A search for charged lepton flavour violation at the Fermilab Mu2e experiment

In the Standard Model, with neutrino masses, the branching ratio for charged lepton-flavour violating (cLFV) processes with muons is unobservable (O(10^{-50)), thus any observation is unambiguous evidence for new physics beyond the SM. Mu2e will extend the sensitivity to cLFV by four orders of magnitude through a search for the neutrinoless conversion of a muon to an electron in the field of a nucleus and will have the most comprehensive reach of the muon cLFV experiments.

One of the key elements of the Mu2e experiment is the Stopping Target Monitor (STM): a HPGe detector that measures the X-rays emitted when the muons are captured by the stopping target which in turn is used to determine the muon flux. This is one of the four detector systems in the experiment and is an analogue of the luminosity monitor at a collider.

Without the STM, Mu2e has no means to normalise the data nor to monitor in real-time the muon flux which is critical in evaluating the performance of the accelerator, solenoids and other detector systems.

In this project, the STM will be commissioned and integrated with the other Mu2e components. Its performance will be optimised using simulated data and the first measurements of the Mu2e flux will be undertaken, alongside an estimate of the flux uncertainty. The physics reach of Mu2e is principally determined by the performance of the straw tracker in reconstructing tracks with a robust momentum scale, with the best possible

resolution in a high rate environment. The resolution will be determined from double-tracked spallating cosmic rays and the momentum scale from a sample of stopped mu^+ decays.