Research Proposal for an Experiment to Search for the Decay μ -> eee

Abstract: We propose an experiment (Mu3e) to search for the lepton flavour violating decay mu+ -> e+e-e+. We aim for an ultimate sensitivity of one in 10^16 mu-decays, four orders of magnitude better than previous searches. This sensitivity is made possible by exploiting modern silicon pixel detectors providing high spatial resolution and hodoscopes using scintillating fibres and tiles providing precise timing information at high particle rates.

• The paper presents a novel experimental design aiming to improve muon decay sensitivity by four orders of magnitude, targeting a branching ratio near 1e-16.
• It employs state-of-the-art HV-MAPS silicon pixel detectors alongside scintillating fiber and SiPM time-of-flight systems for precise tracking and timing.
• The experiment offers a critical test of lepton flavour violation, potentially unveiling new physics beyond the Standard Model.

An Experiment to Search for the Decay $\mu^{+} \rightarrow e^{+}e^{-}e^{+}$

This paper outlines a comprehensive experimental proposal for the Mu3e experiment, aimed at investigating lepton flavour violation (LFV) in the muonic decay $\mu^{+} \rightarrow e^{+}e^{-}e^{+}$. The proposed experiment is set to achieve unprecedented sensitivity levels, targeting one in $\num{e16}$ muon decays, which marks an improvement by four orders of magnitude over any previous searches. This enhanced sensitivity is facilitated through the use of advanced detector technologies, notably modern silicon pixel detectors that offer high spatial resolution and hodoscopes employing scintillating fibers and tiles to deliver precise timing information at elevated particle rates.

Experimental Setup and Objectives

• Modern Silicon Pixel Detectors: The experiment intends to leverage High Voltage Monolithic Active Pixel Sensors (HV-MAPS), which are crucial for high granularity and precision in tracking and vertexing. These sensors allow for the substantial reduction of material through thinning and integration of readout circuitry within the sensor itself.
• Complementary Technology: Time-of-flight systems, consisting of scintillating fiber hodoscopes and Silicon Photo-Multiplier (SiPM) tiles, augment the pixel detectors, aiming to reduce accidental background to below the sensitivity threshold of $\textrm{B}(\mu^{+} \rightarrow e^{+}e^{-}e^{+}) \sim \num{e-16}$.

Implications and Comparisons

The proposed experiment holds significant potential to complement existing searches for LFV, challenging models that suggest new physics beyond the Standard Model (BSM). It promises sensitivity to interactions predicted by supersymmetric models and left-right symmetric models, among others. The paper contends that the Mu3e experiment's unique approach and sensitivity to LFV processes will provide a critical test bed for BSM physics.

• Comparison with Existing Experiments: Comparisons are drawn with ongoing searches at facilities like the Large Hadron Collider (LHC) and other muonic and tau decay experiments. While the LHC probing high-energy processes, the Mu3e experiment aims to offer insights into LFV at much lower energies, potentially revealing new interaction mechanisms.
• Potential Discoveries: LFV in charged lepton processes would decisively point to new physics, especially if not corroborated by discoveries at the LHC. With unprecedented measurement accuracy, the Mu3e experiment holds the potential to set benchmarks in understanding LFV interactions deeply linked to neutrino mass generation and charge parity (CP) violation.

Future Prospects and Development

• Phased Approach: Designed for deployment in phases, the experiment begins with a initial configuration focusing on central silicon detectors, gradually expanding to incorporate the full recurl system and time-of-flight enhancements. The first phase will utilize existing facilities while the second hinges on the development of a high-intensity muon beamline.
• Technological Innovation: The experiment stands on the cusp of technological breakthroughs in sensor and data acquisition systems, with a focus on developments in real-time analysis without hardware triggers and high-throughput data processing.

Conclusion

The Mu3e experiment proposal presented in this paper is a robust exploration into the $\mu^{+} \rightarrow e^{+}e^{-}e^{+}$ decay, combining state-of-the-art detection technology with strategic data handling to push the boundaries of LFV research. By setting ambitious targets and proposing innovative solutions to longstanding technical hurdles, the research outlined here provides a detailed roadmap for exploring LFV with unprecedented precision.