Mu2e

Mu2e
Name	Mu2e
Type	Particle physics experiment
Location	Fermilab
Start	2014
Status	Construction/Commissioning
Collaborators	Fermilab, Brookhaven National Laboratory, Caltech, University of Chicago, University of Virginia

Mu2e Mu2e is a high-sensitivity particle physics experiment based at Fermilab designed to search for charged-lepton-flavor violation by observing coherent neutrinoless conversion of muons into electrons in the field of an atomic nucleus. The experiment aims to improve limits set by earlier searches at TRIUMF, Paul Scherrer Institute, and Brookhaven National Laboratory by several orders of magnitude, offering potential implications for theories such as Supersymmetry, Grand Unified Theory, and models with heavy Majorana fermions. Mu2e is situated within a broad context including experiments like MEG, COMET, Mu3e, and LHC programs at CERN.

Overview

Mu2e is organized around a pulsed negative muon beam delivered to an aluminum stopping target inside a graded solenoid system developed at Fermilab with magnet technology influenced by work at Brookhaven National Laboratory and BNL Alternating Gradient Synchrotron. The collaboration includes institutions such as Caltech, University of Chicago, University of Virginia, University of Washington, Oxford University, University of Manchester, INFN, and KEK. The experiment follows a lineage of charged-lepton-flavor searches including predecessors at TRIUMF and PSI, and contemporary efforts at J-PARC through the COMET experiment.

Physics motivation and theory

Mu2e targets the process of muon-to-electron conversion in the field of a nucleus, a charged-lepton-flavor-violating transition forbidden in the original Standard Model without neutrino mass and highly suppressed even with neutrino oscillations. Observation would indicate physics beyond the Standard Model, consistent with extensions like Supersymmetry, Left-Right Symmetric Model, Seesaw mechanism, heavy Z' boson scenarios, or lepton-flavor-violating couplings from Two-Higgs-Doublet Model variants. The sensitivity goal probes effective mass scales comparable to those accessible indirectly by LHC searches for supersymmetric particles, flavor-changing neutral current constraints from B-factory experiments such as BaBar and Belle, and rare-decay limits from KOTO and NA62. Theoretical frameworks connecting muon conversion rates to other observables include model-independent effective-field-theory treatments, linking to operators studied in Muon g−2 and Electric dipole moment constraints.

Experimental design and detector components

The Mu2e apparatus consists of three superconducting solenoids: the production solenoid inspired by designs at Brookhaven National Laboratory and CERN, the transport solenoid with an S-shaped curved geometry similar to concepts used at ISIS, and the detector solenoid housing the stopping target and detector systems. Key detector components are the straw-tube tracker developed with contributions from JLab groups, a crystal-calorimeter array influenced by BaBar calorimetry R&D, and a cosmic-ray veto system modeled after systems used at Super-Kamiokande and SNO. The beamline uses a production target and pion capture scheme rooted in techniques from NuMI and NA61/SHINE experiments. Instrumentation includes low-mass tracking from Oxford University teams, waveform digitizers from SLAC heritage, and superconducting magnet engineering with expertise from Fermilab and KEK collaborators.

Data acquisition and analysis methods

Mu2e employs a triggered and streaming data-acquisition architecture leveraging digitization platforms similar to those used at CMS and ATLAS calorimeter readout efforts, with real-time filtering informed by algorithms developed at FNAL and BNL. Event selection focuses on isolated, monoenergetic electrons at ≈104.97 MeV, requiring precise momentum reconstruction from the straw tracker and energy/time matching from the calorimeter. Calibration strategies draw on techniques from BABAR, Belle II, and MEG II including use of cosmic rays, pulsed laser systems, and dedicated calibration runs. Analysis frameworks utilize software paradigms from ROOT-based toolkits popularized by CERN experiments, with systematic-error estimation guided by practices at MINOS, NOvA, and DUNE collaborations.

Project status and results

As of the latest phase, Mu2e moved through construction and subsystem commissioning with component delivery and integration milestones at Fermilab sites; magnet tests, tracker assembly, and calorimeter commissioning have been reported by participating institutions including Caltech and University of Chicago. Prototype and test-beam measurements with straw prototypes and crystal assemblies have been performed at facilities like SLAC National Accelerator Laboratory and CERN test beams. No observation of muon conversion has been announced; Mu2e aims to reach single-event-sensitivity improvements over past limits set by experiments at TRIUMF and PSI. Results will complement searches at COMET and indirect constraints from LHC and precision-flavor programs.

Collaborations and funding

The Mu2e collaboration encompasses universities and national laboratories across North America, Europe, and Asia, including Fermilab, Brookhaven National Laboratory, Caltech, University of Chicago, University of Virginia, Oxford University, INFN, and KEK. Funding and oversight involve agencies such as the U.S. Department of Energy, Particle Physics and Astronomy Research Council-style national agencies in partner countries, and institutional contributions analogous to arrangements seen for LHC and DUNE projects. Industrial partners and magnet vendors with histories supplying CERN and KEK have provided key components, and computing resources are coordinated with centers used by Fermilab and CERN experiments.

Challenges and future developments

Major challenges include backgrounds from decay-in-orbit electrons, radiative pion capture, and cosmic-ray-induced events—issues that require shielding, pulsed-beam timing strategies derived from Accelerator Division expertise at Fermilab, and a high-efficiency cosmic veto informed by Super-Kamiokande experience. Magnet quenches, stray-field control, and superconducting-cable procurement echo technical risks faced by LHC magnet programs and ITER procurement lessons. Future developments consider detector upgrades, alternate stopping targets paralleling studies at COMET and Mu3e, and synergies with next-generation flavor experiments and collider constraints from CERN and proposed facilities like the International Linear Collider. The discovery potential would profoundly affect theoretical directions involving Supersymmetry, Grand Unified Theory, and mechanisms for neutrino mass generation.

Category:Particle physics experiments