Nucleon Structure: Spin Content

1
Nucleon Structure Spin Content

• Naohito Saito
• (Kyoto University)

2
Why Nucleon Structure?

• gt99 of Ordinary Matter consists of nucleon
• Need to Understand Fundamental Building Block of
  Universe
• Quarks are confined since 1 usec after Big Bang
• Confinement mechanism is unsolved problem since
  last century

Ordinary Matter
Nucleon 100 Neutrino 0.16 Photon 0.23
3
Why Spin?

• Fundamental Concept in Physics
• Appears in Many Different Levels
• Galaxy to Space-Time Structure
• Fundamental Quantum Number for Elementary
  Particles
• Important in Symmetry Test
• Parity
• Time Reversal

4
Proton Spin Crisis?

• Proton has gone through many crisis
• Mass mu5 MeV/c2, md10 MeV/c2
• Saved by constituent quark model
• Momentum
• Saved by gluon momentum
• Spin
• Helicity SR
• Theory guideline for separation (nf3)
• (Ji, Tang, and Hoodboy)

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Spin Physics Experiments

• COMPASS
• Pol.m
• HERMES
• Pol.e
• JLab Exps
• Pol.e-
• RHIC Spin
• Pol. pp
• BELLE
• ee-

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Probing the Proton Structure

• EM interaction
• Photon
• Sensitive to electric charge2
• Insensitive to color charge
• Strong interaction
• Gluon
• Sensitive to color charge
• Insensitive to flavor
• Weak interaction
• Weak Boson
• Sensitive to weak charge flavor
• Insensitive to color

7
Parton Distribution Functions

• Quark Distributions

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DIS of Lepton from Nucleon

• Structure Probed by Photon
• Function of x and Q2-q2
• ?Charge Squared
• Not distinguish Down and Strange
• Insensitive to Gluon at LO
• Objective is
• Observables are

Proton
Neutron
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Precision Data from DIS

• Precision Data in Wide Kinematical Range
• Q2 evolution agrees with pQCD
• Notes
• Only Fixed Target Spin Experiments in DIS so far
• Need a Collider to extend kinematical coverage

Q2 (GeV2)
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From g1(x,Q2) to DS

• Integrate over x (0,1)!
• Utilize Octet Baryon b-Decay Constants!
• SU(2) OK!
• (Bjorken SR)

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Polarised PDFAsymmetry Analysis Collaboration
M. Hirai, S. Kumano and N. Saito, PRD (2004)

• Valence Dists are determined well
• Sea Dist is poorly constrained
• Gluon can be either gt0, 0, lt0

We need direct constraints on
Gluon and Sea Distributions
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Gluon Polarization
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Gluon Polarization in the Proton

• Hunting for Dg
• Direct Probe
• FNAL E704
• High-pT p0
• High-mass Multi-g pair
• DESY HERMES
• High-pT hadron pair
• Indirect Probe
• NLO Analysis by SMC

PRL 84(00)2584
PLB261(1991)197
PRD58(1998)112002
PLB336(1994)269
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Status of Dg/g(x) from Lepton Scat.

• Photon-Gluon Fusion Process for
• 2-jet
• Open Charm
• New COMPASS results!
• Gluon Pol. Can be , 0, -

Courtesy Vogelsang and Stratmann
15
Constraints on Dg(x) w/ p0 Production

• pp ? p0 X is sensitive to gg?gg and gq?gq

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So What?
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Trial Fit!

• Utilized NLO p0 calculation (thanks to Vogelsang
  and Stratmann)
• to find relevant-x for each pT bin
• Trial Fit to
• Ax (GRSV 1x)
• Axx

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STAR Results on ALL jet!

• High rate ! ( as high as p0)
• More direct access to partonic level kinematics!
• Large Dg (g) seems dead.

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Extend x Range

• ALL(p0) from Run 6/7 (65 pb-1) will extend x
  range to larger x
• STAR jet measurement also provides precision data
• 500 GeV run will cover smaller x-range

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Prompt Photon Production

• Gluon Compton Dominates
• 10 Contamination from Annihilation
• No fragmentation contribution in LO

k2
P1
P2
k1
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Direct photon in Run-2006/7
k2
P1
P2
k1
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What we learned?

• Polarized Collider is a powerful tool to probe
  gluon structure!
• Still need to explore
• Wider x-coverage
• Improved precision
• Higher pT, Prompt Photon, Lepton Scattering needs
  to be understood in consistent framework ? Global
  QCD Analysis
• More data COMPASS, HERMES, RHIC-II, and possibly
  at J-PARC

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Sea Polarization
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DS from SDIS and Global Analysis

• Global Analyses suffer from a lack of DS data
  SU(3) symmetric sea assumed
• In SDIS, with Final State Hadron Detection, DS
  can be extracted

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Single Transverse-Spin Asymmetry
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Single Transverse Spin Asymmetries

• Fermilab E-704 reported Large Asymmetries AN
• Could be explained as
• Transversity x Spin-dep fragmentation (Collins
  effect),
• Intrinsic-kT imbalance (Sivers effect) , or
• Twist-3 (Qiu-Sterman, Koike)
• Or combination of above

Left
Right
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Single Transverse Spin Asymmetry

• Non-zero Single Transverse Spin Asymmetry (SSA)
  requires
• Spin Flip Amplitude
• Phase difference

Spin Flip!
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PHENIX Results Central Rapidity

• ZERO asymmetry

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STAR Results Forward Rapidity

• Large Asymmetry Measured
• To distinguish models, jet axis reconstruction is
  required
• ProbablyAN requires large xBJ, which is only
  achievable in hi-pT in Central rapidity (small
  xsection)
• Global Analysis with HERMES results would be
  beneficial

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BRAHMS new Results on AN

• BRAHMS observed Large AN in charged pion
  production
• Similar Trend to E704, but deviation from ZERO
  starts earlier
• Need to explore further!

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Collins or Sivers?Final
state
Initial state
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BELLE Results for p-pairs for 30fb-1

• Significant non-zero asymmetries
• Rising behaviour vs. z
• cos(f1f2) double ratios only marginally larger
• First direct measurement of the Collins function

z1
z2
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Hi-E Heavy Ion Physics RHIC, LHC
Growing Fields
Hadron Collider Physics TEVATRON LHC
Precision Structure Studies
Dark Matter
Neutron EDM
Mechanism of Confinement Lattice-QCD?Effective
Model
nN elastic scattering
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Workshop on
Hadron Structure at J-PARC
November 30 - December 2, 2005, KEK, Tsukuba,
Japan
http//www-conf.kek.jp/J-PARC-HS05/
Physics Topics Hadron Physics with Neutrino
Beam Hadron Physics with 50 GeV Proton Beam
- Polarized Beam and/or Target Exotic Hadrons
Organizers S. Kumano (KEK) S. Sawada (KEK)
Y. Goto (RIKEN) N. Saito (Kyoto) Contact S.
Sawada shinya.sawada_at_kek.jp
Advisory Committee H. Enyo (RIKEN) T.
Hatsuda (Tokyo) T. Kishimoto (Osaka) O.
Morimatsu (KEK) M. Oka (TITech) K.
Tokushuku (KEK)
O. Hashimoto(Tohoku) K. Imai (Kyoto) J. Kodaira
(KEK) S. Nagamiya (KEK) T.-A. Shibata
(TITech) K. Yazaki (TWCU)
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Backup Slides
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RHIC Spin and HERMES SIDIS

• Complementary!
• RHIC W
• No fragmentation ambiguity
• x-range limited
• Useless for transversity studies
• Otherwise WR!
• HERMES Semi-Inclusive DIS
• Wide x-range
• Could be used for transversity studies

37
Transversity Measurements

• Drell-Yan Production of Lepton Pairs
• Clean, but low statistics (QED process)
• Precision will be improved by L upgrade

PHENIX Muon 200 GeV
O. Martin, A. Schaefer, M. Stratmann, W.
Vogelsang PRD60 (99) 117502
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Transverse Jet Production

• Advantages
• Enormous yield could overcome the smallness of
  asymmetries
• No mixture with Gluonic contribution
• Caveats
• Control of systematics will be the key for the
  success
• ATT0.2 0.1 (if PB70)
• ATT is cos2f moment statistics will be divided
  into several f-bins

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Impact of Ds Measurement

• Improve Knowledge on Spin Flavor Structure of the
  Proton
• Beyond Flavor SU(3) assumption
• Neutron EDM
• n-EDM predicted
• using q-EDM and Dq
• Dark Matter
• Better determination of Dark-Matter reaction

J.Ellis and R.A.Flores PLB377(96)83
J.Ellis and M. Karliner Lecture at Erice School
95 hep-ph/9601280
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nN Elastic Scattering

• Cross section for nN elastic Scattering
• Where (Q2 dropped for brevity)

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BNL-Experiment 734
(L.A.Ahrens et.al PRD35(87)785 Reanalysis G.T.
Garvey et. al PRC48(93)761)

• Measured elastic scattering cross section
• and
• Liquid scintillator Drift Tube 170 t
• 0.5E19 POT for neutrino and 2.5E19POT for
  anti-neutrino
• Q2gt0.40 GeV2

Too High Q2 Cut-off
79 from Carbon
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nN-Elastic Scattering Exp at J-PARC

• On-axis at near detector hall for T2K Experiment
• Utilize both two types of LiqScintillator with
  different H/C mixture for pure proton signal
• e.g Bicron BC510A (H/C1.212) and BC-533
  (H/C1.96)
• Pure Carbon can be extracted for nA Xsection
• e.g. 5x5x5m3 125 t
• 1E21 POT possible in one year (130 days)
• 30 times BNL-E734
• Better with polarity change for

43
Sensitivity for Ds

• Assumptions
• Similar Detection Efficiency to E734
• 7.6 for neutrino-N elastic
• 5.4 for anti-neutrino-N elastic
• However with lower Q2 cut-off 0.1 GeV2
• Achievable with more uniform detector ?
• 25 times more statistics but pure proton only 1/6
• Factor 2 reduction in statistical error
• Systematic control improvements to 5
• E734 7.6 dominated by Beam Flux and Nuclear
  Effects
• Possible to remove Nuclear Effects which could be
  larger in lower Q2 region