UCLA

Quantum ChromoDynamics (QCD) is a corner stone of the Standard Model. In the non-perturbative regime first principle calculation is very difficult but phenomenology is rich. The many-body dynamics of quarks interacting in a deconfined state, Quark Gluon Plasma (QGP), produces emergent phenomena which are not manifest in the theory's Lagrangian. QGP can be created in relativistic heavy ion collisions which provide an opportunity to experimentally probe this exotic QCD matter. Of particular interest is the structure of the QCD phase diagram. While the transition from QGP to hadron gas is a cross-over at high collision energies, RHIC's beam energy scan program covers the high baryon density region of the phase diagram to search for signs of a first-order transition and its accompanying critical point.

Bubbling is a general characteristic of first-order phase transitions and may lead to clusters of quarks which coalesce into final state protons in heavy ion collisions. We construct a new observable, $\Delta \sigma^2$, to search for excess clustering in the azimuthal distributions of identified proton tracks. We show that the effects of detector inefficiency and elliptic flow can be corrected and that the observable behaves as expected in various models and simulations. $\Delta \sigma^2$ is measured in STAR's Beam Energy Scan I data set and a strong repulsive interaction among protons is observed. The repulsion is found to depend heavily on the event multiplicity, increasing dramatically in magnitude as the event multiplicity decreases. Energy dependence, absent in model calculations, is observed in STAR data, suggesting a possible scenario of an energy dependent attractive correlation beneath a repulsive background.