CALIBRATION OF THE MU2E ABSOLUTE MOMENTUM SCALE USING POSITIVE PION DECAYS TO POSITRON AND ELECTRON NEUTRINO

The Mu2e experiment will search for neutrinoless, coherent conversion of a muon into an

electron in the field of an aluminum nucleus (μ−N ! e−N) at the sensitivity level of 10−17.

This conversion process is an example of Charged Lepton Flavor Violation (CLFV), which

has never been observed experimentally before. The Mu2e experiment tracker is designed

to accurately detect the 105 MeV/c conversion electron (CE) momentum in a uniform 1 T

magnetic field. The mono-energetic positrons (e+) at 69.8 MeV from the decay of positively charged

pions (p+) that have stopped in the aluminum stopping target are investigated as a

calibration source to measure the accuracy of absolute momentum scale. The backgrounds

for the calibration arise from μ+ decay-in-flight (DIF) backgrounds and other stopped p+

decays that produce reconstructed e+ tracks mimicking a signal trajectory originating from

the stopping target. The most significant background is the μ-DIF background. Therefore,

we identified the need for a momentum degrader placed at the entry of the Detector Solenoid,

to increase the pion stops in the stopping target and suppress the μ-DIF background. The

material of the degrader is chosen to be titanium (Ti). The thickness of degrader is optimized

by the pion stops efficiency to muon flux efficiency ratio and the 4mm Ti degrader is the

optimized one. The calibration signal and backgrounds are simulated with the 3mm and

4mm Ti degrader. The ratio of S/B is used as a figure of merit, S/B ? 1.85 for the 3mm Ti

degrader and S/B ? 2.93 for the 4mm Ti degrader. The 4mm Ti degrader performs better

than the 3mm Ti degrader in terms of S/B ratio. By fitting the reconstructed momentum

spectra of signal and backgrounds, we extract the signal distribution peak and width of

x0 = 69.268 ± 0.013 MeV/c and ? = 0.324 ± 0.009 MeV/c (with the 3mm Ti degrader),

x0 = 69.263 ± 0.013 MeV/c and ? = 0.299 ± 0.009 MeV/c (with the 4mm Ti degrader).

We also show that the peak shifts by backgrounds for both degraders are within 100 keV/c

momentum scale accuracy requirement.