Measurements of Upsilon Production in p+p, p+Au and Au+Au Collisions at 200 GeV with the STAR Experiment

Quark-gluon plasma (QGP) is composed of deconfined quarks and gluons, a new state of matter, which is predicted to have existed a few millionth of a second after the Big Bang. Under normal conditions, due to the so-called ``color confinement”, quarks and gluons are bound firmly as bound states such as mesons or baryons. However, at extremely high energies and densities, due to the small value of the running coupling of Quantum Chromodynamics (QCD), partons are only bound weakly so that they can move freely on their own. At the very initial times after the big bang, the temperature and energy density was extremely high, the universe was in a state of QGP where all quarks and gluons were deconfined and fly at nearly the speed of light. Today, such an extreme state of matter can be achieved experimentally at Relativistic Heavy-Ion Collider (RHIC) and Large Hadron Collider (LHC). Measurements of quarkonium production are an important tool to study the properties of the Quark-Gluon Plasma (QGP) formed in relativistic heavy-ion collisions. Quarkonium suppression due to the color-screening effect was proposed as a direct evidence of the QGP formation \cite{Matsui:1986dk}. Moreover, different quarkonium states may dissociate at different temperatures depending on their binding energies. This so-called sequential melting phenomenon can help constrain the temperature of the QGP. However, other effects, such as cold nuclear matter effects and regeneration, add additional complications to the interpretation of the observed suppression. Compared to charmonia, bottomonia is much less affected by regeneration contribution at RHIC energies, making them a cleaner probe to the QGP. In this thesis, the bottomonium (Upsilon) productions are studied in p+p, p+Au and Au+Au collisions at a center of mass energy per nucleon pair of 200 GeV with the STAR experiment. In p+p and p+Au collisions, the differential production cross section and nuclear modification factor $R_{pAu}$ as a function of transverse momentum $p_T$ or rapidity $y$, are measured for $0