Speaker
David Hitlin
Description
The Mu2e experiment at Fermilab searches for the charged-lepton flavor violating
neutrino-less conversion of a negative muon into an electron in the field of
a aluminum nucleus. The dynamics of such a process is well
modelled by a two-body decay, resulting in a mono-energetic electron with
an energy slightly below the muon rest mass (104.967 MeV).
If no events are observed in three years of running, Mu2e will set a limit on
the ratio between the conversion and the capture rates, \convrate, of $\leq
6\ \times\ 10^{-17} (@ 90 \%$ C.L.). This will improve the current
limit by four orders of magnitude.
A very intense pulsed muon beam ($\sim 10^{10} \mu/$ sec) is stopped on
a target inside a long evacuated solenoid where the detector is located.
The Mu2e detector is composed of a tracker, an electromagnetic
calorimeter and a veto for cosmic rays externally surrounding the
detector solenoid. The calorimeter plays an important
role in providing excellent particle identification capabilities and an
online trigger filter while aiding the track reconstruction capabilities.
It should keep functionality in an environment where the neutron, proton and photon
backgrounds from muon capture processes and beam flash
deliver a dose of $\sim$ 120 Gy/year in the hottest area.
It will also need to work in 1 T axial magnetic
field and a $10^{-4}$ torr vacuum. The calorimeter requirements are
to provide a large acceptance for 100 MeV electrons and reach at this energies:
+ a time resolution better than 0.5 ns;
+ an energy resolution *O($5\%$)*;
+ a position resolution of *O(1)* cm.
The baseline calorimeter configuration consists of two disks, each one
made of $\sim$ 700 undoped CsI crystals read out by two large area
UV extended Silicon Photomultipliers (SIPM). These crystals emit at 310 nm
with a large light yield (30 pe/MeV) when coupled in air to the SIPMs and
provide a fast response and accurate timing having a time emission
of $\tau \sim$ 20 ns. These crystals match the requirements for stability
of response, high resolution and radiation hardness. SIPM signals are amplified,
shaped and then read out through 200 msps waveform digitizers
optically connected to the DAQ system. We present the calorimeter design,
the experimental tests and the simulation carried out to prove the
validity of the chosen configuration. In particular, we will summarise the
results of the test beam with electron beams in the energy range between
80 and 140 MeV and the irradiation program carried out both with crystals
and SiPM.
