# Nucleon Structure down to the Physical Pion Mass

## First-ever calculation of nucleon structure using full lattice QCD at the physical pion mass

Researchers led by John Negele from MIT’s Center for Theoretical Physics will use their ALCC allocation to perform the first calculation of nucleon structure using full lattice QCD at the physical pion mass. Working with the Budapest-Marseille-Wuppertal (BMW) collaboration and using their 2+1 flavor dynamical stout-smeared O(a)-improved Wilson quark configurations for nucleon structure observables, they will calculate connected diagram contributions to form factors, moments of structure functions, and generalized form factors at 6 pion masses from approximately 338 MeV down to the physical pion mass at lattice spacing ~ 0.12 fm.

The team will study scaling behavior by calculating observables for m ~ 250 MeV at a ~ 0.12, 0.09, and 0.07 fm and finite volume corrections effects by comparing results for mL = 4.1 and 5.5 at m = 296 MeV. They will also conduct an extensive calculation of disconnected quark diagrams and gluon contributions for m ~ 250 MeV at a ~ 0.12 fm.

## Optimized code, exceptional performance

The software for the stout-smeared O(a)-improved Wilson fermion calculations was optimized by Stefan Krieg for the Blue Gene/P. At these lattice sizes, the Dirac operator achieves a maximum of 37 percent of peak and typically runs at approximately 32 percent of peak on one to sixteen racks. The full inverter runs at more than 32 percent of peak. Using their own optimized domain wall inverter, performance is exceptional and provides significant advantage to the U.S. lattice QCD effort of international QCD software development supported by ASCR and NNSA.