This work unexpectedly led to the discovery of lots of new types of particles. It was not until high-energy accelerators such as SLAC probed deep within the neutron and proton that these many new particles were understood and physicists began to have a deep understanding of nuclear forces. There are new particle discoveries still being made today and unanswered questions about them that limit our ability to fully explain all the properties of matter. The SLAC accelerator can be seen as a gigantic electron microscope. It allows us to resolve structures on very small scales and also to set up conditions where new types of particles can be produced and their properties studied.
Higher energy allows resolution of smaller structures.
Hence to resolve small structures we need waves with smaller wavelength. No ordinary microscope can resolve objects that are small compared to the wavelength of visible light. Quantum mechanics tells us that particles also possess wave-like properties and that the wavelength shrinks as the energy grows. Higher energy allows higher mass particles to be produced. Consider the situation where an electron and a positron, each with energy E, travelling in opposite directions meet and annihilate.