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 10 8 increase in computer power AND spectacular algorithmic advances before useful interactions with experiment ...”

• ab initio Chemistry

1. 1930+50 = 1980 2. 0.1 flops  10 Mflops 3. Gaussian Basis functions

• ab initio QCD

1. 1980 + 50 = 2030?* 2. 10 Mflops  1000 Tflops 3. Clever Multi-scale Variable?

“Almost 20 Years ahead of schedule!” *

F

ast Computers + Smart Algorithms + Rigorous QCD Theoretical Analysis = ab initio predictions

Forces in Standard Model

Atoms: Maxwell N=1(charge ) electron + Nuclei Weak N=2 (Isospin) proton + neutron quarks Standard Model: U(1) £ SU(2) £ SU(3) Sub nuclear: Strong N=3 (Color )

Quantum Chromo Dynamics - QCD

• •

QED: theory of electromagnetism QCD : theory of strong interactions – hadronic physics QED Photon,



Charged particles, e,



, u, d,…

2 charges: positive & negative

Photon is

neutral



e ' 1/137

QCD Gluons, G Quarks: u, d, s, c, b, t

3 charges: “red”, “green”, “blue”

Gluons carry

color charge



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: potential), m (mass), ° º A º (vector (4x4 matrices) • Lattice QCD : finite difference • Probability measure: • Observables:

Gauge generation

How to produce gauge fields?

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

– Must be “reversible” – No adaptive time steps Total energy in gauge/quark fields

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

FNAL

Weak matrix elements

JLAB

Hadronic Physics

BNL

RHIC Physics

SciDAC – R&D Vehicle Cluster Prototyping Software R&D Impact on DOE ’ s Nuclear Physics Program

QCD friendly API’s/libs • Application codes

SciDAC Software

• High level (Linpack-like) • Data parallel (C/C++) • Linear algebra, threading, comms • Code generators http://www.usqcd.org

QDP/C++ Expressions

Can form expressions: c  i (x) = U  ij (x+nu) b  j (x) + 2 d  i (x) for all sites x QDP++ code (data-parallel) multi1d< LatticeColorMatrix > LatticeFermion c, b, d; int nu, mu; U(Nd); c = shift(u[mu], FORWARD ,nu)*b + 2*d; 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

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

Lab doubling beam energy Adding new experimental Hall

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% orbital So: ~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 Q 2 = 7 GeV 2

Highly excited energies:

First ever lattice calculation

Pattern of states -> Future work: – Separate out decays – Move to physical regime Possible 5/2 state ½ + 3/2 + 5/2 + ½ Phys. Rev. D79 034505 3/2 5/2 -

Exotic matter?

QED QCD Can we observe exotic matter?

Excited string

Spectroscopy

• Charmonium excited spectrum: J -+ • Exotic matter (1 -+ ) radiative decay :

large

Unknown in experiment GeV Phys. Rev. D77 034501 & to appear PRD If true with light quarks:

Can observe at future JLab Hall D!!

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