Niveditha Ramasubramanian

Disentangling centrality and final state effects due to Quark Gluon Plasma in small system collisions.

Abstract:

Although the traditional nuclear modification factor (RAB) of Pi0s is the most reliable signal to study the formation of Quark Gluon Plasma(QGP), it has some limitations. This RAB of pi0 is a convolution of various effects, such as centrality bias, initial state effects, and final state effects. As a result, it is challenging to factorize these and study the impact of the final state effect (QGP formation) independently.

In order to eliminate potential biases arising from the determination of event activity class, PHENIX conducted measurements of high pT Pi0 and direct photon yields across various event activity bins. The ratio of direct photons to Pi0 for different centrality classes demonstrated consistency with minimum bias, as well as with ratios observed in p+p collisions (scaled appropriately), except for the most central events.

For a more in-depth examination of final-state effects, a double ratio was introduced, normalizing the Pi0 nuclear modification factor with that of direct photons. This approach effectively canceled out initial state effects, which are expected to be similar for Pi0 and direct photon production and biasses from event class determination. Consequently, it offered a clean signal to scrutinize final-state effects. The results unveiled a significant 20% suppression with a confidence level of 4.5 sigma for the most central events when compared to the minimum bias scenario.