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Comparing particle multiplicity predictions: Insights from Pythia, Herwig with the LHCb use case

Saliha Bashir, Agnieszka Obłąkowska-Mucha, Gloria Corti

TL;DR

The paper compares Pythia and Herwig for LHCb forward physics in minimum-bias pp collisions, focusing on soft QCD dynamics and MPI. It uses default and LHCb tuned configurations to compare observables such as charged-particle multiplicity, pT and eta distributions at 7 and 13 TeV, validated via Rivet. The main finding is that Pythia generally provides closer agreement with LHCb data, particularly in forward observables, though strong sensitivity to MPI and hadronization parameters requires careful tuning of either generator. The results highlight the practical impact of generator choice and parameter tuning on forward region predictions and emphasize the need for systematic tuning campaigns for reliable LHCb simulations. Overall, the work reinforces Pythia as the preferred generator for LHCb analyses under current tunes while outlining a path for improving Herwig’s forward predictions through tuning.

Abstract

Monte Carlo Event Generators are tools for simulating outcomes of high-energy collisions and particle production in High Energy Physics (HEP), such as those conducted at the Large Hadron Collider (LHC). Two of the most widely used general-purpose event generators are \textsc{Pythia} and \textsc{Herwig}, both of which play a significant role in understanding particle production, the internal structure of the proton, and the underlying physics of interactions that take place at LHC. This paper focuses on the modelling of low-energy diffractive and minimum-bias proton-proton collisions, where soft QCD effects and MPI dominate particle production, rather than hard perturbative processes. The LHCb experiment focuses on studying heavy quark hadrons, and given its specialization in precision measurements and rare decays, the choice of event generator for simulations is crucial. In this paper, we present a comparison between simulations using \textsc{Pythia} and \textsc{Herwig} with LHCb data. Our analysis demonstrates that \textsc{Pythia} provides a more consistent agreement with experimental measurements across key observables and hence remains the preferred generator for many particle physics analyses at the LHCb experiment, ensuring its continued importance in future high-energy physics research. \keywords{particle decays, rare decays, simulation, parameter sensitivity, generator tuning, simulation tools

Comparing particle multiplicity predictions: Insights from Pythia, Herwig with the LHCb use case

TL;DR

The paper compares Pythia and Herwig for LHCb forward physics in minimum-bias pp collisions, focusing on soft QCD dynamics and MPI. It uses default and LHCb tuned configurations to compare observables such as charged-particle multiplicity, pT and eta distributions at 7 and 13 TeV, validated via Rivet. The main finding is that Pythia generally provides closer agreement with LHCb data, particularly in forward observables, though strong sensitivity to MPI and hadronization parameters requires careful tuning of either generator. The results highlight the practical impact of generator choice and parameter tuning on forward region predictions and emphasize the need for systematic tuning campaigns for reliable LHCb simulations. Overall, the work reinforces Pythia as the preferred generator for LHCb analyses under current tunes while outlining a path for improving Herwig’s forward predictions through tuning.

Abstract

Monte Carlo Event Generators are tools for simulating outcomes of high-energy collisions and particle production in High Energy Physics (HEP), such as those conducted at the Large Hadron Collider (LHC). Two of the most widely used general-purpose event generators are \textsc{Pythia} and \textsc{Herwig}, both of which play a significant role in understanding particle production, the internal structure of the proton, and the underlying physics of interactions that take place at LHC. This paper focuses on the modelling of low-energy diffractive and minimum-bias proton-proton collisions, where soft QCD effects and MPI dominate particle production, rather than hard perturbative processes. The LHCb experiment focuses on studying heavy quark hadrons, and given its specialization in precision measurements and rare decays, the choice of event generator for simulations is crucial. In this paper, we present a comparison between simulations using \textsc{Pythia} and \textsc{Herwig} with LHCb data. Our analysis demonstrates that \textsc{Pythia} provides a more consistent agreement with experimental measurements across key observables and hence remains the preferred generator for many particle physics analyses at the LHCb experiment, ensuring its continued importance in future high-energy physics research. \keywords{particle decays, rare decays, simulation, parameter sensitivity, generator tuning, simulation tools

Paper Structure

This paper contains 15 sections, 2 equations, 15 figures, 6 tables.

Table of Contents

  1. Introduction
  2. General Structure of Monte Carlo Event Generators
  3. Hard process generation
  4. Parton showers
  5. Multiparton Interactions (MPI) and Underlying Events(UE)
  6. Diffraction and forward physics
  7. Hadronisation
  8. Matching and Merging with Fixed-Order Calculations
  9. Event Generation in Pythia
  10. Event Generation in Herwig
  11. Comparison of models in Pythia and Herwig with Default settings at 7 and 13 TeV
  12. Impact of modification of parameters on particle production
  13. Comparison of models in Pythia and Herwig with LHCb settings
  14. Verification of Pythia8 parameters for the LHCb Simulation
  15. Summary and conclusions

Figures (15)

  • Figure 1: Comparison of Pythia and Herwig transverse momentum distributions at $\sqrt{s} = 7~\mathrm{TeV}$ for (a) charged hadrons, (b) pions, (c) kaons, and (d) protons.
  • Figure 2: Comparison of Pythia and Herwig transverse momentum distributions at $\sqrt{s} = 13~\mathrm{TeV}$ for (a) charged hadrons, (b) pions, (c) kaons, and (d) protons.
  • Figure 3: Comparison of Pythia and Herwig pseudorapidity ($\eta$) distributions at $\sqrt{s} = 7~\mathrm{TeV}$ for (a) charged hadrons, (b) pions, (c) kaons, and (d) protons. The error bars represent statistical uncertainties.
  • Figure 4: Comparison of Pythia and Herwig pseudorapidity ($\eta$) distributions at $\sqrt{s} = 13~\mathrm{TeV}$ for (a) charged hadrons, (b) pions, (c) kaons, and (d) protons. The error bars represent statistical uncertainties.
  • Figure 5: Comparison of Pythia and Herwig multiplicity distributions at $\sqrt{s} = 7~\mathrm{TeV}$ for (a) charged hadrons, (b) pions, (c) kaons, and (d) protons. The error bars represent statistical uncertainties.
  • ...and 10 more figures