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Multiple Interactions in Pythia 8

Richard Corke

TL;DR

This work surveys the Pythia 8 MI framework and its essential components, including interleaved $p_{ot}$ ordering, impact parameter, PDF rescaling, beam remnants, primordial $k_{ot}$, and colour reconnection, to model non-diffractive hadronic events. It then introduces two new effects—rescattering and enhanced screening—and provides initial evaluations: rescattering yields only modest changes and does not resolve colour reconnection, while enhanced screening, implemented via activity-dependent scaling of $p_{ot 0}$, produces sizable shifts in $\langle p_{ot} \rangle$ as a function of charged multiplicity and can ease the CR tuning burden. The results point to enhanced screening as a promising avenue for reducing underspecified CR effects, with rescattering offering richer colour and jet topologies for future study. Overall, the paper demonstrates that the Pythia 8 MI framework can accommodate richer physics to better describe underlying-event observables, guiding further tuning and development.

Abstract

Modelling multiple partonic interactions in hadronic events is vital for understanding minimum-bias physics, as well as the underlying event of hard processes. A brief overview of the current Pythia 8 multiple interactions (MI) model is given, before looking at two additional effects which can be included in the MI framework. With rescattering, a previously scattered parton is allowed to take part in another subsequent scattering, while with enhanced screening, the effects of varying initial-state fluctuations are modelled.

Multiple Interactions in Pythia 8

TL;DR

This work surveys the Pythia 8 MI framework and its essential components, including interleaved ordering, impact parameter, PDF rescaling, beam remnants, primordial , and colour reconnection, to model non-diffractive hadronic events. It then introduces two new effects—rescattering and enhanced screening—and provides initial evaluations: rescattering yields only modest changes and does not resolve colour reconnection, while enhanced screening, implemented via activity-dependent scaling of , produces sizable shifts in as a function of charged multiplicity and can ease the CR tuning burden. The results point to enhanced screening as a promising avenue for reducing underspecified CR effects, with rescattering offering richer colour and jet topologies for future study. Overall, the paper demonstrates that the Pythia 8 MI framework can accommodate richer physics to better describe underlying-event observables, guiding further tuning and development.

Abstract

Modelling multiple partonic interactions in hadronic events is vital for understanding minimum-bias physics, as well as the underlying event of hard processes. A brief overview of the current Pythia 8 multiple interactions (MI) model is given, before looking at two additional effects which can be included in the MI framework. With rescattering, a previously scattered parton is allowed to take part in another subsequent scattering, while with enhanced screening, the effects of varying initial-state fluctuations are modelled.

Paper Structure

This paper contains 12 sections, 12 equations, 5 figures, 1 table.

Table of Contents

  1. Introduction
  2. Multiple Interactions in Pythia 8
  3. Interleaved $p_{\perp}$ Ordering
  4. Impact Parameter
  5. PDF Rescaling
  6. Beam Remnants, Primordial $k_{\perp}$ and Colour Reconnection
  7. Rescattering
  8. Rescattering in Pythia 8
  9. Mean $p_{\perp}$ vs Charged Multiplicity
  10. Enhanced Screening
  11. Mean $p_{\perp}$ vs Charged Multiplicity
  12. Conclusions

Figures (5)

  • Figure 1: (a) Two $2 \rightarrow 2$ scatterings, (b) a $2 \rightarrow 2$ scattering followed by a rescattering
  • Figure 2: Average distribution of scatterings, single rescatterings and double rescatterings per event ($\sqrt{s} = 1.96 \!\ \mathrm{GeV}$, minimum bias). Double rescattering is not visible at this scale in the $\mathrm{d} N / \mathrm{d} ( \mathrm{log} \, p_{\perp}^2)$ plot, but is visible in the ratio
  • Figure 3: Mean $p_{\perp}$ vs Charged Multiplicity, $| \eta | \leq 1$ and $p_{\perp} \geq 0.4~\mathrm{GeV/c}$, CDF Run II data against Pythia 6.418 (Tune A) and Pythia 8.114 (default settings) with and without deferred FSR
  • Figure 4: Mean $p_{\perp}$ vs Charged Multiplicity, Pythia 8.114 (deferred FSR), effects of rescattering
  • Figure 5: Mean $p_{\perp}$ vs Charged Multiplicity, Pythia 8.114, effects of the enhanced screening ansatz