<p dir="ltr">The Mu2e experiment will search for neutrinoless, coherent conversion of a muon into an</p><p dir="ltr">electron in the field of an aluminum nucleus (μ−N ! e−N) at the sensitivity level of 10−17.</p><p dir="ltr">This conversion process is an example of Charged Lepton Flavor Violation (CLFV), which</p><p dir="ltr">has never been observed experimentally before. The Mu2e experiment tracker is designed</p><p dir="ltr">to accurately detect the 105 MeV/c conversion electron (CE) momentum in a uniform 1 T</p><p dir="ltr">magnetic field. The mono-energetic positrons (e+) at 69.8 MeV from the decay of positively charged</p><p dir="ltr">pions (p+) that have stopped in the aluminum stopping target are investigated as a</p><p dir="ltr">calibration source to measure the accuracy of absolute momentum scale. The backgrounds</p><p dir="ltr">for the calibration arise from μ+ decay-in-flight (DIF) backgrounds and other stopped p+</p><p dir="ltr">decays that produce reconstructed e+ tracks mimicking a signal trajectory originating from</p><p dir="ltr">the stopping target. The most significant background is the μ-DIF background. Therefore,</p><p dir="ltr">we identified the need for a momentum degrader placed at the entry of the Detector Solenoid,</p><p dir="ltr">to increase the pion stops in the stopping target and suppress the μ-DIF background. The</p><p dir="ltr">material of the degrader is chosen to be titanium (Ti). The thickness of degrader is optimized</p><p dir="ltr">by the pion stops efficiency to muon flux efficiency ratio and the 4mm Ti degrader is the</p><p dir="ltr">optimized one. The calibration signal and backgrounds are simulated with the 3mm and</p><p dir="ltr">4mm Ti degrader. The ratio of S/B is used as a figure of merit, S/B ? 1.85 for the 3mm Ti</p><p dir="ltr">degrader and S/B ? 2.93 for the 4mm Ti degrader. The 4mm Ti degrader performs better</p><p dir="ltr">than the 3mm Ti degrader in terms of S/B ratio. By fitting the reconstructed momentum</p><p dir="ltr">spectra of signal and backgrounds, we extract the signal distribution peak and width of</p><p dir="ltr">x0 = 69.268 ± 0.013 MeV/c and ? = 0.324 ± 0.009 MeV/c (with the 3mm Ti degrader),</p><p dir="ltr">x0 = 69.263 ± 0.013 MeV/c and ? = 0.299 ± 0.009 MeV/c (with the 4mm Ti degrader).</p><p dir="ltr">We also show that the peak shifts by backgrounds for both degraders are within 100 keV/c</p><p dir="ltr">momentum scale accuracy requirement.</p>

<p dir="ltr">The Mu2e experiment is searching for Beyond-Standard-Model, Charged Lepton Flavor Violation (CLFV) in the muon capture reaction μ⁻+ Al → e⁻ + Al. To compare the accessible energy scale of this experiment, the Large Hadron Collider (LHC) is capable of observing new physics at the few TeV mass scale. However, by searching for μ-to-e conversion at a branching ratio sensitivity of 10⁻¹⁷, Mu2e will probe for new physics at mass scales up to 10³ ∼ 10⁴TeV, far beyond the reach of any planned accelerator and surpassing the current world’s best limit by 10⁴ times. In addition, there is no competing Standard Model process that produces this decay to a branching ratio level < 10⁻⁵⁴. To report a reliable result, the number of stopped muons will be normalized to 10% precision utilizing two γ-ray transitions and one x-ray atomic transition. The first, directly proportional to the CLFV signal, is the 1808.7 keV γ-ray emitted promptly in the muon capture process. The second, the 2p→1s atomic transition of muonic aluminum, is the 346.8 keV x-ray line. The third, is the 844 keV γ-ray from the β-decay process. These signals need to be measured in the presence of an energy flux background of 3.2 x 10⁸TeV/sec, consisting of muons, electrons, neutrons, x-rays and γ-rays. Here, two com- 11 photon counting detectors are used in the luminosity measurement. One of them, the LaBr₃ detector, is capable of high rate operation up to and above 800 kcps and energy resolution of 0.7%, producing highly accurate statistical measurements. The other, the HPGe detector is capable of energy resolution of 0.1%, with limited rate capability ∼ 70 kcps, yet producing measurements having low systematic error. Once signals are found within the background, corrections must be understood and applied including: geometric factors, detector efficiency, branching ratio of the observed physics pro- cesses, signal loss due to propagation to the detector, interfering lines, event loss due to pile-up, event loss due to algorithm miscalculation, time evolution of the signal, and others. The normalization measurement will be reported out in real time every 5 to 10 minutes, and a comprehensive off-line analysis will be undertaken using merged data sets.</p>

<p>The ability of the Mu2e experiment to probe, or discover BSM physics in direct CLFV μ+ and π+ decay modes is estimated.</p>