Revision 1

Superseded · first revision · record

Title: Does muon-to-electron conversion occur at rates near 1e-16?
Plain: The SM forbids a muon from turning into an electron without neutrinos, except at hopelessly tiny rates. Seeing it would prove new physics acting on flavor.
Precise: Measure or bound the coherent conversion rate $R_{\mathrm{mue}} = \Gamma(\mu- N \to e- N)/\Gamma(\mu- \mathrm{capture})$ on aluminum at single-event sensitivity $\sim 1e-17$ (Mu2e, COMET), versus the SINDRUM II gold limit of about $7e-13$. Combine with $\mu \to e\gamma$ (MEG II) and $\mu \to 3e$ (Mu3e) to identify the operator responsible if seen.
Settled by: Mu2e at Fermilab and COMET at J-PARC reaching single-event sensitivity near 1e-17.
Kind: well-posed
Answer type: yes-no
References: (empty)
Literature status: open
Status note: MEG II reported $\mathrm{BR}(\mu \to e \gamma) < \sim 1.5e-13$ in 2025 (moderate confidence on the exact value); Mu2e and COMET are expected to start physics running around 2027.
Posed since: (empty)