks to spin structure•Deeply Inelastic Scattering and RHIC•Siberian Snakes and accelerating polarized protons•Our plans and some results•Future directions
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Quark ModelQuark Model QQCCD D
?
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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
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Deeply-inelastic scattering ep → e X :
“inelasticity” Bjorken var.
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Parton Model : Feynman; Bjorken, Paschos
Assume: struck parton carries momentum ξ P
(spin 1/2 !)
Find: ξ ≡ x
ξP
P
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“DGLAP” evolution
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Polarized Deeply Inelastic Scattering
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⇒
⇐
SLAC (E80,130,…), CERN (EMC, SMC, COMPASS), DESY (HERMES), JLab
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• also for scaling violations predicted in QCD:
−=Lg1
−=Lg1
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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”
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L. De Nardo (HERMES)
(doesn’t yet include newCOMPASS results)
2006: Delta Sigma = 0.2
Quarks and anti-quarkscarry 20%
of proton spin
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• What are the detailed patterns of quark & antiquark polarizations ?→ Flavor asymmetries in sea ? Strangeness ?
• What are the origins of large observed single-transverse-spin asymmetries ? What do they tell us about the nucleon ?→ Transverse quark pol.? Correlations spin / parton k ?
Orbital angular momentum? Spatial distributions?T
• What else carries the proton spin ?
→ How are gluons polarized ?→ How large are parton orbital angular mom. ?
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Measuring the proton spin structurewith polarized proton probes
Quarks contribute only 20%!
gluon
quarkpion
quark
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DIS
pphigh pThigh pT
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• this is formalized through “factorization theorems”
measure→ learn about !
calculate
⇒
⇒
universal parton scatt.perturbative QCD
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pp → π XQCDprediction
Cornerstones to the RHIC Spin program
Physical Review Letters 91, 241803 (2003)
0
Mid-rapidity: PHENIX
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Cornerstones (continued)
pp → π X0
Forward rapidity: STAR
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RHIC Spin Physics Program
• Direct measurement of polarized gluon distribution using multiple probes
• Direct measurement of anti-quark polarization usingparity violating production of W+/-
• Transverse spin: Transversity & transverse spin effects: possible connections to orbital angular momentum?
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RHIC Polarized ColliderBRAHMS & 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
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The Problem of Spin Resonances
• Spin precesses about guide field (proton: 200 precessions in one turn at RHIC, 100 GeV)
• Spin precesses about horizontal magnetic fields (for example, focusing fields)
• When the 2 precessions beat, resonance condition: every 500 MeV of acceleration!
• Solution: Siberian Snakes
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Siberian Snakes
1996-2001:Siberian Snakes
2003-4: Warm AGS Snake
Snake: precess spin, leavingbeam direction unchangedat exit of snake
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Siberian Snakes—how they work
• Spin resonance: spin axis is no longer vertical
• Snake 1: precess spin 180 deg. about +45 deg. in horizontal plane• +45 deg. spin component unchanged, vert. and
-45 deg. spin components flipped • Snake 2: precess spin 180 deg. about -45 deg.
• -45 deg. spin component unchanged, vert. and +45 deg. spin components flipped
Two snakes: vertical spin is unchanged, horizontalspin components flip each turn.
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0.002
0.003
0.004
0.005
0.006
0.007
0.008
0.009
0.01
0.011
20 40 60 80 100 120 140 160 180 200 220 240
Beam Energy [GeV]
Asym
met
ry
d
Resonance at Ggamma=138
P=0.61
P=0.36
Polarization measurement on RHIC accelerationramp, June 2006
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Exquisite Control of Systematics
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ALL
(P) Polarization (L) Relative Luminosity(N) Number of pi0s
−+−+++++
−+−+++++
−+++
−+++
+−
=+−
=LNLNLNLN
PPA
YBLL ||
1σσσσ
++ same helicity+− opposite helicity
“Yellow” beam “Blue” beam
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RHIC Polarized ColliderBRAHMS & 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
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STAR spin physics program: ∆G, ∆q/∆qbar, transverse spin
Large acceptance TPCand EMC -1<η<2
FMS
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PHENIX spin physics program: ∆G, ∆q/∆qbar, transverse spin
Muon IDPanels
CentralArms
North MuonArm
South MuonArm
Ring ImagingCerenkov
EM Calorimeter
Muon TrackingChambers
Beam-BeamCounter
Multiplicity/VertexDetector
Time ExpansionChamber
Drift Chambers
Pad Chambers
Time of FlightPanels
Four spectrometer arms with excellent trigger and DAQ capabilities.
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Pi0 reconstruction for ALL
2-3 GeV/cBckgr=17%
3-4 GeV/cBckgr=7%
4-5 GeV/cBckgr=5%
1-2 GeV/cBckgr=45%
Results obtained for four pt bins from 1 to 5 GeV/c
Pi0 peak width varies from 12 to 9.5 MeV/c2
from lowest to highest pt bins
Background contribution under pi0 peak for ±25 MeV/c2 mass cut varies from 45% to 5% from lowest to highest pt bins
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2003+2004 Data
2005 Data:Y. Fukao and K. BoylePANIC Oct. 2005
gggg →
GG
GGALL
ΔΔ∝
gqgq →
GG
qqALL
ΔΔ∝
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Final results on ∆G will come from combined NLO analysis of all channels at RHIC and in DIS
RHIC measurements will span broad range in x with good precision. multiple channels with independent theo. and exp. uncertainties.
Uncertainty through extrapolation to small x
√s=200 GeV incl. π0 prod’n √s=500 GeV incl. jet prod’n
∆G Measurements by 2012see Spin report to DOE http://spin.riken.bnl.gov/rsc/
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unpol.
u
Δd + u → W −
Δu + d → W −
Δd + u → W +
Δu + d → W +
Expected start: 2009
Δq-Δq at RHIC via W production
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Transverse Spin
The RHIC (STAR and BRAHMS) results at forward rapidity demonstrate that large spin effects exist in the perturbative QCD regime.
There are new results from Belle showing large fragmentation asymmetry for polarized quarks.
New HERMES results show large asymmetries for orbital angular momentum effects in polarized proton.
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STAR data
−=1h
• Transverse Physics: Measurement of transversity and study of other transverse spin effects with possible connections to orbital angular momentum
Physics with transverse spin at RHIC
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ϕ1
1hPr
2hPr
ϕ2−π
ϕ1
ϕ2
( ) ( ) ( ) [ ]( ) [ ]( )
( ) ( ) Θ=−=
+=Ω
→ ⊥⊥−+
2cm
21
111
121T
221
21
41y1yyB
zHzHyBddzdzd
Xhheedσ
sin
cos ϕϕLq
2-hadron inclusive transverse momentum dependent cross section:
Net (anti-)alignment oftransverse quark spins
Θ
e+e- CMS frame:
e-
e+
GeV 5210ss
E2z h ., ==
[D.Boer: PhD thesis(1998)]
Quark Analyzing Power at Belle
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J.C. Collins, Nucl. Phys. B396, 161(1993)
φ
q
momentum hadron relative : 2
zmomentum hadron e transvers:
momentum hadron : spinquark : momentumquark :
h
sE
EEppsk
h
qh
h
h
q
=
=⊥
v
v
v
r
qsv
kv
hph v,
Collins Effect:Fragmentation with of a quark qwith spin sq into a spinlesshadron h carries anazimuthal dependence:
⊥hpv
( )φsin∝
⋅×∝ ⊥ qh spk vvv
Quark Analyzing Power
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Results for e+ e- π π X for 29fb-1
• Integrated results:– cos(2φ0) method
(3.06±0.57±0.55)%– cos(2φ1+φ2)
method(4.26±0.68±0.68)%
-- A_N = 20%!
z1
z2
Systematic error
A0
A12
Final results
R. Seidl et al. (Belle), PRL 96, 232002 (2006)
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Plans
• Longitudinal spin---gluon polarization at root(s)=200 GeV to 2009---W parity violating production: anti-quark polarizations by flavor---2009-2012, 500 GeV
• Transverse spin---study quark transversity, quark analyzing power, orbital angular momentum of quarks and gluons in proton
• New detectors: forward spectrometers (2007), micro vertex detectors (2010), endcap cal. (2012)
• Polarized ep and eA Collider: start 2012?
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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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Polarimetry
recoilCarbon
polarizedbeam
scatteredproton
Carbontarget
0.01 < |t| < 0.02 (GeV/c)2
rightleft
rightleft
NB NN
NNA
P+
−⋅−=
1
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Raw asymmetries from carbon polarimeter by bunch (2005)