Search for Charged Lepton Flavor Violation at the Mu2e Experiment

Author: Borrel, Léo L.

Year: 2026

Degree: Dissertation (Ph.D.)

Advisors: Hitlin, David G.; Porter, Frank C.

Committee Members: Wise, Mark B.; Hitlin, David G.; Porter, Frank C.; Filippone, Bradley W.; Echenard, Bertrand

Option: Physics

Abstract

The Muon to Electron conversion experiment (Mu2e) is a particle physics experiment under construction at the Fermi National Accelerator Laboratory (FNAL or Fermilab). Its goal is to search for signature of Charged Lepton Flavor Violation (CLFV) using the channel of neutrinoless muon conversion to electron in an aluminum target. It aims to improve the current limit by four orders of magnitude, reaching a single-event sensitivity of 3 × 10⁻¹⁷. The experimental signal is an electron with a 105 MeVc⁻¹ momentum being ejected from the stopping target. The main backgrounds are Standard Model muon decays in the stopping target, cosmic ray induced electrons, and radiative pion captures.

The Mu2e experiment contains two main detectors: a low-mass highly segmented tracker that measures the momentum of electrons, and an electromagnetic calorimeter that measures the energy of incoming charged particles. The calorimeter is composed of inorganic crystal scintillators that have been prepared and tested at Caltech and INFN before being assembled in the final structure at Fermilab. The Caltech group has also been involved in the energy calibration of the calorimeter through the design, commissioning, and operation of the source calibration system. It uses a neutron generator to activate a fluid containing fluorine atoms that radioactively decay into oxygen and generate 6.13 MeV gamma rays that are used as a reference energy for the calorimeter crystals.

The analysis of the results of the experiment has been developed to use the maximum likelihood estimation method. The choices of probability distribution functions used in the fit are described, and preliminary fitting results are displayed.

Finally, a phenomenological study has been performed to calculate the muon conversion rate of 236 isotopes. It includes the addition of muon X-ray experimental dataset to the existing electron scattering dataset, their combination using Barrett moment matching, the inclusion of permanent quadrupole deformation effect, and the estimation of neutron distribution for even-even nuclei.