Mu2e Run I Sensitivity Projections for the Neutrinoless $\mu^- \to e^-$ Conversion Search in Aluminum

TL;DR

This paper estimates Mu2e Run I's sensitivity to neutrinoless muon-to-electron conversion in aluminum, predicting significant improvements over previous limits through detailed background analysis and optimized detection strategies.

Contribution

It provides the first detailed sensitivity projection for Mu2e Run I, including background estimation, analysis procedures, and optimization methods for the experiment.

Findings

Expected sensitivity of R_{μe} = 1.2 × 10^{-15} at 5σ significance

Projected upper limit of R_{μe} < 6.2 × 10^{-16} at 90% CL if no signal

Achieves a three-order-of-magnitude improvement over previous experimental limits

Abstract

The Mu2e experiment at Fermilab will search for the neutrinoless $\mu^- \to e^-$ conversion in the field of an aluminum nucleus. The Mu2e data-taking plan assumes two running periods, Run I and Run II, separated by an approximately two-year-long shutdown. This paper presents an estimate of the expected Mu2e Run I search sensitivity and includes a detailed discussion of the background sources, uncertainties of their prediction, analysis procedures, and the optimization of the experimental sensitivity. The expected Run I $5 \sigma$ discovery sensitivity is $R_{\mu e} = 1.2 \times 10^{-15}$, with a total expected background of $0.11 \pm 0.03$ events. In the absence of a signal, the expected upper limit is $R_{\mu e} < 6.2 \times 10^{-16}$ at 90% CL. This represents a three order of magnitude improvement over the current experimental limit of $R_{\mu e} < 7 \times 10^{-13}$ at 90\% CL set by the SINDRUM II experiment.