Theory Program for the 12 GeV Upgrade

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Theory Program for the 12 GeV Upgrade

• David Richards

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Outline

• Two Initiatives
• Lattice QCD
• Phenomenology Marc Vanderhaeghen
• Lattice QCD
• Exploiting advances in Lattice QCD
• Science Milestones White Paper to DOE
• Moments of GPDs at physical pion mass
• Nucleon Form Factors at high Q2
• Pion form factor
• Hybrids spectrum and couplings
• Computational program
• Research synopsis Theory and 12 GeV
• Gluonic Excitations and origin of confinement
• Fundamental structure of nuclear building blocks
• Physics of Nuclei
• Standard Model and beyond

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Advances in Lattice QCD - I
Actions with exact chiral symmetry
Advances in high-performance computing
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Advances in Lattice QCD - II

• Achievable precision limited by lightest pion
  mass attained.

Momentum fraction carried by valence quarks
Detmold et al, PRL87, 172001 (2001)
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Advances in Lattice QCD - III

• Exact chiral symmetry at Finite Spacing a

Ginsparg-Wilson
Domain-wall fermions (DWF) action in 5-D
L5
Computationally 20-30 times as demanding
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State of the Art

• Current resources enable us to begin exploiting
  these advances in full QCD
• Employ computationally efficient full QCD
  lattices generated by MILC Collaboration using
  staggered fermions for sea quarks
• Employ DWF for valence quarks
• Pion masses down to less than 300 MeV to yield
• Moments of GPDs and PDFs
• Elastic form factors
• Resonance spectrum of baryons and mesons
• Approach requires introducing multiplicity of
  pions and approximation to fermion determinant

Fully consistent treatment of chiral symmetry for
both sea and valence quarks
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Moments of Flavor-NS PDFs and GPDs - I

• Lattice QCD can compute both moments of GPDs
  with respect to x, and t-dependence

Lattice data m? 740 MeV
Decrease slope decreasing transverse size as x
! 1 - Burkardt
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Moments of Flavor-NS PDFs and GPDs - II

• Nucleon axial charge benchmark computation
• DWF computation removes uncertainty in matching
  to continuum.
• Chiral behavior expected to be mild

DWF Data Two volumes
LHPC Preliminary
Systematic errors on lowest moments lt 5 by 2012
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Elastic Nucleon Form Factors Gp(n)E,M(Q2) - I

• Magnetic Moments, obtained at Q2 0, known
  experimentally to high precision
• Lattice computation at physical quark masses and
  large volumes
• Large Q2 behavior controlled by lattice spacing a
• Need to work on fine lattice

• Importance of pQCD Corrections
• Brodsky
• pQCD computation
• Belitsky, Ji, Yuan

m? 750 MeV
m? 900 MeV
LHPC/SESAM, hep-lat/0404005
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Elastic Nucleon Form Factors Gp(n)E,M(Q2) - II

• Cross-over in ? GpE(Q2)/GpM(Q2) not yet observed.

• Reach Q2 ' 6 GeV2 by 2010
• Reach Q2 ' 10 GeV2 by 2012

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Pion Form Factor I

• Laboratory for observing interplay between
  perturbative and non-perturbative aspects of QCD
• Asymptotic normalization determined
• Small Q2, vector-meson dominance provides
  faithful description
• Free of disconnected contributions
  straightforward lattice calculation

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Pion Form Factor II

• Current and Projected Experimental Situation

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Pion Form Factor III
LHPC, Bonnet et al hep-lat/0411028

• Pion Form factor over Q2 commensurate with
  experiment
• Pion GPDs and transition form factors

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Resonance Spectrum

• Spectrum of hybrids composed of heavy quarks has
  been important lattice focus
• Adiabatic Potentials
• Spectrum of charmonium

• 1 GeV Excitation Energy associated with excited
  string
• 1- lightest hybrid
• Does this interpretation persist to light-quark
  sector

MILC Collaboration
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Photocouplings and Transition FF - I
RSMGC/GM
REM-GE/GM
Alexandrou et al. (2005)

• REM should approach 1 Ji, Yuan
• Compute in Lattice QCD at high Q2

discrepancy currently under theoretical study
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Photocouplings and Transition FF - II

• Photocouplings between hybrid and conventional
  mesons
• GlueX proposal to produce hybrid mesons using
  real photons supported by flux-tube model
  calculations
• No suppression of conventional-hybrid
  photocouplings for hybrids near 2 GeV

Close and Dudek, PRL91, 142001 (2003) PRD 69
034010 (2004)

• Investigate and attempt to verify prediction
  using lattice QCD

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Computational Advances - I

• Program of lattice studies with milestones
  linked to 12 GeV Whitepaper to DOE, Feb 05.

Coarse a 0.11 fm
fine a 0.09 fm
Tflop-year m?-7 a-6
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Computatonal Advances - II

• Cluster Trends
• 1 Tflop-scale clusters currently offering
  1/Mflops price-performance
• Cluster hardware price-performance increasing at
  least as fast as Moores Law (performance/price
  doubling every 18 months)
• Ramped funding model tuned to physics goals
• Each ensemble requires at most 2 years running on
  largest cluster
• Funding above current 1.2M SciDAC NP

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Theory Research Synopsis - I

• Gluonic Excitations and origin of confinement
• Lattice QCD
• Phenomenological quark models
• Chiral dynamics and large Nc QCD
• Properties of exotics relevant to phase-shift
  analysis
• Fundamental structure of nuclear building blocks
• QCD Factorization and GPDs
• EM corrections to hadronic processes
• Lattice QCD
• Models of nucleon structure
• Quark-hadron duality

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Theory Research Synopsis - II

• Physics of Nuclei
• Nuclear effects in DIS
• Effective field theory approaches
• Relativistic description of few-nucleon systems
• Nucleon-nucleon force from QCD
• Standard Model and beyond
• Constraints on lepton-flavor mixing from
  experiments
• Effects of TeV-scale
• Signals for new light particles such as gluinos

• Phenomenology Marc Vanderhaeghen

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Summary

• Lattice QCD
• Emerging computational resources allowing
  exploitation of theoretical advances
• Schedule of computations commensurate with the
  physics goals of the 12 GeV program
• Precise computations of low moments of PDFs and
  GPDs
• Pion form factor and transition form factors
• Spectrum, transitions and decays of exotic mesons
• Understanding of nucleon-nucleon force from
  lattice QCD
• Strong theory effort on the campaigns of the 12
  GeV upgrade

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Theory Research Synopsis - III
How Theory Group research matches the JLab 12 GeV
program
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JLab Theory Center Associate Staff

• Numerous sabbatical visitors (supported by JLab)
  D. Diakanov (Nordita) - from Apr 04 (9
  months)
• J. Tjon (Utrecht) - from Aug 03 (6 months) J.
  Laget (Saclay) from May 04 (4months) D.
  Leinweber (CSSM) from Aug 04 (2 months)
• 1 postdoctoral fellow (external funding)
• Vladimir Pascalutsa (WM, Gross/Vanderhaeghen
  DOE) - from Oct 03
• 8 graduate students3 supported by JLab 2 LSU
  (one LSU support, one SURA Fellowship) 3
  Adelaide University (with AWT)

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Strange Form Factors of Proton - I

• Flavor-separated quantities more demanding than
  those of flavor non-singlet
• Requires computation of disconnected diagrams

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Strange Form Factors of Proton - II

• Recent computation of magnetic moment using
  amalgam of Lattice QCD and Phenomenology

D. Leinweber et al, hep-lat/0406002
Ab initio computations of magnetic moments and
form factors at non-zero Q2.
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Resonance Spectrum - II
MILC, PRD68, 074505 (2003)

• Particles unstable in full QCD at light pion
  masses
• multiparticle contributions
• Method for investigating multiparticle states
  known
• Variational methods
• Basis of interpolating operators
• Determine scattering lengths

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Resonance Spectrum - III

• Higher resonances provide information about
  dynamics and confinement
• Aim to compute masses of lowest few states of
  given quantum number, and learn about their
  structure

Glueballs

• Morningstar and Peardon
• PRD60, 034509
• Quenched glueball calculations provide road-map
• Method more demanding for particles containing
  quarks

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Hybrid Meson Photocouplings - II

• Desire to minimize sources of uncertainty leads
  us to work at the charm quark mass
• no need for chiral extrapolation (poorly
  constrained for hybrids)
• smaller effects due to quenching
• conventional charmonium radiative transition data
  is mature

• Flux-tube model has only small relativistic
  corrections for charmonium hybrids direct test
  of the model