tching-Madgraph

This folder contains the necessary tools and scripts for studying jet matching in Monte Carlo event generation using MadGraph and Pythia. In particular, it focuses on the impact of the parameters xqcut and qcut on the matching between matrix element (ME) jets and parton shower (PS) jets.

What is Jet Matching?

In Monte Carlo event generation:

• Matrix elements (ME) compute hard scattering processes involving multiple final-state partons (jets) at the parton level.
• Parton showers (PS) simulate soft and collinear QCD radiation, which can also result in jets.

To avoid double counting jets generated both by ME and PS, a jet matching scheme is applied (e.g., MLM or CKKW). This ensures that:

• Hard jets are described by the matrix element.
• Softer jets and radiation are handled by the parton shower.

Parameters: xqcut and qcut

• xqcut: A parameter in MadGraph that sets the minimum kT separation between partons in the ME generation. It defines the hardness of the jets in the matrix element.
• qcut: A parameter in Pythia that sets the kT threshold for matching parton shower jets to the matrix element. It must be greater than or equal to xqcut, and should be tuned to ensure a smooth transition between ME and PS.

A poor choice of these parameters can lead to:

• Gaps or spikes in jet multiplicity distributions.
• Overlapping (double counting) or missing jets.
• Mismodeled physical observables (e.g., jet spectra, HT, etc.).

Purpose of This Folder

This directory provides:

• Scripts to automate MadGraph and Pythia runs over a grid of xqcut and qcut values.

  • control_script.py: Script that controls the whole workflow for the jet matching study. Within this file, different booleans in the header allow you to select what you want to do. Before running it, make sure all paths within this file are appropriate for you. Gridpacks and mass point cards are currently set to be generated in the folder where the script is run, while ROOT events and histograms are set to be stored in an EOS directory.
  • generate_gridpacks: Script that generates gridpacks from some input cards. Used in the control script.

• Tools to analyze and validate the smoothness of jet-related observables.

  • plotdjr.C: Generates the differential jet rates (DJR) distributions, which are studied to validate that the matching has been done correctly (should all be smooth for an accurate transition between ME and PS contributions).

• Different scripts are also added for the generation of the different cards, both for the inclusive/dilepton scalar and pseudoscalar models.