Investigations on a Fuzzy Process: Effect of Diffusion on Calibration and Particle Identification in Liquid Argon Time Projection Chambers

TL;DR

This study uses simulations to analyze how ionization electron diffusion affects calibration and particle identification in Liquid Argon TPCs, revealing potential biases and implications for detector performance.

Contribution

It provides the first detailed analysis of diffusion effects on calibration metrics and particle separation, highlighting the need for drift-time dependent corrections.

Findings

Diffusion causes a ~2.5% bias in median charge deposition.

Diffusion causes a ~5.0% bias in MPV of charge deposition.

Impact on muon/proton separation is minimal, but significant for similar-mass particles.

Abstract

Ionization electron diffusion in Liquid Argon Time Projection Chambers (LArTPCs) has typically been considered at the detector design stage, but little attention has been given to its effects on calibration and particle identification. We use a GEANT4-based simulation to study how diffusion impacts these techniques, and give consideration to how this effect is simulated. We find that diffusion can cause a drift-dependent bias to both the median and Most Probable Value (MPV) of charge deposition per unit length distributions. The bias is estimated to be ~2.5% (median) and ~5.0% (MPV) for typical maximum drift times in currently running LArTPCs before adding detector specific considerations such as electric field non-uniformities. This indicates that these metrics should not be used for calibration without care, contrary to the conventional wisdom. The impact of diffusion on the ability of LArTPCs to separate muons and protons is small, and not expected to pose any problems in future detectors. Diffusion may however be a significant source of systematic uncertainty when separating particles of more similar masses (muons and pions, kaons and protons). Separation of such populations may be improved by implementation of a drift-time dependent particle identification.