[Paper Review] Testing Lepton Flavor Universality and CKM Unitarity with Rare Pion Decays in the PIONEER experiment

PIONEER Collaboration, W. Altmannshofer+59 more|arXiv (Cornell University)|Mar 10, 2022

Particle physics theoretical and experimental studies9 citations

TL;DR

The PIONEER experiment proposes a next-generation rare pion decay experiment using a liquid xenon calorimeter and a low-gain avalanche diode (LGAD) active target to measure the charged-pion branching ratio $ R_{e/\mu} $ with 1 part in $ 10^4 $ precision. This measurement tests lepton flavor universality and CKM unitarity, probing new physics up to the PeV scale through deviations from the Standard Model prediction.

ABSTRACT

The physics motivation and the conceptual design of the PIONEER experiment, a next-generation rare pion decay experiment testing lepton flavor universality and CKM unitarity, are described. Phase I of the PIONEER experiment, which was proposed and approved at Paul Scherrer Institut, aims at measuring the charged-pion branching ratio to electrons vs.\ muons, $R_{e/μ}$, 15 times more precisely than the current experimental result, reaching the precision of the Standard Model (SM) prediction at 1 part in $10^4$. Considering several inconsistencies between the SM predictions and data pointing towards the potential violation of lepton flavor universality, the PIONEER experiment will probe non-SM explanations of these anomalies through sensitivity to quantum effects of new particles up to the PeV mass scale. The later phases of the PIONEER experiment aim at improving the experimental precision of the branching ratio of pion beta decay (BRPB), $π^+ o π^0 e^+ ν(γ)$, currently at $1.036(6) imes10^{-8}$, by a factor of three (Phase II) and an order of magnitude (Phase III). Such precise measurements of BRPB will allow for tests of CKM unitarity in light of the Cabibbo Angle Anomaly and the theoretically cleanest extraction of $|V_{ud}|$ at the 0.02\% level, comparable to the deduction from superallowed beta decays.

Motivation & Objective

• To test lepton flavor universality by measuring the charged-pion decay branching ratio $ R_{e/\mu} $ with unprecedented precision of 1 part in $ 10^4 $.
• To probe for new physics beyond the Standard Model through deviations in $ R_{e/\mu} $, sensitive to new particles up to the PeV mass scale.
• To improve the precision of the pion beta decay branching ratio $ \pi^+ \to \pi^0 e^+ \nu(\gamma) $ to test CKM unitarity and extract $ |V_{ud}| $ at the 0.02% level in later phases.
• To develop advanced detector technologies, including ATAR sensors, LXe calorimeters, and SiPM-based readout systems, for high-rate, high-precision tracking and energy measurement.
• To establish a new experimental platform at PSI for rare decay physics, integrating expertise from rare kaon decays, muon experiments, and high-energy collider physics.

Proposed method

• Utilize a low-gain avalanche diode (LGAD)-based active target (ATAR) for high-precision 4D tracking of decay products in rare pion decays.
• Employ a large-acceptance, deep, fast, and uniform liquid xenon (LXe) calorimeter with excellent energy resolution to detect and measure positrons from $ \pi^+ \to e^+ \nu $ decays.
• Implement a cylindrical positron tracker using 300 $\mu$m thick Si strips or LGADs to enhance momentum resolution and particle identification.
• Conduct R&D on ATAR sensor prototypes, focusing on energy resolution and gain suppression mechanisms, with beam tests at $\pi E5$ and $\pi E1$.
• Develop and test LXe calorimeter prototypes to benchmark photon transport simulations, optimize optical segmentation, and evaluate photo-sensor performance and material properties.
• Perform rate testing of FPGA-to-CPU/GPU and CPU-to-CPU optical PCI-express links, and evaluate data compression algorithms for CALO data.

Experimental results

Research questions

• RQ1 Can the charged-pion branching ratio $ R_{e/\mu} $ be measured with a precision of 1 part in $ 10^4 $, enabling a sensitive test of lepton flavor universality?
• RQ2 What is the sensitivity of the $ R_{e/\mu} $ measurement to new physics beyond the Standard Model, particularly at the PeV mass scale?
• RQ3 Can the pion beta decay branching ratio $ \pi^+ \to \pi^0 e^+ \nu(\gamma) $ be measured with sufficient precision to test CKM unitarity and extract $ |V_{ud}| $ at the 0.02% level?
• RQ4 How do detector components such as ATAR sensors, SiPMs, and LXe calorimeters perform under high-rate beam conditions relevant to the experiment?
• RQ5 What are the optimal configurations for the cylindrical positron tracker and LXe calorimeter in terms of segmentation, materials, and readout electronics?

Key findings

• The PIONEER experiment aims to measure $ R_{e/\mu} $ with a precision of 1 part in $ 10^4 $, significantly improving on current experimental limits.
• The experiment is designed to probe new physics effects in lepton flavor universality up to the PeV mass scale through quantum corrections involving new particles.
• Phase I of PIONEER is scheduled to begin in 2029, following successful R&D and prototype testing of key components like ATAR sensors and LXe calorimeters.
• Prototype development includes beam tests at $\pi E5$ and $\pi E1$, with two weeks of beam time approved for 2022 to validate beam conditions.
• LXe calorimeter R&D includes testing photo-sensor performance and optical properties using a 2-liter LXe cryostat at McGill University with a SiPM-based setup.
• The collaboration is developing a prototype APOLLO Command Module and a 4-channel digitizer board for trigger and data acquisition system prototyping.