For physics between QM and String Theory I've heard a lot of different names. Quantum Electrodynamics seems to be the physics of the electron and the photon. Quantum Chromodynamics seems to be the physics of quarks. But High Energy/Nuclear/Particle Physics, Atomic physics, QFT, I don't know what the difference between them is. This forum has one category for HE/N/P physics and another for atomic physics so there has to be some difference.