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.

• The paper develops the Ajaz tune, a sequential nine-parameter PYTHIA 8.316 retune that directly evaluates generator predictions while expanding parameters and observables only when residual tensions require it.
• The retune improves the grouped fit score density from 99.18 for Monash 2013 to 85.67, with especially large gains in CMS 13 TeV charged density and ATLAS 13 TeV underlying-event data.
• The paper shows improved held-out validation, 63.15 versus 68.94 for Monash, while identifying unresolved low-energy minimum-bias tensions and worse performance in some 7 TeV event-shape and underlying-event sectors.

Overview and motivation

This paper presents the "Ajaz tune," a sequential nine-parameter retune of PYTHIA 8.316 built around the Monash 2013 baseline, targeting soft-QCD observables in pp collisions at $\sqrt{s}=0.9$, 2.36, 7, and 13 TeV (2603.21364). The central methodological commitment is that the tune is constructed by direct generator evaluation at explicit parameter points—no interpolation surrogate is used—and that both the active parameter set and the observable basis are enlarged only when the residual score structure shows the current subspace to be too restrictive. The authors position this workflow as complementary to established strategies based on expert tune families, Professor-style interpolation, Bayesian optimization, and high-dimensional or color-reconnection-focused studies (2603.21364).

The motivation rests on the observation that Monash 2013 was never intended to close the tuning problem: its own documentation flagged tensions in strange-particle production, baryon-sensitive observables, and low-$p_T$ structures, while the LHC soft-QCD dataset has since expanded substantially across minimum-bias, identified-hadron, underlying-event (UE), and event-shape measurements (2603.21364).

Fit basis and held-out validation design

The fit basis spans nine analysis blocks across four energies: the ALICE charged-particle block (0.9–7 TeV), ALICE 7 TeV identified hadrons, CMS 7 TeV UE, ATLAS 7 TeV event shapes, CMS 13 TeV charged density, CMS 13 TeV identified hadrons, ALICE 13 TeV light flavor, ATLAS 13 TeV MB tracks, and ATLAS 13 TeV UE. A separate held-out validation basis—two CMS low-energy charged-particle blocks, the ATLAS early UE block at 0.9/7 TeV, and two 13 TeV charged-particle blocks—is excluded from the fit objective throughout, so the final point can be tested on measurements that did not select it. The score is a grouped $\chi^2$-type quantity summed per analysis block with unit weights; the authors are explicit that bin-to-bin experimental correlations are not included, so the score is a controlled ranking metric rather than a formal covariance-based reduced $\chi^2$ (2603.21364). Event-class consistency (inelastic vs NSD selections, leading-object UE definitions) is preserved by grouping observables analysis-by-analysis rather than pooling all bins.

Sequential construction

The construction proceeds through a documented sequence of parameter promotions:

1. Five-parameter localization: {StringZ:aLund, StringZ:bLund, ColourReconnection:range, MultipartonInteractions:pT0Ref, StringFlav:probStoUD}, scanned over 21 points ($3.0\times10^6$ events per point) and refined locally.
2. Mixed-family multi-energy extension: the five-parameter basin failed to transport to the multi-energy objective—the dominant deterioration came from the low-energy charged-particle sector—which motivated promoting MultipartonInteractions:ecmPow. The grouped score dropped sharply and stabilized near $$S_{\mathrm{grouped}}/N_{\mathrm{bin}} \approx 273.69$$.
3. StringPT:sigma activation: a real but modest gain ($273.69 \to 273.41$), showing the main stabilization was already achieved at six parameters.
4. MultipartonInteractions:expPow opening, addressing persistent minimum-bias/UE tension via the MPI overlap profile.
5. StringFlav:probQQtoQ promotion alongside broadening of the 13 TeV flavor basis, adding explicit baryon sensitivity.

The final basis then absorbed the CMS 7 TeV UE, ATLAS 7 TeV event-shape, ATLAS 13 TeV MB track, and ATLAS 13 TeV UE blocks. The report-level benchmark froze both tunes and reran them with $4.8\times10^6$ events each on identical generator version, analysis chain, statistics policy, and score definition (2603.21364).

Final benchmark results

The headline fit-basis result is a grouped score density improvement from $S_{\mathrm{grouped}}/N_{\mathrm{bin}} = 99.18$ for Monash to $85.67$ for Ajaz—a separation of 13.51 units distributed over several physically distinct sectors rather than concentrated in one histogram. The blockwise pattern is summarized below.

Analysis	Monash	Ajaz	Better
ALICE charged-particle	699.426	696.777	Ajaz
ALICE 7 TeV identified hadrons	14.102	8.558	Ajaz
CMS 7 TeV UE	86.400	90.788	Monash
ATLAS 7 TeV event shapes	18.519	57.250	Monash
CMS 13 TeV charged density	26.410	3.423	Ajaz
CMS 13 TeV identified hadrons	7.186	5.943	Ajaz
ALICE 13 TeV light flavor	33.479	25.916	Ajaz
ATLAS 13 TeV MB tracks	26.814	15.067	Ajaz
ATLAS 13 TeV UE	99.675	35.664	Ajaz

The largest single gain is the CMS 13 TeV charged-density block (26.410 → 3.423), which the authors attribute to the retuned MPI sector transferring more successfully to the highest fitted energy. The ATLAS 13 TeV UE improvement (99.675 → 35.664) is similarly large. On the held-out validation basis, Ajaz also improves over Monash ($p_T$0 vs $p_T$1), driven by the ATLAS early UE block and both held-out 13 TeV blocks—notably the ATLAS 13 TeV charged-track validation block, where the grouped score falls from 54,190 to 17,529. This demonstrates that the high-energy gains extend beyond the exact histograms used in optimization (2603.21364).

Residual tensions and physical reading of the shifts

Monash remains better in two fitted sectors: the CMS 7 TeV UE block and, more strongly, the ATLAS 7 TeV event-shape block (18.519 vs 57.250), indicating the Ajaz point does not capture every aspect of 7 TeV global event geometry. The dominant unresolved issue, however, is structural: the low-energy ALICE charged-particle block still dominates the total score numerically for both tunes, with an essentially negligible Ajaz gain there (699.426 → 696.777). The same pattern appears in validation, where Monash remains slightly better in the two CMS low-energy blocks. The authors conclude this is a deeper low-energy minimum-bias problem that neither tune resolves within the present parameter space—an honest limitation stated where it bears on the result.

Physically, the final point moves moderately in StringZ:aLund but more substantially in StringZ:bLund (0.98 → 1.245); it increases ColourReconnection:range strongly (1.80 → 3.177), raises pT0Ref slightly with a harder ecmPow (0.215 → 0.239), prefers a broader overlap profile (expPow 1.85 → 2.017), and reduces StringPT:sigma below its default. The probQQtoQ shift is small, indicating no dramatic baryon-pair displacement is needed. The authors read these as correlated movements across fragmentation, flavor, color topology, and MPI activity rather than isolated adjustments (2603.21364).

Limitations and open questions

The paper concedes several boundaries explicitly. No universality claim is made beyond the tested basis. The score omits bin-to-bin experimental correlations, so quoted significances should not be read as formal reduced-$p_T$2 values. A full tune-uncertainty envelope, alternative weighting schemes, and covariance-aware scoring were outside scope. The low-energy charged-particle sector remains unresolved for both tunes, and the 7 TeV topology-sensitive sectors favor Monash. Open questions left by the work include whether a different MPI overlap or color-reconnection model could reconcile the 7 TeV event-shape deficit without sacrificing the 13 TeV gains, and what additional model freedom would be required to address the persistent low-energy minimum-bias tension.

Conclusion

The paper delivers a controlled, sequentially constructed nine-parameter PYTHIA 8.316 retune that improves the Monash 2013 baseline on a broad common soft-QCD basis (grouped score density 99.18 → 85.67) and generalizes to a held-out validation basis (68.94 → 63.15), with the strongest gains in 13 TeV minimum-bias, UE, and identified-hadron sectors. Its principal contribution is methodological as much as numerical: each parameter promotion is tied to a concrete sector-level tension, keeping the final point interpretable while leaving the remaining limitations explicit.