Lepton Flavour Physics

Mu3e at Paul Scherrer Institut

Mu3e is searching for the flavour violating decay μ→eee as a powerful probe of physics beyond the Standard Model. It will extend sensitivities to this process by four orders of magnitude, sensitive to effects far beyond the direct reach of colliders.

To reach a target sensitivity on the branching ratio of 1E-16, Mu3e uses the world's highest intensity continuous muon beam which is located at PSI. Studying up to 10 billion muon decays per second, the signal we search for is three low-energy electrons coincident in space and time, with a combined energy of the muon mass. Mu3e uses cutting-edge ultra-thin HV-MAPS pixel sensors to minimise the impact of Coulomb scattering on the momentum and position measurements, and ultra-fast timing detectors to reduce combinatoric backgrounds.

Prof. Gavin Hesketh is the Physics Coordinator for Mu3e. Aside from analysis, we are also heavily involved in the commissioning of the experiment and contributing to the construction of the outer pixel layers.

For more information, contact Prof. Gavin Hesketh

Mu2e at Fermi National Accelerator Laboratory

The Mu2e Experiment at Fermilab will search for the lepton flavour violating muon conversion process in the field of a nucleus, μN→eN. This process is effectively forbidden in the Standard Model, but could be enhanced by new physics, making it a powerful probe that complements the purely leptonic process studied at Mu3e. Since the Standard Model prediction is unobservably small, any signal would be clear evidence of new physics at scales far beyond the LHC. To reach a sensitivity of 1E-17, Mu2e will stop around 2E18 muons in aluminium targets, using solenoids and magnets to deliver low-energy muons. The key signature is a mono-energetic electron near 104 MeV, requiring careful suppression and understanding of backgrounds from muon decays and pion capture.

UCL plays a central role in Mu2e through the Stopping Target Monitor (STM), a detector system that measures X-rays emitted when muons are captured by the aluminium target. This system, essential for determining the muon flux and normalising the data, is analogous to a luminosity monitor in collider experiments. UCL led the development of the data acquisition (DAQ) firmware and software, and oversee the commissioning and ongoing monitoring of the STM, ensuring Mu2e can monitor muon flux in real-time and thereby maximise its sensitivity to new physics.

For more information, contact Dr Rebecca Chislett or Dr Alex Keshavarzi