What is the name of the particle that allows Ant-Man to change size?

The particles that allow Ant-Man to change size are called Pym Particles. They were discovered and developed by scientist Hank Pym, the original Ant-Man, and later used by Scott Lang in the Marvel Cinematic Universe.

In the MCU, Pym Particles alter the distance between atoms, allowing a person or object to shrink dramatically or grow to enormous proportions. Ant-Man's suit stores the particles in specialized cartridges, and his helmet and controls help him regulate the transformation. The technology also explains how he can retain considerable strength while tiny and become physically powerful when enlarged.

Pym Particles are not the same as Quantum Particles or the Quantum Realm. The Quantum Realm is a separate subatomic dimension that characters can enter, while Pym Particles are the technology that helps them change size and, in some stories, access that realm. Hank Pym first introduces the science to Scott Lang in Ant-Man, released in 2015.