Speaker
Description
The primary source of high transverse momentum (>4-5GeV/c) direct photons is the initial hard scattering of partons, specifically the quark-gluon Compton scattering process. This has been validated not only in hadron-hadron, but also in heavy ion collisions (see for instance PRL 109 (2012) 152302), where the observed yields were proportional, as expected, to the number of binary nucleon-nucleon collisions as calculated from the Glauber model for various collision centralities. This observation supports that the collision geometry is well defined by the centrality measure we have been using, at least in the case when the colliding ions are large. The definition, however, is not obvious in very asymmetric collisions,
like p+Au. We assume that high transverse momentum direct photons are a "standard candle" for initial hard scattering, and, per extension, for collision geometry not only for p+p and A+A, but also for small-on-large collisions. If true, comparing the centrality dependence of direct photon and hadron production in p+Au is a robust test of the applicability of the Glauber model in such systems. In this poster we will report on the status of the analysis of high transverse momentum direct photons in p+Au collisions at various centralities as well as of the direct photon / hadron ratios and its comparison with hadron production.
