Disentangling centrality bias and final state effects in d+Au collisions

by Niveditha Ramasubramanian (IRFU, CEA, Saclay)

Tuesday 21 Nov 2023, 11:00 → 13:00 US/Eastern

Abstract: PHENIX has conducted a comprehensive study that simultaneously measures the production of direct y and TT0 particles in d+Au collisions at a ?sNN = 200 GeV. This measurement covers a transverse momentum (pT) range spanning from 7.5 to 18 GeV/c. The selection of different event samples is based on event activity, specifically the detection of charged-particle multiplicity at forward rapidity. Direct photon yields have been employed to empirically gauge the contribution of hard-scattering processes within these distinct event samples. Using this empirical estimate, the average nuclear-modification factor, denoted as RydirdAu,EXP, is found to be 0.925 with a statistical uncertainty of ±0.023 and a scale uncertainty of ±0.15. Notably, this value is consistent with unity for minimum-bias (MB) d+Au events. For event classes characterized by moderate event activity, RydirdAu,EXP closely aligns with the MB value within an uncertainty of 5%. These outcomes validate that the previously observed enhancement in high-pT TT0 production, which was noted in small-system collisions with low event activity, is a consequence of a bias in interpreting event activity within the Glauber framework. In stark contrast, for the top 5% of events featuring the highest event activity, RydirdAu,EXP exhibits a significant suppression of 20% relative to the MB value, with a statistical significance of 4.5cr. This suppression may be attributed to final-state effects.