Eur. Phys. J. Plus (2026) 141: 205
https://doi.org/10.1140/epjp/s13360-026-07443-9

Regular Article

Modelling identified hadrons in $\mathit{pp}$ at SPS energies with Pythia 8 Angantyr: subcollision acceptance and string-end topology

Physics Department, Faculty of Science, Umm Al-Qura University, PO Box 715, 21421, Makkah, Saudi Arabia

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Received: 20 October 2025
Accepted: 16 February 2026
Published online: 25 February 2026

Abstract

We study identified hadron production (p, ${\pi }^{\pm }$, $K^{\pm }$) in inelastic proton–proton collisions at $p_{\text{lab}}=20$, 31, 40, 80, and $158\text{GeV}/c$ using Pythia 8.315 with Angantyr, confronted with NA61/SHINE rapidity spectra. Four ingredients are varied in a controlled way: (i) the accepted mix of NN subcollisions via two post-classification settings, DS4-lite$({\epsilon }_{\text{ND}},{\epsilon }_{\text{SD}},{\epsilon }_{\text{DD}},{\epsilon }_{\text{EL}})=(0.6,0.2,0.2,0.2)$ and DS4-strong (0.6, 0.4, 0.4, 0.4); (ii) the string-end hadron formation pattern (diquark vs. popcorn, allowing one intermediate meson); (iii) the level of soft activity through a gentle low-energy MPI retune with elastic-like remnants filtered; and (iv) hadronic rescattering (HR).

At 20–$40\text{GeV}/c$, DS4-lite suppresses the midrapidity proton excess and restores the observed “U” shape, with HR providing a modest additional deepening; DS4-strong improves the central ${\pi }^{\pm }$ and $K^{\pm }$ normalizations. At 80 and $158\text{GeV}/c$, the popcorn option smooths unphysical forward spikes for protons, enhances forward ${\pi }^{\pm }$ and $K^{+}$, and slightly moderates $K^{-}$, while the DS4 strength chiefly controls midrapidity levels. A persistent feature remains at intermediate energies: the large-y$K^{+}$ tail ($y\gtrsim 1.5$) is underestimated at 31 and $40\text{GeV}/c$, pointing to missing forward-baryon/associated-strangeness dynamics rather than to overall soft activity or rescattering effects.

With these minimal, transparent adjustments—DS acceptance plus a controlled string-end topology—the simulations closely follow NA61/SHINE across species and energies in the SPS window.

This study paves the way to a consistent, data-anchored pp baseline for Angantyr extrapolations to p A and A+A, while quantitatively constraining the midrapidity balance of soft activity and baryon stopping across SPS energies.