- The paper introduces a phenomenological model for multiple partonic interactions that integrates both hard and soft QCD scatters into Herwig++.
- The model employs two-parameter tuning with ptmin at 3.4 GeV and mu² at 1.5 GeV² to match CDF Tevatron data and predict LHC scenarios.
- Its implementation coherently interfaces MPI with parton showers and hadronization, ensuring realistic momentum conservation and event simulation.
Simulation of Multiple Partonic Interactions in Herwig++
The paper presents a new phenomenological model of multiple partonic interactions (MPI) integrated into the Herwig++ event generator. This work is particularly relevant for experiments at hadronic colliders where understanding and modeling of the underlying event (UE) is crucial. The underlying event includes all particle activity in a hadron collision not associated with the primary hard scattering process. It is particularly vital for accurate jet energy measurements and overall event characterization in collider experiments like those conducted at the Tevatron and CERN's Large Hadron Collider (LHC).
Model Overview
The authors build upon existing theoretical frameworks and propose a model that typically represents MPI as a process of simple QCD 2→2 scatterings, both hard and soft. The chosen approach does not demand new experimental input for multi-parton distribution functions since these are largely uncharted. Instead, it effectively uses constraints from standard parton distribution functions, assuming factorization and independence to construct its predictions.
The model's validation involves a two-parameter tuning to match it against CDF's UE data from the Tevatron. The parameters $\ptmin$ and μ2, representing the minimum transverse momentum for additional scatters and the inverse hadron radius squared, are crucial. The emergent $\ptmin$ is set at 3.4 GeV and μ2 at 1.5 GeV² by carefully fitting to experimental data.
Implementation in Herwig++
An important implementation aspect within Herwig++ is the organization's impact parameter space. The model follows an eikonal form to represent the spatial structure of partonic interactions and calculates multi-particle production probabilities in collisions. The authors also extend the standard approach by combining it with parton showers and hadronization processes in Herwig++, ensuring a more realistic event simulation.
For Monte Carlo generation, they handle parton extraction by interfacing the MPI with parton showers coherently. The methodology is built on simulating additional partonic scatters and efficiently incorporating parton showers to maintain dynamical integrity without violating momentum conservation.
Numerical Results and Implications
Testing against CDF Tevatron data, the tuned MPI model yields satisfactory agreement across several observables, including charged particle multiplicities and transverse momentum sums. The success extends to reasonable predictions for LHC scenarios. Notably, the paper highlights variations between predictions using different parton distribution functions (PDFs), showcasing a significant sensitivity of the MPI model to PDF choices.
Future Developments
Future iterations of this work promise improvement by integrating softer scatterings below the perturbative limit, potentially enhancing the description of minimum bias events. Moreover, extending the model robustness will accommodate various UE descriptions and empirical adjustments based on LHC data. There is also potential to explore saturation effects and multiparton correlations more systematically, leveraging rich data from contemporary collider experiments.
This paper delivers valuable advancements in accurately simulating hadronic collisions, contributing significantly to both theoretical understanding and experimental practice in particle physics. The authors' integration of MPI modeling within Herwig++ offers a flexible, robust tool for ongoing and future particle physics investigations at high-energy colliders.