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arXiv · 2607.27572

Photon-calibrated event-activity bias and subcollision geometry in d+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV with PYTHIA 8 Angantyr

Muhammad Ajaz·Haifa I. Alrebdi·Muhammad Waqas
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Abstract

Event-activity selections in small nuclear collision systems couple collision geometry to the soft response accompanying a hard scattering. We examine this coupling in d+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV with PYTHIA 8.316 Angantyr, using direct photons, terminal pre-decay neutral pions, and anti-kT jets. A hard-bias factor compares the probability for a hard event to enter an activity class with an $N^{ND}_{coll}$ -weighted minimum-bias reference. The model predicts a depleted most-active class and an enhanced peripheral class for both photons and pions. Photon-tagged events also sample a smaller mean impact parameter and more nondiffractive subcollisions than pion-tagged or inclusive HardQCD events. Comparing fully correlated Angantyr events with an Ncoll-reweighted minimum-bias reference and a factorized diagnostic separates the explicit geometric contribution from residual hard-soft correlations. STAR-like and PHENIX-like particle-level activity proxies preserve the class ordering but give different class probabilities. The calculation is generator-level and does not identify a unique microscopic origin for the residual correlations.

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Muhammad Ajaz, Haifa I. Alrebdi, Muhammad Waqas. 2026-07-30. Photon-calibrated event-activity bias and subcollision geometry in d+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV with PYTHIA 8 Angantyr. https://arxiv.org/abs/2607.27572

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