Study of baryon fluctuations in azimuthal phase space and the search for critical phenomena at STAR

Divergence of correlation length is a universal feature of critical phenomena in phase transitions. In the search for a critical point in the QCD phase diagram, such a divergence may be reflected in particle yield fluctuations in phase space. Fluctuations of baryon multiplicities in heavy-ion collisions within a limited pseudo-rapidity range have been used to search for signs of a critical point in the STAR Beam Energy Scan (BES) Phase I data. Particle yield fluctuations arising from critical phenomena would naturally exist in both longitudinal and azimuthal subvolumes of phase space. In this analysis, proton fluctuations in azimuthal subvolumes are investigated via new observables. Mixed events are utilized as a baseline which allows for the measurement of correlation length proxies that are less sensitive to common experimental complications. Measurements of transformed proton multiplicity distributions in azimuthal partitions of Au+Au collisions from the STAR BES-I program will be presented. The kurtosis of these distributions will be shown as a function of beam energy and azimuthal partition size. The observed trends are compared with those obtained from the AMPT model. Future perspective with the STAR BES-II data will also be discussed.