Computational findings support advancements in large-scale Jefferson Lab experiments

Schematic diagrams showing possible quark propagation channels in pion-pion scattering relevant for the sigma meson.

Schematic diagrams showing possible quark propagation channels in pion-pion scattering relevant for the sigma meson.

Often, researchers aim for a distant horizon with respect to their research goals. When they get there, they plot their course for the next far-off landmark.

Throughout the 20th century, researchers tried to understand the laws that govern the atomic world. Once the exotic realm of protons, neutrons, and electrons became more familiar, researchers set their sights on understanding the even smaller, more fundamental particles that comprise these atomic building blocks.

The Thomas Jefferson National Accelerator Facility (Jefferson Lab) is a hub for the convergence of experimentalists and computational scientists working to understand the subatomic world. To that end, Jefferson Lab commissioned the GlueX experiment to help understand the principles governing the theory of quantum chromodynamics (QCD). Researchers use QCD to better understand the fundamental interactions between quarks—the building blocks of protons and neutrons—and the gluons that bind them in a way similar to how atoms are bound together into molecules by the electromagnetic force.

GlueX is designed to study “hadrons,” composite particles built out of quarks and gluons, one class of which are known as mesons, made of one quark and one antiquark. The simplest mesons are made of a quark–antiquark pair bound by the glue field. One aim of GlueX is to confirm the existence of exotic “hybrid” mesons, quark–antiquark pairs, bound by gluons in an excited state.

When produced, exotic mesons are in existence for only a tiny fraction of a second before they decay. And because of mesons’ short-lived, ultra-small nature, researchers use supercomputer simulations in tandem with experiment to gain a deeper understanding of quark, gluon, and meson behavior. This two-pronged approach recently helped a team confirm the existence of the sigma meson—a mystery in QCD for over 50 years and an essential step toward observing the heavier, exotic particles that the team is seeking.

The team of computational researchers, led by Jefferson Lab’s Robert Edwards, has been using the Cray XK7 Titan supercomputer at the Oak Ridge Leadership Computing Facility (OLCF)—a US Department of Energy (DOE) Office of Science User Facility located at DOE’s Oak Ridge National Laboratory.

Super scattering

In their search for exotic mesons, the Edwards team uses computation to study how particles scatter when they collide.

The behavior of the particles as they scatter is described by a long sequence of linear equations—researchers are actually capturing a particle’s “resonance,” or an energetic “ringing” in the sense of a bell, that lets researchers know the particle was there. Even simulating a modest number of particles requires serious computing power, but the Edwards team simulations are anything but modest.

“We have to do around 400,000 linear equation solves per configuration, and we use about 500 configurations in our simulations,” Edwards said. “This is reliant upon the GPUs on Titan to follow through with these large-scale calculations.”

Even the world’s most powerful supercomputers are unable to simulate the full range of heavy particles the team is trying to capture and study. The team has already invented new computational methods to improve performance of simualtions, and as supercomputers get faster, researchers are able to study heavier, more complex subatomic particles.

Recently researchers focused on confirming the existence of the sigma meson as part of the team’s larger objectives.

Project collaborator Jozef Dudek, who has joint appointments as a staff scientist at Jefferson Lab and an assistant professor of physics at the College of William & Mary, discussed how characterizing the sigma meson was a major step forward toward the team’s research goals.

“The particles we’re interested in are a lot heavier than the sigma,” he said. “These are hypothetical particles, called hybrid mesons, that our earlier calculations indicate ought to exist. The idea of these hybrid mesons is that they don’t just have quarks and antiquarks, but also gluons doing something. These much heavier hybrid mesons decay very quickly into lighter mesons, including things like sigma mesons. If you’re going to understand how to observe these hybrid mesons in experiments via what they decay into, you have to understand the sigma first.”

The numerical results of solving the linear equations can be expressed as diagrams, which can be used again when studying heavier particles. “To get a handle on how these processes are treated in lattice QCD, you want to create these diagrams at the lowest energies before you see them again higher up,” Dudek said.

The team must account for the various particle pairings that can form during a scattering experiment. “You have a hybrid meson that decays into one pair, which could include a sigma meson, but it is also possible that it goes into another pair,” Edwards said. “Knowing the relative probability of these things is something that we can use to inform the experimentalists, to help them know where to look for the exotic mesons.”

In addition to its work on the sigma meson, the Jefferson Lab team performed the first calculation of the radiative decay of an unstable particle—a major and necessary step toward computing the rate at which exotic mesons will be produced in the GlueX experiment.

Through their extensive collaborations with NVIDIA and Intel, the researchers are prepared to use next-generation computers to continue down the path toward heavier, more exotic particles. In addition, Edwards serves as a coprincipal investigator on an Exascale Computing Project award that seeks to improve and expand QCD calculations for exascale machines—supercomputers that are 1,000 times more powerful than current-generation petascale machines (exascale computers will be able to complete a quintillion calculations per second, as opposed to current-generation machines’ quadrillion calculations per second).

Dudek indicated that neither computing nor experiment alone will achieve the team’s end goals, but the combination of the two is a powerful driver toward confirming the existence of exotic mesons and better understanding of the standard model of particle physics. “One of the successes of this work is the combination of physics progress and computing happening together,” he said. —Eric Gedenk

Related Publication: Raul A. Briceño, Jozef J. Dudek, Robert G. Edwards, and David J. Wilson, “Isoscalar ππ Scattering and the σ Meson Resonance from QCD.”  Physical Review Letters 118 (January 9, 2017): 022002, doi:10.1103/PhysRevLett.118.022002.

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