Identified-Hadron Spectra in π+ + Be at 60 GeV/c with Channel-Wise Subcollision Acceptance in PYTHIA 8 Angantyr

Identified-hadron production (p, π±, K±) in π++Be at plab=60GeV/c (s≃10.6GeV) is investigated using Pythia 8.315 (Monash tune) with the Angantyr extension. Differential multiplicities d2n/(dpdθ) are confronted with NA61/SHINE measurements across standard θ bins. Within the fluctuating-radii Double-Strikman (DS) scheme, two unsuppressed opacity mappings are compared to quantify systematics. In addition, a minimal extension is introduced: a flat, post-classification, channel-wise acceptance applied after ND/SD/DD/EL tagging. It acts on primary and secondary πN pairs, keeps hadronization fixed (Lund string), and leaves the internal event generation of each admitted subcollision unchanged. Opacity-mapping variations alone induce only percent-level differences and do not resolve the soft/forward tensions. By contrast, the flat acceptance—interpretable as a reduced effective ND weight—improves agreement across species and angles. It hardens the forward π+ spectra and lowers large-θ yields, produces milder charge-asymmetric changes for π− consistent with the weaker leading feed, suppresses proton yields at all angles (with a residual ∼30% forward high-p deficit), and improves K±, with a stronger effect for K+ than K−. These results show that a geometry-blind reweighting of the subcollision mixture suffices to capture the main NA61/SHINE trends for π++Be at SPS energies without modifying hadronization. The approach provides a controlled baseline for subsequent, channel-balanced refinements and broader π+A tuning.