Extraction and validation of a new set of CMS PYTHIA8 tunes from underlying-event measurements (1903.12179v2)

Abstract: New sets of CMS underlying-event parameters ("tunes") are presented for the PYTHIA8 event generator. These tunes use the NNPDF3.1 parton distribution functions (PDFs) at leading (LO), next-to-leading (NLO), or next-to-next-to-leading (NNLO) orders in perturbative quantum chromodynamics, and the strong coupling evolution at LO or NLO. Measurements of charged-particle multiplicity and transverse momentum densities at various hadron collision energies are fit simultaneously to determine the parameters of the tunes. Comparisons of the predictions of the new tunes are provided for observables sensitive to the event shapes at LEP, global underlying event, soft multiparton interactions, and double-parton scattering contributions. In addition, comparisons are made for observables measured in various specific processes, such as multijet, Drell-Yan, and top quark-antiquark pair production including jet substructure observables. The simulation of the underlying event provided by the new tunes is interfaced to a higher-order matrix-element calculation. For the first time, predictions from PYTHIA8 obtained with tunes based on NLO or NNLO PDFs are shown to reliably describe minimum-bias and underlying-event data with a similar level of agreement to predictions from tunes using LO PDF sets.

• The paper presents the extraction and validation of CMS PYTHIA8 tunes that accurately simulate underlying events in proton-proton collisions using different NNPDF3.1 PDF orders.
• It employs a comprehensive tuning methodology based on UE observables from CDF and CMS data at 1.96, 7, and 13 TeV to ensure consistency across energy scales.
• The study highlights the impact of ISR/FSR and MPI dynamics on jet correlations, offering refined predictive tools for QCD analyses in high-energy physics.

Overview of New CMS PYTHIA Tunes for Underlying Event Simulation

The paper presents a new set of tunes for the CMS PYTHIA event generator, focusing on underlying-event simulations tuned to specific quantum chromodynamics (QCD) phenomena observed in high-energy hadron collisions. These new tunes utilize different orders of the NNPDF3.1 parton distribution functions (PDFs) — namely, Leading Order (LO), Next-to-Leading Order (NLO), and Next-to-Next-to-Leading Order (NNLO) — to more accurately simulate aspects of the underlying event (UE) in proton-proton collisions at multiple energy scales.

Key Developments

1. PDF and Strong Coupling Parameters: The new tunes adopt PDF sets that match the perturbative order of higher-order matrix element (ME) calculations, using consistent values of the strong coupling constant, $\alpha_s(M_Z)$, aligning with the PDF order. For LO tunes, $\alpha_s(M_Z)$ is 0.130, while for NLO and NNLO tunes, it is 0.118.
2. New Tunes and Validation: The tunes — labeled as CP1 to CP5 — were fitted using UE observables at various collision energies, particularly integrating CDF data at 1.96 TeV and CMS data at 7 TeV and 13 TeV. Validation spans comparing the model predictions with observables from LEP event shapes, multijet, Drell-Yan, and top quark production processes, ensuring a robust and versatile tuning.
3. Partonic Interaction Simulations: These tunes efficiently simulate multiple-parton interactions (MPI), beam-beam remnants, and color reconnections, pivotal for accurately modeling soft and semi-hard processes in underlying events.

Numerical Results and Observations

• UE Sensitivity: Predictions from tunes based on higher-order PDFs (CP3, CP4, CP5) describe minimum-bias and UE data with consistent reliability, challenging the long-held notion that higher-order PDFs might under-represent soft QCD events due to their gluon distribution characteristics.
• Consistency Across Energies: The fitting procedure provides tunes with parameters that show good agreement with data across a spectrum of collision energies (1.96, 7, and 13 TeV), especially in soft interaction-sensitive observables like charged particle multiplicity and transverse momentum distributions.
• ISR/FSR and MPI Dynamics: The paper distinguishes that the different orders of running for initial- and final-state radiation ($\alpha_s^{ISR}$ and $\alpha_s^{FSR}$) significantly impact jet correlation and multiplicity, particularly in top quark-antiquark pair production, contributing critical insights into optimal PS-ME merging strategies in MC simulations.

Practical Implications and Speculations

The introduction of these tunes serves multiple practical roles:

• Enhanced Experimental Accuracy: They refine the predictive accuracy of experimental simulations, which is crucial for LHC analyses where UE effects need separation from hard processes.
• Versatile Application: Their applicability in both standalone UE and merged configurations with higher-order ME generators without a significant degradation in predictions broadens their utility in various high-energy physics analyses.
• Future Collider Experiments: Understanding interactions at the fundamental level through these new tunes can guide efficient analysis frameworks for future collider data, potentially informing detectors’ hardware and data acquisition strategies.

Conclusion

The development of these new CMS PYTHIA tunes marks an advanced step in simulating underlying events in high-energy physics, facilitating detailed examinations of QCD processes at various energy scales. Their deployment in conjunction with higher-order MEs addresses some historical challenges in accurately modeling QCD backgrounds, further contributing to the fidelity and robustness of theoretical physics predictions against observed collider data. These enhancements reflect a critical underpinning in the ongoing exploration of fundamental particles and interactions at the quantum level.