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					 The Electron-Ion Collider:
Tackling QCD from the Inside
(of Nucleons and Nuclei) Out

         Christine A. Aidala
      Los Alamos National Lab
           October 26, 2009
Theory of strong interactions: QCD
• Developed in mid-1970’s
• Gluons, the force mediators, are self-interacting
  – High energies  Can perform calculations than in
    Much more complex to perform calculations as
    perturbative expansions in the strong coupling
• Quarks and(similar to QED, but more diagrams
    constant gluons (partons) are the point-like
        contribute due to gluon
  degrees of freedom in QCD self-coupling)
   – But confined in color-neutral, composite particles
   – When probed at high enough energy scales, partons
     behave as (nearly) free particles—asymptotic freedom

                      C. Aidala, LANL, Oct. 26, 2009        2
      pQCD as a theoretical tool
• Perturbative QCD—a powerful tool for about
  30 years now
• Much of its utility stems from the
  approximation in which high-energy hadronic
  processes can be factorized into universal non-
  perturbative terms, measurable in experiment,
  which get convoluted with the perturbative,
  partonic hard-scattering term . . .

                   C. Aidala, LANL, Oct. 26, 2009   3
                     Factorized pQCD
                P1                                                            0
                                                                         Dq ( z )
                                       Hard Scattering Process

                                                 qgqg
                                                 ˆ                             X

  pp   X   qx1   g x2   
                                     ˆ                               qg qg
                                                                              s   Dq
                                                                               ˆ         0
                                                                                                    ( z)

“Hard” probes have predictable rates given:

         – Parton distribution functions (need experimental input)
         – Partonic hard scattering rates (calculable in pQCD)
         – Fragmentation functions (need experimental input)
                                    C. Aidala, LANL, Oct. 26, 2009                                            4
   Parton distribution functions (pdfs)
   and fragmentation functions (FFs)
• Simplest system in which to
  measure pdfs: deep-inelastic
  scattering (DIS) of
  electromagnetic probes off of
• Simplest system in which to
  measure FFs: e+e- annihilation
  to hadrons
• Once measured at a particular
  energy scale (Q2) and for a
  particular momentum fraction
  (x), pQCD tools exist to then
  calculate expectations for pdfs
  and FFs at different Q2 and x
   – DGLAP: linear evolution in Q2
   – BFKL: linear evolution in x

                           C. Aidala, LANL, Oct. 26, 2009   5
Some successes of factorized pQCD
• Using pdfs measured
  (mostly) in DIS and
  FFs measured in e+e-,
  can describe cross
  sections for particle
  production in high-
  energy p+p collisions
   – Next-to-leading order
     (NLO) calculations
                                               Central and forward pion production at RHIC,
  DIS data going into fits (much from          200 GeV
  HERA e+p collider)                           Agreement down to pT of 1-2 GeV/c

                                 C. Aidala, LANL, Oct. 26, 2009                         6
    And some limitations in the
traditional pQCD tools/picture . . .
  “Modern-day ‘testing’ of (perturbative) QCD is as
       much about pushing the boundaries of its
 applicability as about the verification that QCD is the
         correct theory of hadronic physics.”
 – G. Salam, hep-ph/0207147 (DIS2002 proceedings)

                     C. Aidala, LANL, Oct. 26, 2009        7
  Limitations of linear evolution in QCD
• Linear DGLAP evolution
  in Q2
• Linear BFKL evolution in

Linear evolution has a built-
  in high-energy/low-x
• xG rapid rise for decreasing x
  and violation of unitary bound
•  must saturate
   – What’s the underlying                                Need new approach
                        C. Aidala, LANL, Oct. 26, 2009                         8
    Limitations in describing observed
transverse single-spin asymmetries (SSAs)
        Left                                                 AN: difference in cross-section
                                                  1  
                                                             between particles produced to
                                      A 
                                        L           R
                                                             the left and right
                                                  P    
                  Right                 L           R
Theory Expectation:
                     Such large SSAs can’t be generated
               assuming energies
Small asymmetries at highcollinear motion of the partons
(Kane, Pumplin, Repko, PRL 41, 1689–1692 (1978) )                                     π0
               with the nucleon and no gluon exchange
    NA 
                                 AN the Theory
                   betweenO(10-4)scattered parton and the  AN
                   nucleon remnant or outgoing hadron
 Experiment: (no exchange = “leading twist”)
 (E704, FNAL, 1991)

 pp    X                                                                          π-
                   AN O(10-1) Measured!
  s  19 .4 GeV
                                                                                 x 

                            C. Aidala, LANL, Oct. 26, 2009
  Need to go beyond these traditional
           pictures and tools
 Will need the experimental data to test
our new tools and the new understanding
       that goes along with them

              C. Aidala, LANL, Oct. 26, 2009   10
       The Electron-Ion Collider:
  A facility to fulfill the promise of the
  21st century as the century of QCD
• Collider rather than fixed-target facility—reach higher
   – Can use perturbative QCD methods
   – Can study gluon saturation effects
• Electromagnetic rather than hadronic probes
   – Simpler to interpret
   – But will also want to cross-check and test newfound
     understanding against data from (more complex) hadronic
• Hadron beams: polarized protons and polarized light ions;
  heavy ions (compare to HERA: only unpolarized protons)
• Lower energies but higher luminosities than HERA

                         C. Aidala, LANL, Oct. 26, 2009        11
     Goals & Key Questions for the EIC
• Explore a qualitatively new regime in QCD: strong
  color fields in nuclei
   – What are the properties of high-density gluon matter?
      How do the contribute to the gluons the nucleus?
   – How do the gluons quarks and structure ofof QCD
   – How do fastabout the visible matter in our
       bring quarks or gluons interact as they traverse nuclear
• Probe correlations and dynamics of the quarks and
  gluons in the nucleon
   – How do the gluons and sea quarks contribute to the spin structure
     of the nucleon?
   – How are partonic transverse motion and spin correlated?
   – What is the spatial distribution of the gluons and sea quarks in the

• Study how hadronic final states are formed in
   – What is the hadronization time/length for a scattered parton?
   – How does the nucleon remnant hadronize?
                            C. Aidala, LANL, Oct. 26, 2009             12
             Non-linear QCD: Saturation
• Linear BFKL                   proton
 evolution in x
   – Explosion of color
   field as x0??                       N partons                 new 2 partons can recombine into one
                                                                  any partons emitted as energy increases
                                                                  could be emitted off any of the N partons
• New: BK/JIMWLK                                                  Regimes of QCD Wave Function
       based models
   – introduce non-linear effects
    saturation
   – characterized by a scale Qs(x,A)
• Saturation scale Qs
  dynamically generated!
   – Rises with increasing energy or
     decreasing x
        Question: What is the
     relation between saturation
         and the soft regime?
                                 C. Aidala, LANL, Oct. 26, 2009   as~1                 as << 1       13
     e+A: Raising the saturation scale to study
                non-linear effects
     Scattering of electrons off nuclei:
     • Probes interact over distances L ~ (2mN x)-1
     • For L > 2 RA ~ 2A1/3 probe cannot distinguish
       between nucleons in front or back of nucleus
     • Probe interacts coherently with all nucleons

 2        a s xG( x, Qs2 )                    1                                            A
Q ~                               HERA : xG ~ 0.3                    A dependence : xGA ~ 0.3
       RA (  1 for proton)
                                             x                                            x
     Nuclear enhancement factor
                                                    A
     “pocket formula”:                   (Q )  cQ  
                                                A 2                 2
 Enhancement of QS with A  non-linear QCD regime reached at significantly lower
 energy in heavy nuclei than in proton  Easier to access experimentally

                                   C. Aidala, LANL, Oct. 26, 2009                       14
Nuclear enhancement of saturation scale
                                    Note :
                                   Q 2  Qs2       a s  a s (Q 2 )
                                   Q 2  Qs2       a s  a s (Qs2 )
                                      When in saturation regime, running of
                                              strong coupling stops!!
                                  More sophisticated analyses 
                                     If can reach saturation at a point where
                                  moreas << 1, will be ableexceeding
                                          detailed picture to perform
                                           perturbative calculations for
                                  enhancement from pocket formula
                                       unprecedentedly small Q2 values and
                                              compare al., PRL
                                  (e.g. Armesto et to experiment!
                                  94:022002, Kowalski, Teaney, PRD

              C. Aidala, LANL, Oct. 26, 2009                            15
     Universality & geometric scaling
Crucial consequence of non-linear
 evolution towards saturation:
• Physics invariant along trajectories
  parallel to saturation regime (lines of
  constant gluon occupancy)
• Scale with Q2/Q2s(x) instead of x and
  Q2 separately

                                        Geometric Scaling for
                                           e+p collisions over wide
                                           range of Q2
                                       • (Believed to be)
       x < 0.01
                                         consequence of saturation
                                C. Aidala, LANL, Oct. 26, 2009        16
   Nuclear shadowing exhibits geometric scaling as well
         Nuclear shadowing                                       Geometric scaling

                                                                 proton  5


Freund et al., hep-ph/0210139
                                                                          Q 2 / QS ( x )

                                C. Aidala, LANL, Oct. 26, 2009                               17
•   From DIS at HERA:                                 What about on the parton scale?
      – At small-medium Q2,                           Small-x running-coupling BFKL
          Is the wave function of hadrons and nuclei
       σ(NC) >> σ(CC)                                  QCD evolution predicts:
      – For Q2 > MZ2 and MW2,                          QS approaches universal behaviour
       σ(NC) ~ σ(CC)           universal at low x?     for all hadrons and nuclei
         EW Unification                               No dependence on A!!
                   Can we test this experimentally?
    Already a textbook figure ...                      Not only functional form f(QS)
                                                       universal, but even QS(x) itself
                                                       becomes universal

                           Unification                         A.H. Mueller, hep-ph/0301109

    E.C. Aschenauer                  EIC-INT, Seattle, October 2009                           18
Collinear factorization in pQCD:
Long history, relatively well tested

  if want to access QCD dynamics, need to go
beyond the twist-2, collinearly factorized picture.

       Dynamics ↔ (transverse) SSA’s:
 Azimuthal asymmetries generated by S•(p1×p2)

                   C. Aidala, LANL, Oct. 26, 2009     19
             Transverse single-spin asymmetries
                  persist at RHIC energies
ANL ZGS            BNL AGS                    FNAL          RHIC
s=4.9 GeV         s=6.6 GeV                 s=19.4 GeV   s=62.4 GeV
                                            s=200 GeV!


            Effects persist to RHIC energies
       Can probe this non-perturbative structure of
            nucleon in a calculable regime!right                     20
                           C. Aidala, LANL, Oct. 26, 2009
  TMD’s and GPD’s: Recent developments from
 theory in how to look at partons inside hadrons
• Transverse-momentum-dependent distributions (TMD’s):
    – Pioneering work in early ’80’s, but not much activity at that time
    – Sivers function to describe low-energy hadronic SSA’s (1990)
    – (Alternative collinear, higher-twist formulation to describe SSA’s by Qiu and
      The EIC will be the facility to explore these in
      Sterman (1991))
    – Collins argues that Sivers effect prohibited due to time-reversal invariance;
              detail in upcoming decades
      proposes kT-dependent FF (1993)
    – (More on following pages . . .)

• Generalized parton distributions (GPD’s) (1994-7)
• Ji decomposition of nucleon angular momentum; Deeply Virtual Compton
  Scattering to probe GPD’s and access quark orbital angular momentum in
  proton (1997)
    – “Thus there is now a new territory to explore the quark and gluon structure of the
      nucleon besides the traditional inclusive (parton distributions) and exclusive (form
      factors) processes.”

                                    C. Aidala, LANL, Oct. 26, 2009                           21
          TMD’s and collinear, higher-twist
            correlators: Recent progress
Two theory breakthroughs in TMD’s and SSA’s since RHIC turned on
•   Brodsky-Hwang-Schmidt recognize importance of initial- or final-state interactions
    in generating a phase—Sivers function not in fact prohibited by (naïve) time
    reversal (2002)
•   Ji -Qiu-Vogelsang-Yuan unify TMD and twist-3 correlation function approaches
    and clarify the realms of applicability of each (2006)

And very recently—
•        Anticipate more “applications” of these functions
    LO evolution equations for collinear, higher-twist parton correlationnew
    (Kang-Qiu, 2009)
            distribution/correlation functions to other
    First steps moving from LO to NLO in higher-twist correlation functions
             questions in
    (Vogelsang-Yuan, 2009) QCD in upcoming years . . .

Plus a different sort of development—
•   Yuan proposes J/Psi transverse SSA in p+p and SIDIS to test production mechanism
     – Given a non-zero gluon Sivers function, non-zero transverse SSA expected for J/Psi only
       in color-singlet model in p+p, only in color-octet in SIDIS
     – Signals arrival at a certain level of maturity in field of TMD’s—their application to long-
       standing issues in QCD traditionally considered via other approaches
                                     C. Aidala, LANL, Oct. 26, 2009                             22
Leading-twist pdf’s and FF’s, including TMD’s



              Sivers                                          Polarizing FF

            Experimental evidence that these are non-zero!
                   (Most results from past ~5 years)
                             C. Aidala, LANL, Oct. 26, 2009                   23
   Sivers                                  Collins

                BELLE Collins: PRL96, 232002 (2006)

A flurry of new experimental results from fixed-target,
   semi-inclusive DIS and e+e- over last ~5 years.
 Will need more data over wider range of energies to
deepen our understanding of the correlations between
 partonic spin and transverse motion within hadrons.

                      C. Aidala, LANL, Oct. 26, 2009         24
Unpolarized collisions also relevant to
  study TMD’s . . . And vice versa
• Initial attempts have been made to extract the
      TMD’s are relevant when you have distribution
  kT-unintegrated unpolarized gluon both a
     hard scale, allowing pQCD to apply, and a
  from quarkonium pT spectra (hadronic fixed
       soft scale, sensitive to parton dynamics.
  target and TeVatron)
  – PHENIX J/Psi cross sections ready andsoft
     Natural framework to probe effect of waiting to
     scales on this processes in QCD, even at
    be used for hard(thanks to LANL team’s efforts!)
  – Driving interest has been ggHiggs at LHC!
           energy scales as high as LHC!

                 C. Aidala, PHENIX Collaboration Meeting,
                               August 2009
           e+A kinematic coverage and a new
                 Electron-Ion Collider
                                                Well mapped in ℓ+p.
                                                Not so for ℓ+A.

                                                Electron Ion Collider (EIC):
                                                • L(EIC) > 100  L(HERA)
                                                • Electrons
                                                       – Ee = 3 – 20 (30?) GeV
                                                       – Ee = 3 – 11 GeV (JLab)
                                                       – Polarized
                                                • Hadron beams
                                                       – EA = 130 GeV/nucleon
                                                       – EA = 100 GeV/nucleon
Terra incognita: small-x, Q  Qs                       – A= p  U
                                       2               – Polarized p and light ions
                  high-x, large QLANL, Oct. 26, 2009
                            C. Aidala,                                         26
Kinematic coverage for polarized p measurements

World Data on F2p                                      World Data on g1p
 Region of existing g1p data                                    With EIC projections
                               C. Aidala, LANL, Oct. 26, 2009                          27
          EIC accelerator concepts
eRHIC at BNL:                         ELIC at JLab:
Add e ring to RHIC                    Add proton/ion beam to CEBAF

   Both labs aggressively pursuing staged options since
                     C. Aidala, LANL, Oct. 26, 2009                  28
        Medium-energy eRHIC
                 No civil construction required. Possible realization by 2016.
                 Cost review underway this month.

 Opportunity to perform immediate further studies in
p+p, p+A, or A+A collisions based on what’s learned
             from medium-energy DIS.

 Note the medium-energy stage would NOT explore
gluon saturation. Focus on nuclear shadowing, spin-
       momentum correlations of partons, and

                       V. Ptitsyn, EIC-INT, October 2009
                     C. Aidala, LANL, Oct. 26, 2009                      29
            Medium-energy EIC at JLab
Extensive civil construction required
 Longer time scales

                                                        Three compact rings:
                                                        • 3 to 11 GeV electron
                                                        • Up to 12 GeV/c proton (warm)
                                                        • Up to 60 GeV/c proton (cold)

                                                    Lower range of energies than MeRHIC

                                    A. Bogacz, EIC-INT, October 2009
       Physics-driven detector requirements
E.C. Aschenauer, EIC-INT,
October 2009

 • ep-physics
       – the detector needs to cover inclusive (ep -> e’X)  semi-inclusive (ep -
         > e’hadron(s)X)  exclusive reactions (ep -> e’p
             •   large acceptance absolutely crucial
             •   particle identification (,K,p,n) over wide momentum range
             •   excellent vertex resolution (charm)
             •   particle detection for very low scattering angle
                    – around 1deg in e and p/A direction
                     in big contradiction to high focusing quads close to IP
       – small systematic uncertainty for e/p polarization measurements
       – very small systematic uncertainty for luminosity measurement
 • eA-physics
       – requirements very similar to ep
             • most challenging get information on recoiling heavy ion
              from exclusive and diffractive reactions.

                                            C. Aidala, LANL, Oct. 26, 2009      31
Detector design and simulations for both
staged proposals underway.
Significant consideration necessary to
integrate MeRHIC detector into existing
hall (with new civil construction, JLab
medium-energy design would have no
such constraint).

                                                          M. Lamont, EIC-INT, October 2009

                             C. Aidala, LANL, Oct. 26, 2009                             32
                                   Basic design for full-
                                   energy detector

                                 M. Lamont, EIC-INT, October 2009

C. Aidala, LANL, Oct. 26, 2009                             33
• JLab- and RHIC-based groups (including JLab and RHIC users, not
  just lab employees) established and working to
   – Fully develop physics cases for lower-energy, staged implementations
     of the EIC
       • For detailed feasibility studies, need to first create Monte Carlo tools to
         simulate the recently developed physics ideas that go beyond traditional pQCD!
   – Work out details (and costs!) of the lower-energy accelerator facilities
     and detectors
   – Develop the overall physics case and strategy for going from a lower-
     energy EIC to the full-energy EIC by the early 2020’s
• Efforts ramping up in anticipation of the 2012 Nuclear Physics
  Long-Range Plan
   – EIC workshop at EINN (“Electromagnetic Interactions with Nucleons
     and Nuclei) conference in Greece in late September
   – First INT workshop this fall, another planned for next fall
   – e+A mini-symposium planned for APS “April” Meeting in February
   – ....

                               C. Aidala, LANL, Oct. 26, 2009                        34
                       QED vs. QCD

EIC will be main tool in untangling the complexities
         of QCD in the upcoming decades

                                        precision measurements
    observation & models
                                        & fundamental theory
                           C. Aidala, LANL, Oct. 26, 2009        35
  Electroweak physics at the EIC?
• This talk focused on exploring QCD, but
  potential electroweak program for EIC also
  being investigated (K. Kumar, W. Marciano,
  M. Ramsey-Musolf, . . .)
  – Very high luminosities for both electrons and
    positrons desirable
  – Will need to determine if physics gain worth the

                    C. Aidala, LANL, Oct. 26, 2009     36
• The EIC will be a critical facility in tackling the
  rich complexities of QCD at a new level in the
  21st century!

• More information:
   – Official EIC website:
   – e+A working group:
   – Talks from recent INT workshop available at:

                       C. Aidala, LANL, Oct. 26, 2009          37

C. Aidala, LANL, Oct. 26, 2009   38
             Key Measurements in e+A
•   Momentum distribution of gluons G(x,Q2)
           Extract via scaling violation in F2: δF2/δlnQ2
           Direct measurement: FL ~ xG(x,Q2) (requires √s scan)
           2+1 jet rates
           Inelastic vector meson production (e.g. J/ψ)
           Diffractive vector meson production ~ [xG(x,Q2)]2
•    Space-time distributions of gluons in matter
    Exclusive final states (e.g. vector meson production ρ, J/ψ)
    Deep Virtual Compton Scattering (DVCS) - σ ~ A4/3
    F2, FL for various A and impact parameter dependence
•    Interaction of fast probes with gluonic medium?
    Hadronization, Fragmentation
    Energy loss (charm, bottom!)
•   Role of colour neutral excitations (Pomerons)
 Diffractive cross-section σdiff/σtot (HERA/ep: 10% , EIC/eA: 30%?)
 Diffractive structure functions and vector meson production
 Abundance and distribution of rapidity gaps
                                  C. Aidala, LANL, Oct. 26, 2009       39
 Connection to RHIC & LHC Physics
                                          GPb ( x) / GD ( x)
Matter at RHIC
 – Thermalizes fast (0 ~ 0.6 fm/c)
 – We don’t know why and how
 – Initial conditions?  G(x, Q2)
Role of saturation?
 – RHIC → forward region
 – LHC → midrapidity
    • bulk (low-pT matter) & semi-hard
                                                                   LHC       RHIC
Jet Quenching:
 – Need Reference: E-loss in cold matter
 – No HERMES data for
    • charm energy loss                                EIC provides essential new input:
    • in LHC energy range                              • Precise handle on x, Q2
                                                       • Means to study exclusive effects
                                  C. Aidala, LANL, Oct. 26, 2009
                        Diffractive Physics in e+A
    Diffractive event
‘Standard DIS event’

                         Curves: Kugeratski, Goncalves,
                         Navarra, EPJ C46, 413                                                   `
                                                                            xIP = mom. fraction of
                                                                            pomeron w.r.t. hadron
                           momentum transfer:
                    P’     t = (P-P’)2
• HERA/ep: 15% of all events are hard diffractive
• Diffractive cross-section in e+A ?                                    Activity in proton direction
➡Predictions: ~25-40%?
• Look inside the “Pomeron”
➡Diffractive structure functions
➡Diffractive vector meson production:
dσ/dt ~ [xG(x,Q2)]2 !!
• Distinguish linear evolution and saturation models
                            Curves: Kugeratski, Goncalves,
                                          C. Aidala,
                            Navarra, EPJ C46, 413 LANL, Oct. 26, 2009                                41
        Hadronization and Energy Loss
  • nDIS:
   –   Clean measurement in ‘cold’ nuclear

   – Suppression of high-pT hadrons
       analogous but weaker than at RHIC

Fundamental question:
When do coloured partons get neutralized?

Parton energy loss vs.
(pre)hadron absorption

  Energy transfer in lab rest frame
  EIC: 10-1600 GeV HERMES: 2-25 GeV
  EIC can measure heavy flavour energy loss
                                   C. Aidala, LANL, Oct. 26, 2009   42
                 F2 : Sea (Anti)Quarks Generated by
                            Glue at Low x
                                                               F2 will be one of the first
                                                               measurements at EIC
                                                               nDS, EKS, FGS:
                                                               pQCD-based models with
                                                               different amounts of

                                                              Syst. studies of F2(A,x,Q2):
                                                               GA(x,Q2) with precision
                                                               distinguish among

d 2 epeX 4a 2        y2                  y2                
             4 
                  1  y   F2 ( x, Q ) 
                                                   FL ( x, Q )2
  dxdQ  2
            xQ         2     C. Aidala, LANL, Oct. 26, 2009 
                                               2                                            43
     FL at EIC: Measuring the Glue Directly
                                                                     FL  a sG ( x, Q 2 )
                                                                FL requires  s scan
                                                                Q2/xs = y

                                                                Ldt = 5/A fb-1 (10+100) GeV
                                                                     = 5/A fb-1 (10+50) GeV
                                                                     = 2/A fb-1 (5+50) GeV

                                                                statistical error only

                                                                          GA(x,Q2) with great
d 2 epeX 4a 2        y2                  y2                
                1  y   F2 ( x, Q ) 
                                                   FL ( x, Q )2
  dxdQ2     xQ4 
                        2                  2
                                  C. Aidala, LANL, Oct. 26, 2009
                   Charm at an EIC

                                                                         Based on HVQDIS model, J. Smith
•   Allows multi-differential measurements of heavy flavour
•   Covers and extends energy range of SLAC, EMC, HERA, and
                                    wide range
    JLAB allowing for study ofLANL, Oct. 26, 2009 of formation lengths
                         C. Aidala,                                      45
    50% of Momentum Carried by Gluons …
       But Still Plenty of Gluon Puzzles
                                                        g               xS > xg ???
If sea quarks come from gluon splitting,
how can the gluon and sea distributions
diverge as they appear to at low Q2??

         A low Q2 puzzle …
                                   C. Aidala, LANL, Oct. 26, 2009                     46
    Longitudinal Structure Function FL
• Experimentally can be determined
• Highly sensitive to effects of gluon

        + EIC alone
        + 12-GeV data

                                 C. Aidala, LANL, Oct. 26, 2009                              47
                                                       (includes systematic uncertainties)
         DG Via Open Charm and Dijets at EIC

  Projected data on Dg/g with an
  EIC, via g + p  D0 + X
                        K- + +

  Advantage: measurements directly
  at single Q2 ~ 10 GeV2 scale!
• Uncertainties in xDg smaller than 0.01
• Measure 90% of DG (@ Q2 = 10 GeV2)

                                   C. Aidala, LANL, Oct. 26, 2009                   48
   Precisely Image the Sea Quark Polarization
Spin-Flavor Decomposition of the Light Quark Sea

               u              u                             u

                                          u                          d
  | p   =      u +            u     +                      + …
                                                           u             Many models
                                          u                          d   predict
               d              d                             d
                                                                         Du > 0, Dd < 0
                     RHIC-Spin region

                                    C. Aidala, LANL, Oct. 26, 2009                 49
    Towards a 3D spin-flavor
    landscape                           Wu(x,k,r)
                     x = 0.01                 x = 0.40               x = 0.70
                                 m                                                              m

     p                           x                                          p
           TMD                                                                     GPD          B

Link to                                             GPDu(x,x,t) Link to
                                                                ~     ~
Orbital         fWigner function: Probability to find a u(x) quark with a Orbital
                  1,g1,f1T ,g1T
Momentum                                             Hu, Eu Hu, Eu
        Fourierh1, h1T ,h1L ,h1momentum position r and with ,momentum k
                 certain polarization at transfer
                transform in                                              Momentum

                 f1(x)               u(x)                   F1u(t)
                 g1, h1                                     F2u,GAu,GPu
                                     Du, du
                                 Parton                  Form Factors
                                     C. Aidala, (parton) 2009
         gives transverse position of quarkLANL, Oct. 26, with longitud. mom. fraction x   50
           Transverse Spin Asymmetries:
           What can be expected at EIC?
• Larger x range than
  measured by existing
   COMPASS ends at ~ 0.01,
    go lower by almost one                                AUT
    order of magnitude, but
    asymmetries become small
• Have some overlap at
  intermediate x to test
  evolution of Collins function
  and higher twist but at higher

                         C. Aidala, LANL, Oct. 26, 2009              51
Boer-Mulders                                 SPIN2008

  C. Aidala, PHENIX Collaboration Meeting,
                August 2009
       Measurements to test gauge-link
            formalism in pQCD
       (“Sivers process dependence”)
A few different observables now proposed for
• Drell-Yan SSA (Collins 2002, Yuji’s talk)
• Photon-jet SSA (Bacchetta et al., PRL99,
  212002 (2007))
• W SSA (Kang and Qiu, arXiv:0903.3629)

                C. Aidala, PHENIX Collaboration Meeting,
                              August 2009
                   Photon-jet SSA using FOCAL + VTX
    J. Lajoie                                                                        2008 Sivers distribution fits
                                                                   Blue: abs. value of gluon Sivers
            ~5 GeV trigger, 200 pb-1, <P> = 0.65                   Light Blue: abs value of Boer-Mulders
            Assumes FOCAL efficiencies and 0                      Red: quark Sivers (with process dependence)
                                                                   Green: quark Sivers (no process dependence)
            contamination are same at 200/500GeV.

                                       200GeV                                                    500GeV

                       Simulations in increasing detail in progress


                  pTg>5GeV                                             pTg>5GeV
                  -1<hj<1                                              -1<hj<1

                                                hg                                                          hg
            (need to worry about signal contamination/efficiency with with hJET cut)
                                               PHENIX FOCAL Workshop                                                 54
Z’s as clean theoretically as D-Y. FOCAL on both
sides would significantly enhance Z capabilities in
   PHENIX, but need a quantitative estimate to
      determine if SSA would be measurable.

                 C. Aidala, PHENIX Collaboration Meeting,
                               August 2009
          Deep-Inelastic Scattering:
          A Tool of the Trade in Probing
           the Partons within Nucleons

     [Mention that gets more complex if we go
            beyond traditional picture.
       Connect to hadron-hadron, e+e- 
• Probe nucleon with an electron or muon beam
• Interacts electromagnetically with (charged)
  quarks and antiquarks
• “Clean” process theoretically—quantum
  electrodynamics well understood and easy to
  calculate!      C. Aidala, LANL, Oct. 26, 2009   56
             DIS Kinematic Variables
    Deep Inelastic Scattering:                 Measure of resolution power:
                                               Q 2  q 2  (k   k  ) 2

                                               Measure of momentum fraction
                                               of struck quark
                                                x          “Bjorken x”
                                                   2 pq
                                               Measure of inelasticity
                                                   Ee  Ee
    “Perfect” Tomography                             Ee
Inclusive DIS: Measure only energy and scattering angle of outgoing e
Semi-inclusive DIS: Measure outgoing e & some final-state hadrons
Exclusive DIS: Measure entire final state
                           C. Aidala, LANL, Oct. 26, 2009                     57
What Do We Know About Glue in Matter?
        d 2 ep eX 4ae2.m.          y2             y2          
                              1  y   F2 ( x, Q )  FL ( x, Q )
                                                    2             2
  DIS :                         
         dxdQ    2
                     xQ4              2             2           

                                    Access the gluons in DIS via
                                    scaling violations:
                                    dF2/dlnQ2wave function xu
                                                and linear
                                          Gluons dominate

                                    DGLAP evolution in Q2
                                             xG ( 1 ) !
                                     G(x,Q2)             xd        v

                                                 xS ( 1        )

                         C. Aidala, LANL, Oct. 26, 2009                     58
            Other progress in pQCD
            calculational techniques
         (Often) important for experiments to
           measure theoretically tractable
        “Luckily” for us, pQCD an ever-more-
                    powerful tool!
         “Modern-day ‘testing’ of (perturbative) QCD is as
               much about
One (very!) recent example: pushing the boundaries of its
Almeida, Sterman, Vogelsang arXiv:0907.1234
       applicability as about the verification that QCD is the
Cross section for di-hadron production vs. invariant mass using
                   correct theory of hadronic physics.”
threshold resummation (rigorous method for implementing pT
        – G. Salam, hep-ph/0207147 (DIS2002 proceedings)
and rapidity cuts on hadrons to match experiment)
                          C. Aidala, LANL, Oct. 26, 2009          59
                                 eRHIC at BNL
                                                                      (RHIC II)
       24-250 GeV protons
       30-100 GeV/n ions

                                    3-10 (20) GeV electrons
                                 Main ERL (2 GeV per pass)
                                                                         Add energy-recovery
                                                                         linac to RHIC
                                                    Four e-beam

•   Peak luminosity 2.6  1033 cm-2s-1 in electron-hadron collisions
•   Electron beam polarization not affected by energy
•    5 meter “element-free” straight section for detectors
•   Ion beams up to U
•   Ability to take full advantage of electron cooling of the hadron beams
•   Can run hadron-hadron collisions in RHIC simultaneously

                                     C. Aidala, LANL, Oct. 26, 2009                            60
                             ELIC at JLab
     30-225 GeV protons                                 Snake
     30-100 GeV/n ions                                                 Add hadron beam
                                                                       facility to CEBAF

                                                   3-9 GeV electrons

 Visionary green-field design:
• Peak luminosity up to ~8  1034 cm-2s-1 through short ion bunches
• “Figure-8” lepton and ion rings
•  3m “element-free” straight sections
• Ion beams up to Au
• Superconducting RF ion linac concept for all ions
• 12 GeV CEBAF accelerator serves as injector to electron ring

                                   C. Aidala, LANL, Oct. 26, 2009                          61
   EIC Recent History: White Papers 2007
                                       • The Electron Ion Collider
                                         White Paper
Available at:                          • The GPD/DVCS White Paper
• NSAC LRP2007 home page
• Rutgers Town Meeting page
                                       • Position Paper: e+A Physics
•                 at an Electron Ion Collider
                                       • The eRHIC machine:
                                         Accelerator Position Paper
                                       • ELIC ZDR Draft

                         C. Aidala, LANL, Oct. 26, 2009          62

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