[Paper Review] The Muon Counter System for the MicroBooNE experiment

S. R. Soleti|arXiv (Cornell University)|Apr 26, 2016

Neutrino Physics Research1 references3 citations

TL;DR

The MicroBooNE experiment developed a small external muon counter system (MuCS) using two scintillator-based detectors above the liquid argon TPC to measure cosmic ray trajectories with 2 cm spatial resolution. The system enables precise calibration, background understanding, and validation of reconstruction and trigger efficiencies through coincidence timing and 2D track reconstruction, achieving good agreement with Monte Carlo simulations for angular and spatial distributions.

ABSTRACT

The MicroBooNE experiment is a liquid argon TPC experiment designed for short-baseline neutrino physics, currently running at Fermilab. Due to its location near the surface, cosmic muons can be a source of backgrounds to many analyses and having a good understanding of the cosmic rays will be very valuable for the experiment. These proceedings describe the physics motivation, setup, and performance of a small external muon counter system, which will provide improved calibration for the liquid argon TPC and better understanding of the cosmogenic background.

Motivation & Objective

• To reduce cosmogenic background in MicroBooNE by improving cosmic ray characterization.
• To provide a clean dataset of through-going cosmic muons for calibration and reconstruction validation.
• To measure the spatial and angular distribution of cosmic rays using external muon counters.
• To cross-check trigger efficiency and reconstruction performance using timing and track extrapolation.
• To support liquid argon purity and PMT gain calibration using minimum ionizing particles.

Proposed method

• Two identical muon detectors, each with two bilayers of 48×4×1 cm³ scintillator strips, are placed above the TPC to detect cosmic rays.
• Scintillator strips are oriented perpendicular to each other to enable 2D (x,y) track reconstruction with 2 cm spatial resolution.
• Optical fibers collect light from the strips and route it to two multi-anode photomultipliers (MAPMTs) for readout.
• Cosmic ray tracks are reconstructed by matching MuCS hit times with flash times from the TPC's 32-PMT array.
• Through-going muons are identified via 4-bilayer coincidence, enabling clean selection of cosmic ray events.
• Cosmic ray trajectories are extrapolated from TPC tracks to verify hits in the MuCS, improving reconstruction consistency.

Experimental results

Research questions

• RQ1 How accurately can the MuCS measure the 2D spatial and angular distribution of cosmic muons?
• RQ2 To what extent does the MuCS improve the calibration of the liquid argon TPC and PMT response?
• RQ3 Can the MuCS be used to validate trigger and reconstruction efficiencies in MicroBooNE?
• RQ4 How well do the measured cosmic ray angular distributions agree with Monte Carlo simulations?
• RQ5 What is the impact of detector misalignment on the measured cosmic ray distributions, and can it be corrected?

Key findings

• The MuCS achieves a spatial resolution of 2 cm in both x and y directions due to staggered bilayer positioning.
• The measured angular distributions of cosmic rays—azimuthal angle φ peaked at 0° and polar angle θ peaked at 30°—show good agreement with Monte Carlo simulations.
• The distribution of starting y-coordinates exhibits a slope consistent with the known 3D offset between the two MuCS boxes.
• The starting x-coordinate distribution is flat except for a small deviation in the first bin, matching the expected horizontal shift between detectors.
• The fraction of cosmic rays passing through the MuCS without hitting the TPC is 0.3%, indicating high detection efficiency for the system.
• The MuCS provides a clean dataset of cosmic muons suitable for validating trigger efficiency, reconstruction algorithms, and detector performance.