[Paper Review] Snowmass2021 Whitepaper: Muonium to antimuonium conversion

Ai-Yu Bai, Yu Chen+22 more|arXiv (Cornell University)|Mar 22, 2022

Muon and positron interactions and applications4 citations

TL;DR

This paper proposes the MACE experiment to search for muonium-to-antimuonium conversion, a rare charged lepton flavor violation process signaling new physics beyond the Standard Model. By leveraging high-intensity muon beams, a magnetic spectrometer, and advanced tracking and timing techniques, MACE aims to improve sensitivity to the conversion probability by over two orders of magnitude beyond the current PSI limit of $8.3 \times 10^{-11}$ at 90% CL.

ABSTRACT

The spontaneous muonium to antimuonium conversion is one of the interesting charged lepton flavor violation processes. It serves as a clear indication of new physics and plays an important role in constraining the parameter space beyond Standard Model. MACE is a proposed experiment to probe such a phenomenon and expected to enhance the sensitivity to the conversion probability by more than two orders of magnitude from the current best upper constraint obtained by the PSI experiment two decades ago. Recent developments in the theoretical and experimental aspects to search for such a rare process are summarized.

Motivation & Objective

• To search for charged lepton flavor violation (cLFV) via the rare muonium-to-antimuonium conversion process, a clear signature of new physics beyond the Standard Model.
• To address the unresolved question of neutrino mass generation by probing cLFV processes linked to seesaw mechanisms, particularly in type-II and hybrid seesaw models.
• To significantly improve the sensitivity to muonium-to-antimuonium conversion, surpassing the current experimental bound by more than two orders of magnitude.
• To develop a conceptual design for a high-precision experiment using intense slow muon beams, optimized magnetic spectrometers, and advanced background discrimination techniques.
• To assess the feasibility of detecting this rare process by simulating detector responses, optimizing signal identification, and minimizing backgrounds from rare Standard Model processes and cosmic rays.

Proposed method

• Utilize a high-efficiency muonium formation process in vacuum using a beam of slow muons to create muonium atoms.
• Employ a magnetic spectrometer to detect and identify charged particles from muon decay and antimuonium annihilation, enabling precise momentum and charge reconstruction.
• Implement high-precision timing and position reconstruction to identify the interaction vertex and distinguish signal events from background via time-of-flight measurements.
• Apply advanced tracking algorithms and a refined figure of merit to optimize physics sensitivity and distinguish signal events from background sources.
• Use Monte Carlo simulations to model detector response, including secondary electron emission in microchannel plates (MCPs), and validate with theoretical models.
• Apply a dual-signal identification strategy: detect energetic electrons from muon decay and positrons from atomic shell emission in antimuonium annihilation to confirm conversion events.

Experimental results

Research questions

• RQ1 Can the muonium-to-antimuonium conversion process be observed with current and next-generation experimental techniques, given its predicted rarity?
• RQ2 What is the maximum sensitivity achievable with the proposed MACE experiment in probing the muonium-to-antimuonium conversion probability?
• RQ3 How can rare Standard Model processes, such as the three-body decay $\mu^+ \to e^+ \nu_e \bar{\nu}_\mu$ with additional $e^+e^-$ pairs, contribute to background in the signal region?
• RQ4 To what extent can detector energy resolution and timing resolution suppress intrinsic backgrounds from continuous energy spectra in rare decays?
• RQ5 Can the combination of magnetic spectrometry, time-of-flight, and vertex reconstruction effectively discriminate signal events from cosmic ray and beam-related backgrounds?

Key findings

• The current experimental upper bound on the muonium-to-antimuonium conversion probability is $P \lesssim 8.3 \times 10^{-11}$ at 90% confidence level, set by the PSI experiment in 1999 and unchallenged for over two decades.
• The MACE experiment is designed to improve sensitivity to the muonium-to-antimuonium conversion probability by more than two orders of magnitude, reaching the $10^{-13}$ level or better.
• Preliminary Monte Carlo simulations indicate that the detector system can be optimized to distinguish signal events from backgrounds using the time-of-flight and closest approach distance in the magnetic spectrometer.
• The detector response for secondary electron emission in MCPs shows good agreement with theoretical models, with minor systematic deviations, supporting the feasibility of high-precision signal detection.
• Backgrounds from rare Standard Model processes such as $\mu^+ \to e^+ \nu_e \bar{\nu}_\mu e^+ e^-$, with a branching ratio of $(3.4 \pm 0.4) \times 10^{-5}$, are expected to be suppressed by detector energy resolution and can be further mitigated via event reconstruction.
• Theoretical models, including type-II and hybrid seesaw scenarios, predict that double-charged Higgs bosons can mediate muonium-to-antimuonium conversion at tree level, providing a strong motivation for the search.