Stephen Horvat

Confinement is a phenomenon where quarks and gluons are only found in bound color-neutral states, or hadrons. Experiments at the Brookhaven National Laboratory (BNL) and the European Organization for Nuclear Research (CERN) have measured and published key signatures for the formation of a state of nuclear matter where quarks are temporarily de-confined in the hot, dense aftermath of heavy-ion nuclear collisions at \sqrts\ = 200,GeV. This de-confined state corresponds to the theoretically predicted quark gluon plasma (QGP). One reported QGP signature was the suppression of high momentum particles using nuclear modification factors. STAR is now analyzing data produced in the RHIC Beam Energy Scan (BES) which spans \sqrts\ = 7.7 - 62.4,GeV.

This dissertation reports the collision energy dependence of nuclear modification factors. The high-transverse-momentum (high-\pt) suppression reported at higher \sqrts\ is seen to turn off and be replaced by an enhancement of high-\pt\ particle production as the collision energy is reduced. The physics that leads to this strong enhancement competes against and obscures the physics leading toward suppression. Only when the suppression is stronger than the enhancement do the nuclear modification factors provide a clean signature of QGP formation. This dissertation also outlines a new procedure to better disentangle suppression effects from enhancement effects. In this procedure, the centrality dependence of enhancement and suppression effects are exploited with the goal of establishing a limit for the minimum collision energy needed to produce a QGP in central heavy-ion collisions.