Letter Of Intent for a future $μ^+ \to \mathrm{e}^+ γ$ experiment at the High Intensity Muon Beam facility at PSI

TL;DR

This letter outlines a staged, two-phase plan to develop a future μ^+ → e^+ γ experiment at PSI's High Intensity Muon Beam facility, aiming to improve the MEG II sensitivity by more than an order of magnitude within a decade. It presents two complementary detector concepts centered on photon reconstruction via photon conversion with active converters (baseline) or an advanced calorimeter approach, paired with either gaseous drift chambers or silicon pixel tracking for positrons, and outlines two magnetic-field geometries (single solenoid vs solenoid-toroid). A concrete experimental roadmap (Phase-0 proof-of-concept, Phase-I at conventional beams, Phase-II at HIMB) and a detailed sensitivity study are provided, showing potential BR reach down to ~1.5 × 10^-14 in Phase I and a few ×10^-15 in Phase II, along with NP-scale implications as a function of κ_D. The project emphasizes complementary NP sensitivity across μ → e γ, μ → e e e, and μN → eN channels, and it plans to leverage Mu3e experience and ongoing R&D in high-rate tracking and fast timing to enable a robust, scalable search for charged lepton flavor violation with practical impact for beyond-Standard-Model physics.

Abstract

Searches for charged lepton flavor violation in the muon sector stand out among the most sensitive and clean probes for physics beyond the Standard Model. Currently, $μ^+ \to \mathrm{e}^+ γ$ experiments provide the best constraints in this field for a wide range of models while, in the coming years, new experiments investigating the processes of $μ^+ \to \mathrm{e}^+ \mathrm{e}^+ \mathrm{e}^-$ and $μ\to \mathrm{e}$ conversion in the nuclear field are anticipated to reach comparable or higher sensitivities. The High-Intensity Muon Beam (HIMB) facility at PSI, which is expected to deliver muon beam intensities up to two orders of magnitude higher than the existing beam lines, offers a unique opportunity to significantly enhance the sensitivity of $μ^+ \to \mathrm{e}^+ γ$ searches. The discovery potential could be substantially boosted and a sensitivity comparable to that of all the other projects could be reestablished, which is essential for discriminating among competing new-physics scenarios should an observation occur in any of the channels. In this document, we express our interest in developing a $μ^+ \to \mathrm{e}^+ γ$ experimental program at HIMB, with the goal of improving, within the next decade, the sensitivity of the $μ^+ \to \mathrm{e}^+ γ$ search by more than one order of magnitude relative to the expected final result of the current leading experiment, MEG II. This effort would ensure that PSI retains its leadership in this field.

Figures (12)

• Figure 1: NP scale sensitivity of the different LFV muon channels as a function of a parameter $\kappa_D$ which tunes the relative magnitude of dipole and four-fermions coefficients in an effective field theory approach ($\kappa_D = 0$ for a pure dipole interaction) from Davidson:2022nnl
• Figure 2: A sketch of the MEG II experiment with a simulated event.
• Figure 3: Overview of the HIPA accelerator complex including the HIMB beam lines and the new isotope production target station TATTOOS. The new target station TgH will replace the current TgM and the beam lines will replace $\uppi$M1 and $\uppi$M3. Adapted from CDR.
• Figure 4: A sketch of a detector for $\mathrm{\muup^+ \to e^+ \gammaup}$ decays with a silicon pixel tracker for positrons, and active conversion layers with $e^+e^-$ trackers for photons, within a single, long solenoid.
• Figure 5: A sketch of a detector for $\mathrm{\muup^+ \to e^+ \gammaup}$ decays with a central tracker for positrons within a solenoid, and active conversion layers with $e^+e^-$ trackers for photons in an external toroidal field. The hexagonal configuration is only indicative, it could be as well octagonal.