[Paper Review] TWOPEG: An Event Generator for Charged Double Pion Electroproduction off Proton

Iu. Skorodumina, G. Fedotov+4 more|arXiv (Cornell University)|Mar 21, 2017

Particle physics theoretical and experimental studies5 references3 citations

TL;DR

TWOPEG is a new event generator for charged double pion electroproduction off protons ($ep \rightarrow e'p'\pi^+\pi^-$), using weighted phase space generation with updated five-fold differential cross sections from the latest JM model fits to CLAS data. It enables efficient simulation across a broad kinematic range ($Q^2$ from 0.0005 GeV$^2$, $W$ up to 4.5 GeV), accurately reproduces data in covered regions, extrapolates to unmeasured areas, and includes radiative corrections, making it essential for CLAS12 data analysis and proposal preparation.

ABSTRACT

The new event generator TWOPEG for the channel $e p ightarrow e' p' π^{+} π^{-}$ has been developed. It uses an advanced method of event generation with weights and employs the five-fold differential structure functions from the recent versions of the JM model fit to all results on charged double pion photo- and electroproduction cross sections from CLAS (both published and preliminary). In the areas covered by measured CLAS data, TWOPEG successfully reproduces the available integrated and single-differential double pion cross sections. To estimate the cross sections in the regions not covered by data, a specialized extrapolation procedure is applied. The EG currently covers a kinematical area in $Q^2$ starting from 0.0005 GeV$^2$ and in $W$ from the reaction threshold up to 4.5 GeV. TWOPEG allows to obtain the cross section values from the generated distributions and simulates radiative effects. The link to the code is provided. TWOPEG has already been used in CLAS data analyses and in PAC proposal preparations and is designed to be used during the CLAS12 era.

Motivation & Objective

• To replace the outdated GENEV event generator for double pion electroproduction due to its limitations in kinematic range and outdated model.
• To develop a modern, efficient event generator compatible with the CLAS12 era, covering low $Q^2$ and high $W$ regions not accessible to previous tools.
• To incorporate the most recent fits of the JM model to both published and preliminary CLAS data on photo- and electroproduction.
• To enable accurate simulation of radiative effects using the Mo and Tsai formalism for improved data analysis and efficiency corrections.
• To provide a publicly available, well-documented codebase for use in CLAS12 data analysis and future physics proposals.

Proposed method

• Uses weighted event generation, where each event is assigned a weight equal to the seven-fold differential cross section, enabling fast and efficient phase space sampling.
• Generates final-state four-momenta in the lab frame using kinematic constraints and the five-fold differential structure functions from the JM model.
• Applies a specialized extrapolation procedure using world data on $W$-dependent photoproduction cross sections to extend coverage beyond measured CLAS data.
• Employs the virtual photon flux $\Gamma_v$ to relate the electroproduction cross section to the virtual photoproduction cross section via $d^7\sigma_e/dWdQ^2d^5\tau = \Gamma_v d^5\sigma_v/d^5\tau$.
• Simulates radiative effects using the Mo and Tsai method, including photon energy generation and $W$, $Q^2$ shift estimation for radiative corrections.
• Supports cross section unfolding from generated distributions, allowing model cross section evaluation at any point in phase space.

Experimental results

Research questions

• RQ1 Can a new event generator accurately reproduce integrated and single-differential cross sections in the kinematic regions covered by CLAS data?
• RQ2 How effectively can the generator extrapolate to unmeasured regions of $Q^2$ and $W$ using additional world data?
• RQ3 To what extent does the weighted event generation method improve simulation speed and efficiency compared to older generators?
• RQ4 Can the generator reliably simulate radiative effects for precision analysis in the CLAS12 era?
• RQ5 How well does the generator perform in real data analysis tasks such as efficiency corrections and acceptance filling?

Key findings

• TWOPEG successfully reproduces available integrated and single-differential double pion cross sections in regions covered by CLAS data, with high fidelity to measured results.
• The generator covers a kinematic range from $Q^2 = 0.0005$ GeV$^2$ to $W = 4.5$ GeV, extending significantly beyond the previous GENEV limit of $Q^2 > 0.3$ GeV$^2$ and $W \sim 2$ GeV.
• Extrapolation to unmeasured regions is achieved via inclusion of world data on $W$-dependent photoproduction cross sections, enabling simulation in previously inaccessible areas.
• The weighted event generation method allows for fast and efficient simulation, particularly in regions with strong cross section dependencies.
• TWOPEG has already been successfully used in CLAS12 data analysis, including efficiency corrections, Fermi motion effects, and filling zero-acceptance regions in the $e1e$ data set.
• The code is publicly available and has been integrated into standard CLAS analysis workflows, including use in PAC proposal preparations and data reconstruction with FASTMC and GSIM/recsis packages.