Nuclear Physics in the SciDAC Era

Robert Edwards Jefferson Lab

SciDAC 2009

Comparison of Chemistry & QCD : K. Wilson (1989 Capri):

“lattice gauge theory could also require a 108 increase in computer power AND

spectacular algorithmic advances before useful interactions with experiment ...”

• ab initio Chemistry1. 1930+50 = 19802. 0.1 flops 10 Mflops3. Gaussian Basis functions

• ab initio QCD1. 1980 + 50 = 2030?*2. 10 Mflops 1000 Tflops3. Clever Multi-scale Variable?

*Fast Computers + Rigorous QCD Theoretical AnalysisSmart Algorithms + = ab initio predictions

“Almost 20 Years ahead of schedule!”

Forces in Standard Model

electron

proton

neutron

quarks

-

+

+

Atoms: MaxwellN=1(charge)

Nuclei WeakN=2 (Isospin)

Sub nuclear: StrongN=3 (Color)Standard Model: U(1) £ SU(2) £ SU(3)

Quantum Chromo Dynamics - QCD

• QED: theory of electromagnetism• QCD: theory of strong interactions – hadronic physics

QED QCD

Photon, Gluons, G

Charged particles, e, , u, d,… Quarks: u, d, s, c, b, t

2 charges: positive & negative

3 charges: “red”, “green”, “blue”

Photon is neutral Gluons carry color charge

e ' 1/137 s ' O(1)

• Highly non-linear theory – can only use perturbation theory at high energy

Quark+anti-Quark in Meson

Energy in glue

3 Color 3 quarks in Proton

QCD

• QCD: Dirac operator: Aº (vector potential), m (mass), °º (4x4 matrices)

• Lattice QCD: finite difference

• Probability measure:

• Observables:

Gauge generation

How to produce gauge fields?

• Hamilton’s eq’s - 1st order coupled diff. eq’s)

• Bummer!– Must be “reversible”– No adaptive time steps

Total energy in gauge/quark fields

Momentum

Cost Scaling• Cost: reasonable statistics, box size and “physical” pion

mass• Extrapolate in lattice spacings: 10 ~ 100 PF-yr

PF-years

USQCD National Effort• US Lattice QCD effort: Jefferson Laboratory, BNL and FNAL

FNALWeak matrix

elements

BNL

RHIC Physics

JLAB

Hadronic Physics

SciDAC – R&D Vehicle

Cluster Prototyping Software R&D

Impact on DOE’s Nuclear Physics Program

SciDAC Software

QCD friendly API’s/libs• Application codes

• High level (Linpack-like)

• Data parallel (C/C++)

• Linear algebra, threading, comms

• Code generators

http://www.usqcd.org

QDP/C++ Expressions

Can form expressions:

ci(x) = U

ij(x+nu) bj(x) + 2 d

i(x) for all sites x

multi1d<LatticeColorMatrix> U(Nd);LatticeFermion c, b, d;int nu, mu;

c = shift(u[mu],FORWARD,nu)*b + 2*d;

QDP++ code (data-parallel)

Template based Shifts use QMP for face comms Level-1 BLAS-like linear algebra core

Critical code: Dirac operator/inverter

• Critical codes: develop special API and libraries

• Example: Dirac operator

[ ]

Threading/Multi-core

• Hybrid threads/MPI

• Impact:– Coalesce

messages– Better perf.– Cache-

coherency latency EXPENSIVE

Scaling on Cray XT4 (ORNL)

Socket level threading improved performance

threads+mpi

mpi

Work involving RENCI

Acceleration

• Deflation & multigrid – big speedups

JLab/W&M (SciDAC) + TOPS

Nuclear Physics & Jefferson Lab

• Lab doubling beam energy• Adding new experimental Hall

CD-3 JLab Receives DOE Approval to Start Construction of $310 Million Upgrade

CD-3 JLab Receives DOE Approval to Start Construction of $310 Million Upgrade

Nuclear Structure• Fundamental questions

– Size, shape, distribution of charge and current in hadrons– Quark and gluon distributions– How does nucleon spin arise from quarks and gluons?– What role do strange quarks play in nucleon structure?

• Status– Basic nucleon properties calculated with 5-10% precision.– Pursuing higher precision, more demanding properties.

• NP 2014 milestone– Perform lattice calculations in full QCD of nucleon form

factors, low moments of nucleon structure functions and low moments of generalized parton distributions, including flavor and spin dependence.

Nuclear Structure

Spin of the proton?

~41% quark spin (u+d)

~0% orbitalSo: ~59% from glue

(&/or strange)

Most of mass & spin not from quarks

Caveats:• Missing terms

(disconnected)

Phys. Rev. D77 094502

Spectroscopy

Spectroscopy reveals fundamental aspects of hadronic physics.– Essential degrees of freedom?– Gluonic excitations in mesons - exotic states of matter?

• Status.– Can extract excited nucleon energies & identify spins, – Pursuing calculations in full QCD with realistic quark

masses.

• Crucial complement to 12 GeV program at JLab.– Excited nucleon spectroscopy.– GlueX: flagship search for gluonic excitations.

Nucleon spectrum

NP2012 milestone:• Spectrum & E&M transitions up

to Q2 = 7 GeV2

Highly excited energies:First ever lattice

calculation

Pattern of states ->

Future work:– Separate out decays– Move to physical regime

½+ 3/2+ 5/2+ ½- 3/2- 5/2-

Possible 5/2- state

Phys. Rev. D79 034505

Exotic matter?

Can we observe exotic matter? Excited string

QED

QCD

• Charmonium excited spectrum: J-+

• Exotic matter (1-+) radiative decay: large

Spectroscopy

If true with light quarks: Can observe at future JLab Hall D!!

Unknown inexperiment

GeV

Phys. Rev. D77 034501 & to appear PRD

Outlook• Software infrastructure developed for Lattice QCD

– Enabled effective utilization of INCITE resources• Lattice QCD’s impact on Nuclear Physics

– Nucleon structure (protons, neutrons)– Spectroscopy

• Results relevant to U.S. DOE experimental programs• Unifying Nuclear Physics research