Speaker
Description
High pT direct photons are electrically and color neutral, produced in
initial hard scattering. By measuring the nuclear modification factor
R_AA in large ion collisions, the neutral property has been confirmed,
as the R_AA of direct photons remains unity, while, in contrast, jet quenching and energy
loss of parent partons are observed for final state hard hadrons. The size
of the medium, characterized by centrality, relates to the energy loss.
However, the size of the medium in small systems, such as p+Au and d+Au
collisions, might be too small to impose large energy loss on partons.
Nonetheless, the nuclear modification of final state hadrons shows
suppression in central, and, surprisingly, enhancement in peripheral
collisions. One possible reason of this surprising result might be that
the centrality determination is possibly biased in small-on-large
collisions. Therefore, the colorlessness, i.e. lack of interaction of high
pT direct photons with the medium can be used to study
the appropriateness of the centrality determination, as its
nuclear modification factor is independent of centrality and pT.
In this talk, we will present the status of the analysis of high pT direct
photons in p+Au and d+Au collisions, and discuss its implication with
respect to the centrality-dependent hadron spectra.
