The physics of hadron-hadron collisions is very complex involving both perturbative and non perturbative QCD. It is imperative to study p - p collisions in detail to provide a variety of measurements against which the theoretical calculations can be tested. Direct jet measurements, for instance, help address fundamental questions of the fragmentation process. They also form a critical baseline for comparisons of results from heavy-ion studies, where fragmentation functions are expected to be modified due to interactions with the hot and dense medium. Finally, it is important to understand how the beam-beam remnants, multi-parton interactions, and initial- and final-state radiation combine to produce the particles observed in the underlying event. I present results from p - p collisions at 200 GeV collisions as measured by the STAR experiment.
Evolution of jet shapes and fragmentation functions in Au+Au collisions at $\sqrt{s_\mathrm{NN}}=200$ GeV with the STAR experiment at RHIC