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Meson mission

by Trish Barker

Using supercomputing resources provided by the National Science Foundation’s Blue Waters and XSEDE projects and the Department of Energy’s INCITE program, a team of researchers has performed the most precise calculation to date of a key quantity in high-energy physics, a field that asks fundamental questions about the nature of matter and our universe. This work on the ratio of the leptonic decay constants of the K and pi mesons was recently published in Physical Review Letters.

The USQCD Collaboration brings together many U.S. scientists who use large-scale computers for calculations in lattice quantum chromodynamics, a theory of the strong interaction between elementary particles (quarks and gluons). Robert Sugar of the University of California-Santa Barbara, the principal investigator for the team’s Petascale Computing Resource Allocation (PRAC) on the Blue Waters supercomputer, explains that much of their work is aimed at looking for gaps in the Standard Model of high-energy physics that has been around since the 1970s.

“What’s both wonderful and frustrating is, in those 40 years or so, there’s been a huge amount of experimental data, and it all fits, just beautifully,” he says. “But no one in high-energy physics believes that’s the end of the story. What we want to do is understand what the current theory predicts, and determine whether there are any deviations from it that would give us some hints about an even more fundamental theory.”

Lattice QCD calculations carried out on large computational resources help shed light on the results of particle and nuclear physics experiments.

“Many of the properties of the strong interactions, the nuclear forces, can only be studied through large-scale numerical simulations. To make the results of the calculations more accurate and more realistic requires larger and larger scale simulations,” Sugar says. “We replace all of space and time with grids. By making the grid spacing smaller and smaller, we make our approximations more and more accurate. Blue Waters allows us to work on very fine grids with very realistic parameters. Blue Waters and other petascale computers enable us to do the most realistic simulations that we’ve ever attempted.”

XSEDE resources at the National Institute for Computational Science and the Texas Advanced Computing Center, as well as systems at the Argonne Leadership Computing Facility (ALCF), the National Center for Atmospheric Research, and the National Energy Research Scientific Computing Center, were used in this project to perform simulations on coarser grids, and the Blue Gene/Q at the ALCF is being used along with Blue Waters on simulations on the finest grid. “You need data from simulations with a range of grid spacings so you can extrapolate to the zero grid spacing (continuum) limit where physics lies,” Sugar says.

The work published in April in Physical Review Letters deals with how quickly two sub-atomic particles—the Pi and K mesons—decay into an electron and a neutrino. The US QCD team’s calculation is the most precise ratio of those two decay constants that has been obtained in a numerical simulation.

“That in turn allowed us to make a very precise test of the Standard Model,” Sugar says. “It would have been more exciting, of course, if the standard model was wrong, but it was right on! It’s passed every test so far. We hope eventually it will break down!”

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