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The Mu3e experiment aims to search for the lepton-flavour violating decay μ+ → e+e+e− with a sensitivity to one signal decay in 1016 muon decays at a muon stopping rate of 2 × 109 muons/s. With currently available rates of 108 muons/s, a sensitivity on the branching ratio of 10-15 is the aim of the first phase. This will allow for tests of new physics models with enhanced branching ratios for lepton-flavour violating processes with an unprecedented precision. The experiment must be operated at very high muon rates all the while suppressing the background of the decay and accidental electron-positron combinations. Therefore, a tracking detector made of thin pixel sensors with additional scintillating fibres and tiles for precise time measurement will be built. The development of the subdetectors is ongoing while detector construction is still in preparation.

The workshop “Shedding light on X17” brings together scientists looking for the existence of a possible new light particle, often referred to as X17. This hypothetical particle can explain the resonant structure observed at ∼  17 MeV in the invariant mass of electron-positron pairs, produced after excitation of nuclei such as 8 Be and 4 He by means of proton beams at the Atomki Laboratory in Debrecen. The purpose of the workshop is to discuss implications of this anomaly, in particular theoretical interpretations as well as present and future experiments aiming at confirming the result and/or at providing experimental evidence for its interpretation.

The observation of lepton flavour violation (LFV) in the charged lepton sector would be an unambiguous sign of physics beyond the Standard Model (BSM), and thus, it is the channel of choice for many BSM searches. LFV searches in muon decays in particular benefit from the fact that muons can be easily produced at high rates. There is a global effort to search for LFV at high-intensity muon sources to which the upcoming Mu3e experiment at the Paul Scherrer Institute (PSI) will contribute. The Mu3e Collaboration aims to perform a background-free search for the LFV decay \\(\\mu^+\\to e^+e^-e^+\\) with an unprecedented sensitivity in the order of \\(10^{-15}\\) in the first phase of operation and \\(10^{-16}\\) in the final phase - an improvement over the preceding SINDRUM experiment by four orders of magnitude. The high muon stopping rates and low momenta of the decay electrons make high demands on momentum and time resolution and on the data acquisition. The innovative experimental concept is based on a tracking detector built from novel ultra-thin silicon pixel sensors and scintillating fibres and tiles as well as online event reconstruction and filtering in real time.

The Mu3e experiment aims to search for the lepton-flavour violating decay \\(\\mu^+\\rightarrow\\ \\text{e}^+ \\text{e}^+ \\text{e}^-\\) with a sensitivity to one signal decay in \\(10^{16}\\) muon decays at a muon stopping rate of \\(2\\times 10^9 \\frac{\\mu}{\\text{s}}\\). With currently available rates of \\(10^8 \\frac{\\mu}{\\text{s}}\\), a sensitivity on the branching ratio of \\(10^{-15}\\) is the aim of the first phase. This will allow for tests of new physics models with enhanced branching ratios for lepton-flavour violating processes with an unprecedented precision. The experiment must be operated at very high muon rates all the while suppressing the background of the decay \\(\\mu^+\\rightarrow\\ \\text{e}^+ \\text{e}^- \\text{e}^+ \\overline{\\nu}_\\mu \\nu_\\text{e}\\) and accidental electron-positron combinations. Therefore, a tracking detector made of thin pixel sensors with additional scintillating fibres and tiles for precise time measurement will be built. The development of the subdetectors is ongoing while detector construction is still in preparation.

The Mu3e experiment aims to find or exclude the lepton flavour violating decay \\(\\mu^+\\to e^+e^-e^+\\) with a sensitivity of one in 10\\(^{16}\\) muon decays. The first phase of the experiment is currently under construction at the Paul Scherrer Institute (PSI, Switzerland), where beams with up to 10\\(^8\\) muons per second are available. The detector will consist of an ultra-thin pixel tracker made from High-Voltage Monolithic Active Pixel Sensors (HV-MAPS), complemented by scintillating tiles and fibres for precise timing measurements. The experiment produces about 100 Gbit/s of zero-suppressed data which are transported to a filter farm using a network of FPGAs and fast optical links. On the filter farm, tracks and three-particle vertices are reconstructed using highly parallel algorithms running on graphics processing units, leading to a reduction of the data to 100 Mbyte/s for mass storage and offline analysis. The paper introduces the system design and hardware implementation of the Mu3e data acquisition and filter farm.

The MuPix7 is a prototype high voltage monolithic active pixel sensor with 103 times 80 um2 pixels thinned to 64 um and incorporating the complete read-out circuitry including a 1.25 Gbit/s differential data link. Using data taken at the DESY electron test beam, we demonstrate an efficiency of 99.3% and a time resolution of 14 ns. The efficiency and time resolution are studied with sub-pixel resolution and reproduced in simulations.

Mu3e is a novel experiment searching for charged lepton flavor violation in the rare decay \\(\\mu^+ \\rightarrow e^+e^-e^+\\). Decay vertex position, decay time and particle momenta have to be precisely measured in order to reject both accidental and physics background. A silicon pixel tracker based on \\(50\\,\\mu\\)m thin high voltage monolithic active pixel sensors (HV-MAPS) in a 1 T solenoidal magnetic field provides precise vertex and momentum information. The MuPix chip combines pixel sensor cells with integrated analog electronics and a periphery with a complete digital readout. The MuPix7 is the first HV-MAPS prototype implementing all functionalities of the final sensor including a readout state machine and high speed serialization with 1.25 Gbit/s data output, allowing for a streaming readout in parallel to the data taking. The observed efficiency of the MuPix7 chip including the full readout system is \\(\\geq99\\%\\) in a high rate test beam.

The Mu3e experiment searches for the lepton flavour violating decay mu+ -> e+e-e+, aiming for a branching fraction sensitivity of 10^-16. This requires an excellent momentum resolution for low energy electrons, high rate capability and a large acceptance. In order to minimize multiple scattering, the amount of material has to be as small as possible. These challenges can be met with a tracker built from high-voltage monolithic active pixel sensors (HV-MAPS), which can be thinned to 50 um and which incorporate the complete read-out electronics on the sensor chip. To further minimise material, the sensors are supported by a mechanical structure built from 25 um thick Kapton foil and cooled with gaseous helium.

The observation of lepton flavour violation (LFV) in interactions involving charged leptons would be an unambiguous sign of physics beyond the Standard Model of particle physics. Given that muons can be produced at high intensities, searches for LFV with muons are particularly sensitive. In a global initiative, ongoing and upcoming experiments are aiming to discover physics beyond the Standard Model in the three golden muon LFV channels: \\(\\mu\\to e\\gamma\\), \\(\\mu\\to eee\\) and \\(\\mu\\)-to-\\(e\\) conversion on nuclei. With innovative detector concepts and new muon beam lines, these experiments will be able to investigate muon LFV in the coming years with sensitivities improved by up to four orders of magnitude compared to past searches. The current status of muon LFV searches is discussed and the ongoing MEG II and DeeMe experiments as well as the upcoming Mu2e, COMET and Mu3e experiments are presented.

The Mu3e experiment at the Paul Scherrer Institut will search for the lepton-number-violating decay \\mueee, extending the sensitivity by four orders of magnitude compared to existing limits. This probe of new physics is complementary to the existing collider, dark matter and neutrino particle physics programmes, and part of a global programme investigating the charged lepton flavour sector. As well as the main \\mueee search, Mu3e will also extend the sensitivity to low-mass dark photons, and additional flavour-violating decays involving long-lived or stable particles.