• cosmics
• sensitivity
• physics
• particle
• Mu2e

The Mu2e experiment at Fermilab searches for the conversion of a negative muon into
an electron in the field of a nucleus. This process violates charged-lepton flavour conservation and is heavily suppressed in the Standard Model (SM), with a branching ratio
< 10^-50. Any evidence of it would be a clear indication for new physics beyond the SM.
Mu2e sets out to achieve a single event sensitivity of 3 x 10^-17 on the ratio
between the rate of muon conversions and the rate of muon captures in the nucleus.
This will improve by 4 orders of magnitude the previous limit for the process, allowing to test the predictions of different extensions of the SM.
The muon beam is obtained from the decays of pions and kaons produced in the interactions of an 8 GeV proton on a tungsten target. Muons are directed by a graded magnetic field to an aluminum target, located in the last part of the detector, where they eventually stop
and convert.
The result of the conversion is a monochromatic electron of 105 MeV/c momentum.
A straw tube tracker and an electromagnetic calorimeter are optimized to identify
this electron against the possible sources of background events.
A Cosmic Ray Veto(CRV) system, covering the last part of the detector, is used to suppress the dominant background due to cosmic rays.
During the last year, the Mu2e run plan has undergone important modifications: instead of
the continuous running for 3 years at full proton beam intensity, the experiment will run in
two phases. In the first phase, before the Fermilab accelerator shutdown for the neutrino upgrade, there will be a reduced concrete shielding and a reduced proton beam intensity and only
1/10 of the statistics will be collected. During the shutdown the detector will eventually be
refurbished, replacing the parts most sensitive to ageing or radiation damage.
This thesis will present an updated estimate of Mu2e sensitivity corresponding to the first period of data taking and a guess of the final sensitivity at the end of the second phase.
A large simulation campaign has produced the dataset samples corresponding to the conversion electron (CE) signal and the main sources of background: cosmic rays, decays-in-orbits(DIO), radiative pion captures (RPC), and antiprotons.
In the first part of this work some preliminary selection cuts have been defined to ensure a correct event reconstruction and CE signal identification.
The contribution of the main background sources has then been evaluated. As in the previous analyses, the cosmic background resulted to be the dominant one.
For this reason the following work has been focused on the rejection of cosmics background.
Two datasets of cosmics have been produced corresponding to the events with or without a sizeable energy deposit in the CRV.
A larger statistics has been produced for the events with low or null energy deposits. In this category of events the largest contribution to the background comes from muons entering from the solenoid hole and interacting with the solenoid walls producing a 105 MeV/c electron. The only chance to reduce this background is to enlarge the CRV coverage in the front part of the detector.
The study of the cosmic background component with a sizeable energy deposit in the CRV has put in evidence special topologies of events that can be suppressed with a dedicated selection. A particular category of events escaping the CRV time coincidence is given by muons interacting in the calorimeter front disk and producing an upstream muon or electron in the tracker that is then reflected back by the magnetic field gradient and reconstructed as a downstream electron track. The presence of an additional cluster in the calorimeter, corresponding to the first muon interaction, can be used to identify these events. Even if the high CRV energy deposit cosmic dataset statistics needs to be increased, this calorimeter only selection appears to have a very good efficiency in rejecting the residual cosmic background while the inefficiency on the CE signal, also considering the beam particles pileup, is negligible. This selection can also be useful in the future to validate an improved upstream track reconstruction.
Once the final set of selection cuts has been determined the number of signal and background
events expected for Mu2e Phase 1 has been obtained. A scan of the possible time and momentum selection window has been performed to get the optimized 5 sigma discovery reach. The corresponding 90% C.L. upper limit have also been obtained confirming the expectations that, after Phase 1, Mu2e will be able to improve by a factor 10^3 today's limit.
In the last section a projection of the final Mu2e sensitivity has been made: the Mu2e goal of a factor 10^4 improvement of the current world limit is confirmed.