The Lattice Initiative at Jefferson Lab Robert Edwards Jefferson Lab

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The Lattice Initiative at Jefferson LabRobert
EdwardsJefferson Lab

• JLab in a close partnership with MIT has formed
  the Lattice Hadron Physics Collaboration (LHPC)
• Members
• R. Brower, C. Rebbi (Boston U.), C. Morningstar
  (CMU), S. Chandrasekharan (Duke), R.
  Fiebig (FIU), F.X. Lee (GWU),
• R. Edwards, D. Richards, C. Watson (JLab),
• S.J. Dong, T. Draper, K.F. Liu (Kentucky),
• X. Ji, S. Wallace (Maryland),
• P. Dreher, J. Negele, A. Pochinsky (MIT),
• M. Burkhardt (NMSU), E. Swanson (Pittsburg), H.
  Thacker (Virginia)

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Structure and Interactions of Hadrons

• Quark and gluon structure of hadrons
• Spectroscopy of conventional and exotic states of
  hadrons
• Interactions between hadrons
• Fundamental aspects of QCD including confinement
  and chiral symmetry breaking

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SciDAC Initiative

• DOE Scientific Discovery through Advanced
  Computing Initiative develop software/hardware
  infrastructure for next generation computers
• U.S. Lattice QCD Collaboration consists of 64
  senior scientists. Research closely coupled to
  DOEs experimental program
• Weak Decays of Strongly Interacting Particles
• Babar (SLAC)
• Tevatron B-Meson program (FNAL)
• CLEO-c program (Cornell-proposed)
• Quark-Gluon Plasma
• RHIC (BNL)
• Structure and Interactions of Hadrons
• Bates, BNL, FNAL, JLab, SLAC
• Project 6M for 2001-2003, 30 JLab, 30 FNAL,
  15 BNL, 25 universities
• Software development hardware prototyping
  efforts no direct physics support

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National Computational Infrastructure for
Lattice Gauge Theory

• Project 6M for 2001-2003, 30 JLab, 30 FNAL,
  15 BNL, 25 universities
• Unify software development and porting efforts
  for diverse hardware platforms
• Hardware prototyping efforts clusters, QCDOC
• No direct physics support

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Realization of QCD on a lattice

• Approximate continuous space--time with a 4-dim
  lattice, and derivatives by finite differences.
  Theory formulated in Euclidean space.
• Quarks on sites, gluons on links. Gluons
  represented by 3x3 complex unitary matrices Um
  (x) exp(iga Am(x)) elements of the group SU(3).

• Gaussian integration over anti-commuting fermion
  fields y resulted in det(M(U)) and M-1(U)
  factors.
• Gauge action composed of U fields. Approximates
  continuum

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Some Lattice QCD Successes
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More Successes and Future Expectations
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Nuclear Physics Future Expectations
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Precision Tests of the Standard Model

• Lattice calculations of weak matrix elements are
  needed to relate experimental results to
  underlying parameters of the Standard Model
• Multiple measurements of the same Standard Model
  parameters in different experiments and
  calculations will lead to crucial consistency
  tests
• In many cases the greatest challenge is to reduce
  the uncertainties in the lattice calculations

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Constraints on Standard Model Parameters

• ? and ? in Wolfenstein parameterization (1 sigma
  confidence level)
• For SM to be correct, they must be in overlap of
  solid bands
• Left figure constraints today
• Right figure constraints with existing
  experimental errors and only improvement in
  lattice uncertainties to 3

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Confinement and Model Predictions - Static Quark
Potentials

• Models propose different mechanisms for
  confinement
• Static quark potential (potential between
  infinitely massive quarks forming mesons) in
  different representations can discriminate among
  the models
• Perturbative Casimir scaling hypothesis well
  describes non-perturbative lattice data
• for Casimir CD in representation D3,6,8,
• Claimed to rule out models like Bag and Instanton
  scaling different
• Flux tube counting also inconsistent

Bali, 99
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Hadron Spectrum Benchmark of Lattice QCD

• Spectrum of lowest lying states is the benchmark
  of LQCD
• Most extensively pursued lattice calculation
• Quenched spectrum agrees with experiment to 10
• Inconsistency in meson sector apparently resolved
  in full QCD
• Systematic uncertainties
• Finite volume V ? ?
• Continuum extrapolation a ? 0
• Chiral extrapolations MPS? M?
• Calculation 50 Gflops-years.
• In 1999 largest NERSC allocation 2 Gflop-years

GF11, CPPACS 99
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Excited Baryons

• Describing N spectrum gives vital clues about
  dynamics of QCD and hadronic physics
• Role of excited glue
• Quark-diquark picture
• Quark interactions
• Open mysteries
• Nature of Roper?
• ?(1405) mass?
• Missing resonances?
• History of lattice studies of excited baryons
  quite brief. Recent work using improved gauge and
  fermion actions

Lattice Representations Continuum spin reducible
under three irreducible ray representations of
the cubic group Rep. Continuum spin reps
G1 1/2, 7/2, H 3/2, 5/2, 7/2, G2 5/2, 7/2,
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Gluonic States of Matter

• Glueballs quenched glueball
• Surprising result masses closer to 2 GeV instead
  of 1 GeV
• Hybrid mesons big focus of JLab (and lattice
  group!)
• Spin exotic mesons are JPC states not accessible
  in quark model
• Characterized by excited glue or perhaps
  four-quark states
• Lattice calculations of light exotic meson states
  still first generation (noisy)!
• Lightest 1- exotic roughly 2GeV
• Considerably higher than experimental candidates
  1.4, 1.6 GeV

Morningstar Peardon 99
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Moments of Nucleon Quark Distributions

• JLab/MIT-Adelaide 1st three non-trivial moments
  of non-singlet unpolarized quark distribution
  u-d in the proton

• Calculation 10s of Gflops-years
• Chiral extrapolation sensitive to small quark
  mass
• Factor of 2 decrease in error bars in 2 weeks!
• Prediction for transversity dist

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Hardware Plans

• Simplifying features of lattice QCD calculations
  make building specially designed computers far
  more cost effective than buying commercial ones
• Uniform grids
• Regular, predictable communications
• Two hardware tracks
• QCD On a Chip (QCDOC)
• Commodity Clusters
• Each track has its own strength
• Each track may prove more optimal for different
  aspects of our work
• The two track approach positions us to exploit
  future technological advances, and enables us to
  retain flexibility

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Commodity Clusters

• Market forces are producing rapid gains in
  processor and memory performance
• Moores Law ? 60 growth in performance per year
• Pentium 4 currently provides exceptional
  performance for QCD
• Market for interconnects is growing
• Open Source System Software
• Flexible programming environment
• SciDAC Scalable Systems Software
• Targeted price-performance
• JLab acquisitions
• NOW 128 node/myrinet P4 cluster 130 Gflops
• Late summer probably 256 P4 node/3-dim. GigE
  mesh gt 200 Gflops

FY 2002 FY 2003 FY 2004 FY 2005 FY 2006
/Mflops 3.3 2.0 1.2 0.9 0.7
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Deployment Plan

• QCDOC
• FY 2003 1.5 Tflops (Columbia)
• FY 2003-4 5.0 Tflops (BNL)
• Clusters
• FY 2002-3 0.5 Tflops (FNAL, JLab)
• FY 2004 1.0 Tflops (FNAL, JLab)
• FY 2005 6.0 Tflops (FNAL, JLab)
• FY 2006 8.0 Tflops (FNAL, JLab)
• Planning for 22 Tflops by 2006
• Hope to obtain funding from HEP, NP, and SciDAC
  programs
• Funding at a higher level would accelerate
  research, and enable U.S. leadership in lattice
  QCD

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The Competition

• Theorists in Europe and Japan are moving rapidly
  to obtain resources comparable to those we
  propose
• The APE Collaboration will begin deploying
  multi-teraflops computers in 2003
• UKQCD will acquire a 5.0 Tflops (sustained) QCDOC
  in 2003
• DESY plans to acquire a 20.0 Tflops (peak) APE
  NEXT in 2004
• We need to act now to deploy the infrastructure
  required for terascale simulations of QCD

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Conclusions

• JLab lattice group actively pursuing calculations
  of
• Excited baryon spectroscopy
• Exotic/hybrid meson spectroscopy
• Elastic EM nucleon electric and magnetic form
  factors
• Anticipate calculations of ????
• Precise calculations commensurate with
  experimental program require
• Measure a large number of correlators
• Sufficiently light pions to resolve pion cloud
• Large physical volumes
• Continuum extrapolation
• Full QCD
• SciDAC efforts
• Software/hardware infrastructure development
• Follow-on deployment of large terascale systems