Title : Validation of Glauber model in centrality determination for small systems.

The initial motivation to study d+Au collision was to decouple the effects of cold nuclear matter effects in nuclear modification factor (RAA) in heavy ion collision and use this as a control experiment.
From the year 2013, several experimental results indicate the formation of Quark Gluon Plasma (QGP) in small systems. Suppression in the nuclear modification factor RAA of Pi0 and jets is observed in the central d+Au collisions, which could be attributed
to formation of QGP droplets but, along with this, the results also
indicate a counter-intuitive enhancement of RAA in peripheral events.

These results can be further validated (or invalidated) by studying direct photons in different centrality bins. Since the QGP, even if formed, is transparent to high pT direct photons from initial hard scatterings, the high-pT photon RAA should be unity and independent of centrality.
Furthermore, the ratio of direct photon yields to pi0 yields should exhibit the same centrality dependence as the pi0 RAA. Deviation from these expectations is a strong indication that the definition of centrality using Glauber model, which was successful in large systems, may not be valid
for small systems, like d+Au or p+Au.

In this talk, I will highlight preliminary results from d+Au collisions
and the status of analysis in p+Au system.