Characterization of gold+gold collisions at center of mass energy = 200 GeV from STAR: From meson production in ultra-peripheral collisions to high-pT azimuthal correlations in central collisions

Physicists at the Relativistic Heavy Ion Collider have concluded that in high-energy heavy-ion collisions a strongly coupled medium consisting of deconfined quarks and gluons has been observed. In these collisions, the incoming partons undergo hard scatterings losing energy as they traverse the medium created and fragmenting into back-to-back hadron jets. A main analysis technique used to probe this dijet production has been di-hadron correlations. Measurements have indicated that there is a strong suppression of the away-side high- pT particle yield providing direct evidence that high- pT partons lose energy as they traverse the strongly interacting medium. However, since the momentum of the trigger particle is not necessarily a good measure of the jet energy, di-hadron correlations have limited sensitivity to the shape of the fragmentation function. A suppression is seen, but the details are unclear. In this thesis, the possibility to better constrain the initial parton energy by using clusters of multiple high-pT hadrons in a narrow cone as the 'trigger particle' in the azimuthal correlation analysis is presented. Results from this analysis of multi-hadron triggered correlated yields in Au+Au collisions at sNN = 200 GeV from STAR are presented. The yields presented are comparable to di-hadron correlation yields possibly indicating that both methods select a similar underlying jet energy. Finally, comparisons are made to analogous measurements in PYTHIA p+p simulations with the same trend observed.;Another measurement presented in this thesis is the quantum mechanical interference effect observed in ultra-peripheral collisions (UPCs) where the electromagenetic interactions between ions dominate as opposed to the strong interactions probed in the aforementioned hard scattering processes. This thesis presents a measurement of destructive interference in the cross section of photoproduced rho0 mesons in Au+Au UPC collisions at sNN = 200 GeV from STAR. In an ultra-peripheral collision a rho 0 meson is produced when a photon from one nucleus fluctuates to a qq¯ pair and scatters off the second nucleus. Since the photon is as likely to come from one nucleus as the other, the two processes interfere quantum mechanically, and destructive interference in the cross section is observed for low pT rho0 mesons.