Design and Status of the Mu2e Crystal Calorimeter

Abstract

The Mu2e experiment at Fermilab searches for the charged-lepton flavour violating (CLFV) conversion of a negative muon into an electron in the field of an aluminum nucleus, with a distinctive signature of a mono-energetic electron of energy slightly below the muon rest mass (104.967 MeV). The Mu2e goal is to improve by four orders of magnitude the search sensitivity with respect to the previous experiments. Any observation of a CLFV signal will be a clear indication of new physics.The Mu2e detector is composed of a tracker, an electromagnetic calorimeter and an external veto for cosmic rays surrounding the solenoid. The calorimeter plays an important role in providing particle identification capabilities, a fast online trigger filter, a seed for track reconstruction while working in vacuum, in the presence of 1 T axial magnetic field and in an harsh radiation environment. The calorimeter requirements are to provide a large acceptance for 100 MeV electrons and reach at these energies: (a) a time resolution better than 0.5 ns; (b) an energy resolution < 10% and (c) a position resolution of 1 cm.The calorimeter design consists of two disks, each one made of 674 undoped CsI crystals read by two large area arrays of UV-extended SiPMs. We report here the construction and test of the Module-0 prototype. The Module-0 has been exposed to an electron beam in the energy range around 100 MeV at the Beam Test Facility in Frascati. Preliminary results of timing and energy resolution at normal incidence are shown. A discussion of the technical aspects of the calorimeter engineering is also reported in this paper.

Development, construction and tests of the Mu2e electromagnetic calorimeter mechanical structures

“…The main detectors employed by Mu2e are a straw-tube tracker [2] and an electromagnetic calorimeter [3] located inside a vessel with a 10 −4 Torr vacuum level, and surrounded by a large superconducting solenoid which generates an axial magnetic field of 1 T. The main calorimeter function is providing complementary information to the tracker to achieve a powerful μ/e separation which is crucial to extract the conversion electron signal from the expected overwhelming background [4]. The calorimeter is also exploited in a calorimeter-seeded track finder algorithm which improves track reconstruction efficiency and makes the algorithm more robust in high detector occupancy conditions.…”

Introduction to Charged Lepton Flavour Violation

“…The Mu2e apparatus [164], shown in Figure 18, consists of three main superconducting solenoids; The first two, named production and transport solenoid in Figure 18, are used to generate a high intensity, low-momentum, muon beam starting from a 8 GeV proton beam. The third solenoid, named "Detector Solenoid" in Figure 18, contains an Al stopping target, where the muons are stopped to generate the muonic atoms, and downstream to it we have a low-mass straw-tube tracker [165], followed by a pure-CsI crystal calorimeter [166]. Both detectors are left un-instrumented in the inner 38 cm to avoid any interaction with the largest majority (> 99%) of the low momenta electrons coming from the muon DIO processes in the stopping target.…”

Towards the construction of the Mu2e electromagnetic calorimeter at Fermilab

“…A radioactive source and a laser system provide energy calibration with 6 MeV photons and monitoring of the gains and of the timing offsets respectively. Moreover, the calorimeter performance was tested in 2017 at the Beam Test Facility [4] with an electron beam with energies ranging between 60 MeV and 120 MeV [5]. In Figure 2 the resulting energy and timing resolution, are reported.…”