Centrality Dependence of γdir and π0 Production in d plus Au Collisions

Collision centrality is usually characterized by the event activity in heavy-ion collisions, based on the fact that the bulk productions are always soft even in the presence of a hard collision, assuring the proportionality between centrality (impact parameter) and soft particle production. In large-on-large ion collisions, studying the nuclear modification factor, it is observed that jets and final state hadrons show energy loss and the energy loss goes higher as the events go central, explained by the fact that in central collisions the bigger-sized media would absorb more energy. High pT direct photons produced in initial hard scattering, due to the property of colorlessness, are unaffected by the colored media. Using centrality based on the Glauber model, in small systems, e.g., p + Au or d + Au collisions, where only small QGP droplets are expected, surprisingly, high-transverse-momentum neutral hadrons are seen to be suppressed in central events, while enhanced in peripheral events. In this talk, we will show that correlating high pT direct photon gamma(dir) and pi(0) production in d + Au collisions proves that biases in centrality determination inherent in using standard Glauber model could be found and corrected by defining binary collisions based on color-free photon production N-coll(EXP) = gamma(dir)xAu|gamma(dir)(pp) .