ABSTRACT
For the past 100 years, the most progress at understanding the nature of the subatomic world and to some extent how it is connected to the cosmos has been gained by experiments which scatter one type of particle off of some type of matter, be it molecule, atom, or other particle. The experiment suggested by Ernest Rutherford in 1909 aimed alpha particles decaying from unstable radon toward a gold foil. His finding that some projectiles unexpectedly scattered into large angles gave the first indication that the atom was not a ball of nearly uniform density but rather included a hard core, what we now call the nucleus. To probe deeper inside the nucleus and then deeper inside the constituents of the nucleus, neutrons and protons, better beams of projectiles were needed with better collimation, more intensity, and most of all higher energy. Given the dual particle–wave nature of objects at the quantum scale, higher energy projectiles yield shorter wavelengths, which allow probing smaller dimensions. This is analogous to moving from visible light to ultraviolet to x-ray wavelength microscopes to “see” smaller and smaller objects. The need for these higher energy beams was satisfied with the invention of particle accelerators, cyclotrons, synchrotrons, and linear accelerators. The projectiles of choice were electrons and protons, primarily due to the ease of obtaining such bare particles. These scattering experiments through the 1970s were of a fixed target type, just like Rutherford’s. The projectile beam was aimed at a target, which was at a fixed position in the laboratory. The objective of the experiment was to detect the scattered projectile and possibly any new particles that were created in the scattering event. Due to the famous Einstein relationship E = mc2, inelastic scattering events have the potential to convert some of the energy of the incoming projectile into new matter, that is, particles. In this way many new particles were discovered, possibly adding confusion at first but leading the way to a deeper understanding of this part of nature.
