Abstract
The Mu2e experiment will search for the charged-lepton flavor violating (CLFV) neutrino-less conversion of a negative muon into an electron in the field of a nucleus. The conversion process results in a monochromatic electron with an energy of 104.97 MeV, slightly below the muon rest mass. The goal of the experiment is to improve the previous upper limit by four orders of magnitude and reach a SES (single event sensitivity) of $3 \times 10^{-17}$ on the conversion rate, a 90\% CL of $8 \times 10^{-17}$, and a $5\sigma$ discovery reach at $2 \times 10^{-16}$. The experiment will use an intense pulsed negative muon beam. The pulsed beam is essential to reducing backgrounds. The other essential element is a sophisticated magnetic system composed of three consecutive solenoids that form the muon beam. Mu2e will use an aluminum target and examine $\sim10^{18}$ stopped muons in three years of running. The Mu2e experiment is under design and construction at the Fermilab Muon Campus. The experiment will begin operations in 2022 and will require about three years of data-taking. Upgrades to other materials than aluminum are already being planned. This article is written specifically for younger researchers to bridge the gap between conference presentations and detailed design reports, and examines issues not covered in the former without the details of the latter.
Highlights
Mu2e will search for the charged-lepton flavor violating process μ−N → e−N by measuring the ratio μ−N → e−N Rμe = μ−N → all muon captures (1)The expected reach, as of this writing, will have a single-event sensitivity of 3 × 10−17, a 90% CL of 8 × 10−17, and a 5σ discovery sensitivity of 2 × 10−16
We have not yet discussed the detectors, it is useful to note that the Mu2e detectors are annular: the muon beam and stopping target are centered along the magnetic axis of the solenoid and the detectors are downstream of it, arranged in annuli
The Z dependence comes from photonic contributions to muon-toelectron conversion, first calculated in Feinberg et al [12]; the coherent rate goes as Z5 and normalizing to the total capture rate reduces the dependence to Z4
Summary
The intensity of the FNAL muon beam is ∼ 10, 000 times greater than those of the prior generation of muon-to-electron conversion experiments;. We will repeatedly discuss those items and the reader should keep them in mind 1 This examination of Mu2e provides an opportunity to complement global review articles, conference presentations, and detailed design reports. We will examine: (1) experimental considerations: the most significant backgrounds and how they determine the design of the experiment; (2) the formation of the muon beam and the solenoid system used to make it; (3) the detector and monitoring systems. We focus on aspects of Mu2e not normally covered in general talks, such as the extinction system and stopping target monitor. It generally adheres to the design presented in the TDR with occasional updates
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