Understanding Nucleon Sturture

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Where Does the Nucleon Get Its Spin?

• Ming X. Liu
• P-25
• Quark polarization
• Gluon polarization

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The Challenge the Spin Crisis

• Understanding the nucleon sub-structure
• QCD sum rule violation and the spin
  crisis(1987)

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A little Bit of History

• DIS cross section
• _at_Bjorken limit in parton model

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QCD Sum Rules

• Bjorken sum rule
• Ellis-Jeffe sum rule

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More on Spin Crisis

• pQCD non-conservation of axial-vector current
  due to QCD axial anomaly
• Redefine axial-vector current to be conserved

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Gluon Polarization?

• Proton Spin Puzzle and ?G
• Asymptotic limit

Gluons may play a significant role !
Similar result was obtained by Gross and Wilczek
for un-polarized quarks and gluons in 1974
constituent quark model
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Experimental Data on Proton Structure
unpolarized
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Polarized Quark and Gluon Distributions
EMC, SMC at CERN E142 to E155 at SLAC HERMES
at DESY
up quarks
Quark spin contribution to the proton
spin Spin Crisis The rest from gluons and
orbital angular momentum. Measurement of gluon
and sea distributions a major focus of RHIC
spin program.
gluon
sea quarks
down quarks
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A Race Toward Gluon Polarization Measurements

• Semi-inclusive DIS
• HERMES _at_ DESY
• high-pT hadron pairs
• SMC _at_ CERN
• high-pT hadron pairs
• COMPASS _at_ CERN
• high-pT hadron pairs
• open charm

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RHIC-Spin Physics New Frontier of Nucleon
Structure Research

• Proton Spin - a major puzzle from polarized DIS
  experiments
• Proton Spin Decomposition
• Origin of Proton Spin
• gluon, sea quarks, orbital angular momentum ?
• DIS cant directly probe gluons and anti-quarks
• A new tool RHIC-SPIN
• a polarized proton collider
• quark-gluon, quark-quark and gluon-gluon
  interactions

Experimentally
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_at_RHIC-SPIN

• Polarized hadron collisions
• double longitudinal spin asymmetry
• leading-order gluon interactions
• direct-photon production
• heavy-flavor production

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The Relativistic Heavy Ion Collider at
Brookhaven National Laboratory
R-HI New state of matter QGP De-confinement po
larized proton Nucleon Spin Structure Spin
Fragmentation pQCD RHIC is a QCD lab
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Polarized Proton Operation at RHIC
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Siberian Snakes To Maintain Polarization
Effect of depolarizing resonances averaged out
by rotating spin by 180 degrees on each turn

• 4 helical dipoles in each snake
• 2 snakes in each ring
• - axes orthogonal to each other

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RHICs Experiments
Transverse spin only (No rotators)
Longitudinal or transverse spin
Longitudinal or transverse spin
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History of Spin Runs
2005 First long longitudinal-spin
polarized-proton run Figure of merit (LP4) more
than 40 times larger than that of previous runs
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Recorded luminosity in Run5
Longitudinal 4pb-1 Transverse 0.16pb-1
3.8 pb-1 BBC live
Pol 50
0.11 pb-1 BBC scaled
Estimated using BBC live triggers with ?BBC22mb
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What can we learn from such measurements ?

• Asymmetries
• Polarized Quark and Gluon Distributions - LO
• Proton Structure
• QCD dynamics

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The PHENIX Detectors

• Philosophy
• high resolution high-rate at the cost of
  acceptance
• trigger for rare events
• Central Arms
• h lt 0.35, Df p
• g, p0, e, p-, ... Identified
• Momentum, Energy
• Muon Arms
• 1.2 lt h lt 2.4
• Momentum (MuTr)

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ALL measurements from early runs

• p0 results from Run 3 and 4
• mixture of qq, qg, and gg

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?g 1.84 ?g 0.42 at Q21(GeV/c)2 best fit to
DIS data
data prefers the GRSV-std curve
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p0 ALL (run5 expectation)
SPIN2004 Y.Fukao
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A New Probe Heavy Quarks

• Sensitive to gluon polarization Dg(x)
• Gluon Fusion dominates at LO
• PHYTHA estimate

GeV Charm Beauty
200 955 8515
500 973 928
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J/? production mechanism

• LO NRQCD

mix of various processes
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J/? production mechanism (cont.)

• NLO RNQCD

final state mixture ratios fixed from other
data
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NLO NRQCD and PHENIX data

• Theoretical predictions of J/? production at RHIC
  are in good agreement with the PHENIX data COM
  process dominant
• PRD 68 (2003) 034003 G. Nayak, M. Liu, F.
  Cooper
• PRL 93 (2004) 171801 F. Cooper, M. Liu, G.
  Nayak

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More on J/? Production Mechanism- J/Psi
polarization - a key probe?- Spin dynamics in
pQCD
a 1 transversely polarized a -1
longitudinally polarized a 0 no polarization
hep-ph/0106120 M. Kramer
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NRQCD Calculations of ALL

• NRQCD LO (hep-ph/0509335 G. Nayak et al.)
• Helicity dependent double-spin asymmetry

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ALL Measurement with J/????-

• Experimental approach
• Polarization47 (average)
• R is from Beam-Beam-Counter scalers
• N_Jpsi Yield

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J/????-
dimuon mass distribution (Run5 data)

• Very clear J/y peak is observed from muon arms

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Experimental Complexity-time variation-

• Detector performance

• Beam relative luminosity

Raw J/Psi yield per collision
Time
Time
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J/? Yield Measurements

• Fill by fill Charge(-) charge(,--)
• Detector efficiency variation canceled out
• Have Open charm, DY and other background (lt10)
• GaussianExponential background
• Used to estimate the background under J/Psi peak

Dimuon mass distributions
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Raw Asymmetries Fill by Fill N_J/Psi gt 10 per
fill 2 sigma cut 2.80 lt Mass lt 3.44
ltRLgt 1.00892 ALL (raw) -0.007 ? 0.013
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Signals
Raw Asymmetry Per Fill N_Jpsi gt 10 Two pT bins
0.0-1.4 GeV and 1.4 10 GeV 2.80 lt Mass lt 3.44,
2 sigma cut ALL(raw) 0.0011 ? 0.0175 ALL(raw)
-0.0145 ? 0.0179
0 lt pT lt 1.4
1.4 lt pT lt 10
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Statistical Test

• Beam polarization shuffling
• random beam polarization values
• repeat 10,000 times

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Background
Background Raw Asymmetry Left of the peak 2.0 lt
Mass lt 2.5 ALL(raw) 0.0107 ? 0.0220 N 2068 ?
45.5 Under the peak ALL(raw) -0.0351 ?
0.0318 N 988 ? 31
-
, --
Dimuon mass distributions
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ALL vs pT

• First spin physics result from Run5
• J/y produced via almost pure gluon fusion
• sensitive to gluon polarization
• statistically limited.

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Sea Quark Polarization Flavor Asymmetry

• Nucleon is a complex system
• Sea quarks and gluons correlated
• QCD dynamics

LANL/E866
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Sea quark polarization

• Pion cloud model
• Chiral soliton model

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Quark Polarization
Bunce G. et al, hep-ph/0007218
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Summary and Outlook

• First measurement of double spin asymmetry with
  J/Psi from longitudinally polarized p-p
  collisions
• Almost pure gluon fusion
• 10k J/psi, statistically limited
• Expect x100 more statistics in the future
• Theoretical work in progress
• No gluon polarization
• Maximum (,-) gluon polarization
• Excellent QCD test ground with polarization
• Work in progress
• J/Psi polarization measurement
• Open charm - much larger statistics from run5 and
  will tell us more about gluon polarization
• Where does the nucleon get its spin?
• Still dont know but RHIC-SPIN will help us to
  find the answer
• and we will learn a lot more about the nature of
  strong interactions

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Other ALL prospects
Direct photon
Jet ALL
h, electron, m, L, ...
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Double spin asymmetry ALL(preliminary) results in
inclusive jet production in pp collisions at
sqrt(s)200GeV
jet cone0.4, 0.2lthjetlt0.8
)
) Predictions B.Jager et.al,
Phys.Rev.D70(2004) 034010
Results limited by statistical precision Total
systematic uncertainty 0.01 (STAR) beam pol.
(RHIC)
DNP2005 J. Kiryluk,MIT
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Asymmetries from J/Psi
ALL Asymmetry
J/Psi Polarization
PRD 56 (1997)A. Tkabladze O. Teryaev
PRL 87 (2001) F.Yuan K. Chao
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Spin physics results

• Cross sections(not exactly spin, but important
  cross-check for data/theory reliability)
• p0 and direct photonBoth agree well withNLO
  pQCD calc.

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Gluon Polarization and Heavy Quarks

• Heavy Quark production at 200 GeV
• Distinct Experimental signatures
• high Pt lepton
• 2nd vertex
• Large asymmetry and pQCD

ALL
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Spin Structure of the Proton Status
Parton Distributions
(well known)
(moderately well known)
(moderately well known)
(basically unknown)