[Paper Review] Measurement of the longitudinal diffusion of ionization electrons in the MicroBooNE detector

Collaboration, The MicroBooNE, P. Abratenko+181 more|arXiv (Cornell University)|Apr 15, 2021

Dark Matter and Cosmic Phenomena23 references20 citations

TL;DR

This paper presents the first measurement of the longitudinal electron diffusion coefficient, $D_L = 3.74^{+0.28}_{-0.29}$ cm²/s, in a large-scale (85-tonne) liquid argon time projection chamber (LArTPC), MicroBooNE, operating in a neutrino beam at an electric field of 273.9 V/cm. Using ~70,000 cosmic ray muon tracks, the study employs waveform analysis and drift-time-dependent pulse broadening to extract $D_L$, validating the method with simulations and quantifying systematic uncertainties from detector response and drift velocity.

ABSTRACT

Abstract: Accurate knowledge of electron transport properties is vital to understanding the information provided by liquid argon time projection chambers (LArTPCs). Ionization electron drift-lifetime, local electric field distortions caused by positive ion accumulation, and electron diffusion can all significantly impact the measured signal waveforms. This paper presents a measurement of the effective longitudinal electron diffusion coefficient, DL, in MicroBooNE at the nominal electric field strength of 273.9 V/cm. Historically, this measurement has been made in LArTPC prototype detectors. This represents the first measurement in a large-scale (85 tonne active volume) LArTPC operating in a neutrino beam. This is the largest dataset ever used for this measurement. Using a sample of ∼70,000 through-going cosmic ray muon tracks tagged with MicroBooNE's cosmic ray tagger system, we measure DL = 3.74+0.28 -0.29 cm2/s.

Motivation & Objective

• To measure the effective longitudinal electron diffusion coefficient $D_L$ in a large-scale liquid argon time projection chamber (LArTPC) under realistic neutrino beam conditions.
• To validate the measurement method using simulated data and assess systematic uncertainties from detector response, drift velocity, and waveform summation.
• To resolve discrepancies between existing $D_L$ measurements and theoretical models by providing a high-statistics measurement in a fully operational LArTPC.

Proposed method

• Selected ~70,000 through-going cosmic ray muon tracks using the MicroBooNE cosmic ray tagger system to isolate clean, long-drift tracks.
• Extracted signal waveforms from the Y-plane wire readout and deconvolved them to remove detector response effects.
• Fitted the variance of the pulse shape as a function of drift time to the relation $\sigma^2(t) = \sigma_0^2 + 2D_L t$, where $D_L$ is the longitudinal diffusion coefficient.
• Validated the analysis pipeline on simulated datasets to confirm sensitivity and consistency of the $D_L$ extraction method.
• Quantified systematic uncertainties by varying key inputs: detector response function, drift velocity, transverse diffusion, and waveform summation method.
• Combined uncorrelated systematic uncertainties in quadrature to determine the final $D_L$ uncertainty.

Experimental results

Research questions

• RQ1 What is the value of the longitudinal electron diffusion coefficient $D_L$ in a large-scale LArTPC operating in a neutrino beam?
• RQ2 How do systematic uncertainties from detector response and drift velocity affect the $D_L$ measurement?
• RQ3 How does the measured $D_L$ compare to theoretical models (Atrazhev-Timoshkin) and prior experimental data (Li et al., ICARUS)?
• RQ4 Is Coulomb repulsion among drifting electrons a significant source of systematic error in $D_L$ measurements?
• RQ5 Can electron diffusion be used to reconstruct the $t_0$ time of interaction in single-waveform events?

Key findings

• The measured longitudinal electron diffusion coefficient in MicroBooNE is $D_L = 3.74^{+0.28}_{-0.29}$ cm²/s at an electric field of 273.9 V/cm.
• The measurement is consistent with the Atrazhev-Timoshkin theoretical prediction and the ICARUS experiment, but lies below the Li et al. parametrization.
• Systematic uncertainties are dominated by the detector response function (6.5%) and drift velocity (±3.9%/−4.1%), with all others sub-dominant.
• No significant non-linearity in pulse width scaling with $\sqrt{t}$ was observed, indicating negligible contribution from Coulomb repulsion among drifting electrons.
• The measurement is the first in a large-scale (85-tonne) LArTPC operating in a neutrino beam, using the largest dataset to date for this quantity.
• The result reduces uncertainty in $D_L$ compared to previous measurements, supporting improved modeling in future LArTPC physics analyses.