UC Riverside

Anisotropic flow in heavy-ion collisions describes the collective motion of participants and products of the collisions, most commonly in the plane transverse to the axis of the colliding beams. Mathematically, flow is decomposed into a Fourier series of simple shapes which contribute to the overall motion, with the coefficients ($v_n$) describing the magnitude each shape contributes on average. The third harmonic in the series is known as triangular flow ($v_3$) and has previously been shown to develop due to event-by-event fluctuations that randomly produce triangular shapes in the initial collision geometry. This dissertation describes the first study of $v_3$ using the STAR detector at the five lowest center of mass collision energies of gold nuclei in the second stage Beam Energy Scan program at the Relativistic Heavy Ion Collider. A form of $v_3$ which does not develop from fluctuations ($v_3\{\Psi_1\}$) was found and measured at the lowest energy of $\sqrt{s_{NN}}=3.0$ GeV. The source of this $v_3\{\Psi_1\}$ was investigated for the first time using simulated collisions and was found to arise due to two crucial components: geometry and stopping of nucleons in the collision producing initial triangular shapes, and a potential within the equation of state of the medium produced in the collisions. The strength of $v_3\{\Psi_1\}$ was found to decrease with increasing energy, becoming consistent with zero in the region of 3.9 -- 4.5 GeV. A comparison to simulation currently suggests that the initial triangular shape does not vanish and cannot solely explain the disappearance of $v_3\{\Psi_1\}$. This work provides a multitude of new measurements to improve both heavy-ion simulations and our understanding of the equation of state for dense nuclear matter.