"The researchers explain that they chose a particular plasma simulation, “particle in cell” because it's a well-studied problem on many-core architectures."
No they don't. Particle-in-cell isn't a problem at all, let alone a well studied one, it's a category of solver type. Just look at the very first line of the abstract:
"particle-in-cell laser-plasma simulation is an important area of computational physics"
There's a huge difference between a specific well-studied problem and a broad technique that is used to investigate both well-studied and never-before-studied problems.