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

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.

The expected impact of the proposed can be summarized in two main areas. First in terms of the direct economic and technological outputs which support industry and R&D outside of our own research base. Second in terms of the societal impact beyond the research itself.

The economic and technological impact is driven by the strong way in which this proposal links into R&D activities of the different groups in the development of novel HV-CMOS pixel sensors, ultra-low-mass solutions for mechanical supports, electrical services and cooling and fast electronic for high bandwidth data processing. In working directly with UK and worldwide companies to deliver technological solutions to meet the very challenging demand of this experiment, valuable expertise will be transferred from researchers and engineers on the project to these companies.

Just as the planar silicon silicon technology we developed for the LHC, HL-LHC and other particle physics experiments is today finding its way into new applications; for example in proton therapy applications, neutron detection or high resolution mass spectrometry. We fully expect that as HV-CMOS sensors are likely to replace planar silicon sensors in future particle physics experiments, they will do the same, a bit further down the line, for such commercial applications. The detectors similar to the ones developed for Mu3e, for example, could offer a lower cost alternative to the planar sensors used in these applications today. An added benefit of the new HV-MAPS sensors and the high bandwidth daq solutions developed to read them out, for such applications is that their implementation in new applications requires substantially less infrastructure than planar sensors (no separate readout ASICs, no custom front-end electronics, etc) whilst offering a comparable performance. These aspects in combination with the reduced cost per sensor of the CMOS process also open the door to the development of custom solutions for new applications.

Our societal impact is manifested through outreach activities and through generating public interest in science in general. A positive result from the experiment would constitute a major breakthrough in fundamental physics and would be expected to generate substantial public interest. Previous important discoveries received intensive attention from the press, had a substantial impact in raising public interest in fundamental physics and encouraged young people to consider physics as a degree topic or to take it as a topic at the earlier stages of their education.

The groups involved in the proposal have a strong track record on outreach activities, with members of this proposal participating in these programmes. We will use the work done for Mu3e to showcase how exciting science is both at a fundamental and technological level. Activities in which we will participate will include: programmes focussed on school children and their parents; events organised to showcase technology developments to industry; participation in science exhibitions, etc.