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

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.

Figures (7)

• Figure 1: Representative minimum-bias observables on the final fit basis: low-energy ALICE charged-particle pseudorapidity distributions at 0.9 and 2.36 TeV, the CMS 13 TeV charged-density observable, and the ATLAS 13 TeV minimum-bias track multiplicity distribution. Ajaz improves the high-energy minimum-bias sector clearly, while the low-energy ALICE tension remains substantial for both tunes.
• Figure 2: Representative identified-hadron observables on the final fit basis: ALICE 7 TeV pion and kaon spectra, the ALICE 13 TeV pion spectrum, and the CMS 13 TeV $(p+\bar{p})/(\pi^+ + \pi^-)$ ratio. Ajaz improves the ALICE hadron sectors clearly, while Monash remains stronger in at least part of the heavier-species composition structure of the CMS 13 TeV block.
• Figure 3: Representative UE and event-shape observables on the final fit basis: a CMS 7 TeV transverse charged-density distribution, an ATLAS 7 TeV transverse-thrust distribution, an ATLAS 13 TeV trans-max mean-$p_{\mathrm{T}}$ observable, and a CMS 7 TeV transverse scalar-$p_{\mathrm{T}}$ density. Ajaz improves the 13 TeV ATLAS UE sector, while Monash remains better in the CMS 7 TeV UE block and the ATLAS 7 TeV event-shape block.
• Figure 4: Representative low-energy charged-particle distributions from the ALICE charged-particle block at $\sqrt{s}=0.9$ and $2.36$ TeV. These panels illustrate the sector that continues to dominate the total score for both tunes.
• Figure 5: Representative identified-hadron comparisons at 7 and 13 TeV. The grouped panels show the 7 TeV ALICE hadron-spectrum sector together with 13 TeV CMS and ALICE hadron-sector distributions, illustrating the broader flavor-sector improvement of the Ajaz tune at high energy.