[Paper Review] Operation and performance of MEG II detector

MEG II collaboration, K. Afanaciev+83 more|arXiv (Cornell University)|Oct 18, 2023

Particle physics theoretical and experimental studiesPhysics and Astronomy3 citations

TL;DR

This paper presents the operational performance and sensitivity assessment of the MEG II experiment, a high-precision search for the lepton-flavour-violating decay $μ^+ \to e^+\gamma$ at the Paul Scherrer Institute. Utilizing a novel liquid xenon calorimeter, a high-rate RPC tracker, and advanced data acquisition, MEG II achieves a projected sensitivity of $6.0 \times 10^{-14}$, representing a 10-fold improvement over the previous MEG limit.

ABSTRACT

The MEG II experiment, located at the Paul Scherrer Institut (PSI) in Switzerland, is the successor to the MEG experiment, which completed data taking in 2013. MEG II started fully operational data taking in 2021, with the goal of improving the sensitivity of the mu+ -> e+ gamma decay down to 6e-14 almost an order of magnitude better than the current limit. In this paper, we describe the operation and performance of the experiment and give a new estimate of its sensitivity versus data acquisition time.

Motivation & Objective

• To achieve a sensitivity of $6.0 \times 10^{-14}$ for the rare lepton-flavour-violating decay $\mu^+ \to e^+\gamma$, improving on the previous MEG experiment's limit.
• To commission and operate a next-generation detector system with enhanced tracking, calorimetry, and trigger capabilities at PSI.
• To reduce background contributions, particularly from $\gamma$-ray backgrounds, through advanced shielding and detector design.
• To validate the performance of key components such as the liquid xenon scintillation detector and MPPC-based photodetectors under high-rate conditions.
• To establish a real-time, high-bandwidth data acquisition and trigger system capable of handling the expected event rates with minimal dead time.

Proposed method

• Employing a liquid xenon scintillation calorimeter with MPPC-based photodetectors to achieve high energy resolution and fast timing for electron and photon detection.
• Implementing a high-rate, ultra-low-mass resistive plate chamber (RPC) tracker with diamond-like carbon electrodes to measure electron trajectories with high precision.
• Utilizing a custom FPGA-based trigger system integrated with the WaveDAQ data acquisition framework to enable real-time event selection and low-latency data handling.
• Applying a novel X-ray scanning technique for in-situ alignment of photodetectors to maintain optimal energy resolution and position reconstruction.
• Using Geant4-based Monte Carlo simulations and ROOT-based analysis frameworks to model detector response and optimize sensitivity.
• Implementing a software stack based on Slurm, MIDAS, and containerized workflows (Docker/Singularity) for scalable, reproducible data processing and analysis.

Experimental results

Research questions

• RQ1 What is the achievable sensitivity of the MEG II experiment for the $\mu^+ \to e^+\gamma$ decay after full commissioning?
• RQ2 How do the performance characteristics of the liquid xenon calorimeter and RPC tracker contribute to background suppression and signal identification?
• RQ3 What is the effective data acquisition efficiency and trigger rate capability of the WaveDAQ system under operational conditions?
• RQ4 How does radiation damage affect the long-term performance of MPPC photodetectors in the liquid xenon calorimeter?
• RQ5 To what extent can in-situ alignment techniques improve the energy resolution and position reconstruction of the calorimeter?

Key findings

• MEG II achieved full operational data taking in 2021, with a projected sensitivity of $6.0 \times 10^{-14}$ for the $\mu^+ \to e^+\gamma$ decay, representing a 10-fold improvement over the previous MEG limit.
• The liquid xenon calorimeter demonstrated excellent energy resolution and stability, with real-time particle identification achieved via pulse shape analysis of MPPC signals.
• The high-rate RPC tracker with diamond-like carbon electrodes achieved a rate capability exceeding 100 kHz/cm², enabling efficient tracking of high-multiplicity events.
• The WaveDAQ data acquisition system achieved a live-time efficiency above 99% and sustained data rates of several MHz, with low dead time and high reliability.
• In-situ X-ray scanning enabled sub-millimeter alignment precision of photodetectors, significantly improving energy resolution and position reconstruction.
• Radiation damage studies showed that MPPC sensitivity degradation in the liquid xenon environment was within acceptable limits for the planned data-taking duration.