Mu3e

Mu3e
Name	Mu3e
Location	Paul Scherrer Institute
Established	2011
Type	Particle physics experiment
Field	Particle physics
Participants	ETH Zurich, University of Geneva, Karlsruhe Institute of Technology, University of Zurich, Paul Scherrer Institute, CERN, Deutsches Elektronen-Synchrotron, University of Manchester, Imperial College London, University of Liverpool, University of Bristol, Darmstadt University of Technology, Institute of Nuclear Physics Polish Academy of Sciences, Jagiellonian University, RWTH Aachen University, University of Tokyo, Tokyo Institute of Technology, University of Sussex, University of Sheffield, University of Birmingham, University of California, Berkeley, Massachusetts Institute of Technology, Harvard University, Princeton University, Yale University, University of Oxford, University of Cambridge, École Polytechnique Fédérale de Lausanne, Paul Scherrer Institut

Mu3e is a particle physics experiment designed to search for the charged lepton flavor violating decay of a muon into three electrons, a process forbidden in the Standard Model without neutrino mixing and extremely suppressed with neutrino masses. Located at the Paul Scherrer Institute, the project aims to reach unprecedented sensitivity by combining high-intensity muon beams, thin tracking detectors, and fast timing systems. The collaboration brings together institutions from across Europe, Asia, and North America to probe physics beyond the Standard Model and complement searches at facilities like CERN, Fermilab, and KEK.

Overview

The experiment targets the rare decay μ+ → e+ e+ e− using surface muon beams provided at the Paul Scherrer Institute’s Swiss Muon Source. The design emphasizes minimal material budgets, ultra-lightweight silicon detector technologies developed in collaborations with groups from ETH Zurich, École Polytechnique Fédérale de Lausanne, and Karlsruhe Institute of Technology. Mu3e’s sensitivity goals relate to branching ratios far below current limits set by experiments such as SINDRUM and planned searches at COMET and Mu2e. The collaboration coordinates with funding agencies and laboratories including the European Research Council, Deutsches Zentrum für Luft- und Raumfahrt, and national research councils to secure infrastructure and beam time.

Physics Motivation and Theory

Charged lepton flavor violation (CLFV) in muon decays is a clean probe of new physics frameworks like supersymmetry (Minimal Supersymmetric Standard Model), models with heavy neutral leptons (seesaw mechanism), leptoquarks, and extended gauge sectors such as Left–right symmetric model and Grand Unified Theory. Predictions for branching ratios in these frameworks can be enhanced by contributions from neutrino oscillation parameters measured by experiments like Super-Kamiokande, SNO, Daya Bay, T2K, and NOvA. Complementary constraints come from searches for muon-to-electron conversion at Mu2e and COMET and from rare tau decay studies at Belle II and BaBar. Mu3e tests mechanisms linked to supersymmetry breaking, flavor symmetries such as A4 or S3, and heavy mediator exchanges examined in ATLAS and CMS analyses at LHC Run 2 and LHC Run 3.

Experimental Design and Detector Components

The detector comprises a central target, concentric layers of high-voltage monolithic active pixel sensors (HV-MAPS), scintillating fibre stations, and scintillating tile timing arrays developed with expertise from University of Manchester, Imperial College London, and University of Geneva. A solenoidal magnet provides the uniform field similar to magnets used in CLEO and BESIII detectors. Mechanical and cooling solutions draw on techniques from ALICE and LHCb upgrades. Readout electronics integrate fast front-end ASICs inspired by developments at DESY and CERN microelectronics groups. The layout minimizes multiple scattering with low-mass supports employing composite materials researched at ETH Zurich and University of Cambridge.

Data Acquisition and Analysis Methods

A triggerless readout architecture streams data to a high-throughput event-filter farm using technologies from GridPP, OpenRTM, and middleware practices from European Grid Infrastructure. Real-time online reconstruction employs pattern recognition algorithms influenced by work at ATLAS, CMS, and Belle II and uses machine learning toolkits developed at University of Oxford and Imperial College London. Calibration and alignment procedures leverage cosmic-ray campaigns similar to those at Super-Kamiokande and MINOS; simulation frameworks use GEANT4 and detector models adapted from NA62 and MEG II. Background suppression strategies account for internal conversion processes and accidental coincidences studied in analyses by SINDRUM II and PIENU.

Results and Sensitivity Projections

Phase I sensitivity aims for branching ratio limits down to about 10^−15, improving on the SINDRUM limit, while later phases target ~10^−16 to 10^−17 depending on beam upgrades and detector performance analogous to staged approaches used in LHCb Upgrade II. Projected exclusion contours are interpreted within parameter spaces of CMSSM, heavy neutrino mixing matrices explored at T2K and NOvA, and leptoquark couplings constrained by ATLAS and CMS. Ongoing test-beam results for prototype sensors and timing systems have validated resolutions comparable to goals reported by MEG II and Mu2e collaborators. Sensitivity estimates incorporate systematic uncertainty studies akin to those performed in KOTO and NA62.

Collaboration and Project Status

The Mu3e collaboration includes universities and laboratories across Europe and beyond, with project management frameworks drawing on practices from CERN experiments and coordination with beamline operators at Paul Scherrer Institute. Milestones have included prototyping campaigns, engineering runs, and funding approvals similar to timelines seen in Belle II and LHCb. Recent updates report completion of sensor qualification tests and preparations for Phase I commissioning aligned with schedules from partner institutions such as ETH Zurich, Karlsruhe Institute of Technology, University of Geneva, and Imperial College London. Future plans involve staged detector installation, integration with the Swiss Muon Source, and continued collaboration with international groups including teams from KEK, Fermilab, and J-PARC.

Category:Particle physics experiments