Design and Fabrication of a Novel Large-area, High-efficiency Cosmic Ray Veto Detector for the Mu2e Experiment

The Muon to Electron Conversion (Mu2e) experiment is a high-precision
experiment being mounted at Fermilab, which will search for coherent,
neutrino-less muon-to-electron conversion in the presence of an atomic
nucleus. Such a process would exhibit charged lepton flavor violation
(CLFV), which has not yet been observed. Mu2e is designed to improve
the sensitivity by four orders of magnitude over the present limits. In the
search for beyond the standard model (BSM) physics, Mu2e is uniquely
sensitive to a wide range of models and indirectly probes mass scales
up to the energy scale of 10^4 TeV. By design, the backgrounds for the
experiment will be well understood and kept at a sub-event level, which
in the event of the observation of muon-to-electron conversion, will be
direct confirmation of BSM physics.
A significant background comes from processes initiated by cosmic-ray
muons, which will produce approximately one CLFV-like event
per day. In order to reduce this rate to less than one event over the
lifetime of the experiment, a large and highly efficient cosmic-ray veto
(CRV) detector is needed. The overall efficiency must be no less than
99:99%, a requirement that must be maintained in the presence of intense
backgrounds produced by proton and muon beams. A novel detector was
designed that employs long scintillator strips with embedded wavelength
shifting fibers, read out using silicon photomultipliers. The combination
of a high-light yield scintillator and state-of-the-art photosensors results
in a compact, highly efficient detector that is easy to manufacture and
whose design is being used or considered for use in a host of other
planned experiments. The design, fabrication, and performance of the
CRV is presented.