Design and Feasibility of a Scintillating-Fiber Detector to Improve Energy Resolution and Sensitivity in the Mu2e Experiment

The Mu2e experiment at Fermilab will search for the neutrinoless conversion of muons into electrons, a charged lepton–flavor–violating process that would signal new physics beyond the Standard Model. Improving the energy resolution of the 105 MeV conversion electron is essential for reducing backgrounds and enhancing sensitivity, a goal also critical for the future Mu2e-II upgrade. A key limitation arises from the detector’s inability to distinguish electrons with downstream z-momentum (forward tracks) from those reflected upstream (backward tracks). This ambiguity introduces an asymmetry in the reconstructed energy spectrum, as backward electrons traverse the target and surrounding material twice, losing more energy and degrading momentum resolution. To address this, we perform detailed Geant4 simulations using scorer meshes to map the radiation environment near the target, generating spatial flux distributions of charged and neutral particles to evaluate material doses and detector survivability. These results guide the design of a proposed scintillating-fiber detector around the Stopping Target. Complementary tests at Boston University using Kuraray SCSF-78, SCSF-81, and SCSF-3HF fibers coupled to Hamamatsu SiPMs measure light yield, decay time, and timing resolution to optimize the detector configurations that can enhance Mu2e’s energy resolution and sensitivity.