The Mu2e crystal calorimeter and improvements in the $\mu^-\mbox{N} \to e^-\mbox{N}$ search sensitivity

Abstract

The Mu2e experiment will search for Charged Lepton Flavor Violation (CLFV) looking at the conversion of a muon into an electron in the field of an aluminum nucleus. About $$7\cdot 10^{17}$$ muons, provided by a dedicated muon beam line in construction at the Fermi National Accelarator Laboratory (Fermilab), will be stopped in 3 years in the Aluminum target. The corresponding single event sensitivity will be $$2.5\cdot 10^{-17}$$. The Standard Model of particle physics, even extendend to include the finite neutrino masses, predicts the ratio Rμe between muon conversions and muon nuclear captures to be $$\sim 10^{- 52}$$. Several extensions of the Standard Model predict Rμe to be in the range of $$10^{-14} - 10^{-18}$$. % The current best experimental limit, set by the SINDRUM II experiment is $$7 \cdot 10^{-13}$$ @ $$90\%$$ CL. The Mu2e experiment plans to improve this experimental limit by four order of magnitude to test many of the possible extensions of the Standard Model. To reach this ambitious goal, the Mu2e experiment is expected to use an intense pulsed muon beam, and rely on a detector system composed of a straw tube tracker and a calorimeter made of pure CsI crystals. The calorimeter plays a central role in the Mu2e measurement, providing particle identification capabilities that are necessary for rejecting two of the most dangerous background sources that can mimic the μ⁻N → e⁻N conversion electron: cosmic muons and $$\bar{p}$$ induced background. The calorimeter information allows also to improve the tracking performance. Thanks to a calorimeter-seeded track finder algorithm, it is possible to increase the track reconstruction efficiency, and make it more robust with respect to the occupancy level. Expected performances of the calorimeter have been studied in a beam test at the Beam Test Facility in Frascati (Rome, Italy). A reduced scale calorimeter prototype has been exposed to an electron beam, with energy varying from 80 to 140 MeV, for measuring the timing resolution and validate the Monte Carlo prediction. A timing resolution $$\sigma_{\rm t}<200$$ ps @ 100 MeV has been obtained. Combination of the background rejection performance, and the improvements in the track reconstruction, have then been used in the calculation of the expected Mu2e sensitivity.