Sequential retuning of PYTHIA~8.316 to a global soft-QCD basis in pp collisions at $\sqrts=0.9-13$ TeV

TL;DR

This paper presents a comprehensive nine-parameter retune of PYTHIA 8.316 for soft-QCD observables in proton-proton collisions across 0.9 to 13 TeV, improving agreement with experimental data through a staged, multi-energy fitting process.

Contribution

The paper introduces a novel sequential retuning methodology for PYTHIA 8.316, enhancing its accuracy for soft-QCD observables across multiple energies and datasets.

Findings

Significant improvement in fit quality for 13 TeV minimum-bias and underlying-event data.

Better agreement with identified-hadron spectra and ratios in several sectors.

Remaining tensions in low-energy charged-particle data and some 7 TeV observables.

Abstract

We present a sequential nine-parameter retune of PYTHIA 8.316, built on the Monash 2013 baseline, for soft-QCD observables in proton--proton collisions at $\sqrt{s}=0.9$--$13$ TeV. The tune is constructed with direct generator evaluation and developed in stages: an initial five-parameter localization in the hadronization-sensitive sector, successive extensions driven by multi-energy minimum-bias tensions, and a final expansion to a broader soft-QCD fit basis. The fitted set includes minimum-bias charged-particle data, identified-hadron spectra and ratios, underlying-event observables, and event-shape distributions. A separate held-out validation basis is kept outside the fit. The final tune retunes nine non-perturbative parameters controlling fragmentation, strangeness suppression, diquark production, color reconnection, MPI regularization and its energy dependence, the impact-parameter overlap profile, and transverse-momentum generation in string breaking. On the common fit basis, the grouped score density improves from $99.18$ for Monash 2013 to $85.67$. The strongest gains occur in the 13 TeV minimum-bias and underlying-event sectors, with further improvement in the ALICE identified-hadron blocks and a smaller gain in the CMS 13 TeV identified-hadron sector. Monash remains better in the CMS 7 TeV underlying-event block and the ATLAS 7 TeV event-shape block, while the low-energy charged-particle sector remains the main unresolved tension for both tunes. The held-out validation basis supports the same picture. Monash remains slightly better in the low-energy CMS validation blocks, but the present tune performs better in the ATLAS early underlying-event block and in both held-out 13 TeV charged-particle sectors. We interpret it as a controlled soft-QCD retune that improves a broad set of observables while keeping its limitations explicit.