A Matter of Scale: Small, Smaller, Smallest?

François Corriveau

Ever since Rutherford, scattering has been a widely used and successful particle physics method to investigate the structure of matter. The particle accelerator HERA, located in Hamburg, Germany, can probe the proton structure down to 0.000000000000000001 meter, one thousandth of its size. Collisions of point-like electrons or positrons on protons yield detailed information on the composition of the proton in terms of quarks and gluons. They also test the interaction processes and challenge our understanding of them on a wide kinematical scale. An historical review of critical experimental phenomena will be presented. Specific examples will illustrate how scattered quarks and gluons may be observed as jets of particles at HERA. The next question, “Does the quark itself have a sub-structure?”, will be addressed.