Mu3e experiment: a search for lepton flavour violation in the decay of a muon to three electrons.

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The Mu3e experiment will study a very large numbers of muon decays. For this the team behind the experiment will build a detector that can measure with unprecedented accuracy the daughter particles produced in these decays. The experiment will look for a very specific type of decay in which an anti-muon decays to 2 positrons and an electron. Crucially, in this a process the anti-muon loses its "flavour" to become a positron. Although other particles such as quarks and neutrinos are known to do this, such a change of flavour has never been observed for the charged leptons (electrons, muons or taus). If the well-established standard model (SM) of particle physics is the complete story, processes involving the violation of the conservation of charged lepton flavour will not be observed. However, particle physicists expect that to address some big questions in physics, such as why in our universe matter dominates over antimatter, new models going beyond the standard model are needed. A plethora of such model have been formulated but substantive evidence to corroborate any of these remains absent. To date, previous experiments have excluded that muon decays involving charged lepton flavour violation occur even once in 1,000,000,000,000 decays. The results of the Mu3e experiment will be more sensitive by a factor 10,000 again, potentially adding 4 zeros to this already very impressive result; or - much better - finding evidence of the sought after decay process and with that, direct evidence of new physics. Another key question particle physicist are trying to answer is to understand the nature of the mysterious dark matter. The existence of dark matter is strongly established from astronomical observations, but its nature remains unknown. The unique capabilities of the Mu3e experiment to study with great precision the decays product of a very large number of muons, also allows the members of the team that will build and operate the detector to search for hypothetical dark matter candidate particles at low masses. Such particles would not have been be seen in any other experiment to date or planned.