MEG (experiment)

MEG (experiment)
Name	MEG
Location	Paul Scherrer Institute
Country	Switzerland
Started	2008
Completed	2013
Field	Particle physics
Focus	Charged lepton flavour violation

MEG (experiment) was a particle physics experiment designed to search for the charged lepton flavour violating decay μ+ → e+γ at ultra-low branching ratios. Located at the Paul Scherrer Institute and using the muon beams delivered by the Swiss Muon Source, MEG combined precision detectors and large-scale collaborations to challenge predictions from extensions of the Standard Model (particle physics), such as Supersymmetry, Grand Unified Theory, and models with Seesaw mechanism neutrino masses.

Overview

The experiment was conceived to improve previous limits set by experiments at TRIUMF, Los Alamos National Laboratory, and Paul Scherrer Institute predecessors by several orders of magnitude. The collaboration drew on expertise from institutions including CERN, INFN, KEK, University of Oxford, Institut de Física d'Altes Energies, Max Planck Institute for Physics, and University of Tokyo. The central novelty was combining a high-intensity continuous muon beam with a liquid xenon calorimeter and a magnetic spectrometer based on a gradient-field superconducting magnet, enabling simultaneous precision measurement of positron momentum, timing, and photon energy.

Experimental Apparatus

The apparatus integrated multiple subsystems developed by groups from University of Pisa, University of Zurich, Paul Scherrer Institute, Ecole Polytechnique, University of Geneva, and Nagoya University. A superconducting magnet known as the COBRA magnet, employing techniques from Brookhaven National Laboratory cryogenics and KEK magnet engineering, provided a graded magnetic field for positron tracking. The positron spectrometer used low-mass drift chambers inspired by designs from Stanford Linear Accelerator Center and DESY, while timing counters adapted fast scintillator technology from Fermi National Accelerator Laboratory and Imperial College London. The photon detector was a 900-liter liquid xenon calorimeter using photomultiplier tubes developed in collaboration with teams from Rutherford Appleton Laboratory, CEA Saclay, and University of California, Berkeley. The beamline incorporated electrostatic separators and solenoids based on designs from TRIUMF and Los Alamos National Laboratory muon facilities.

Science Goals and Methods

MEG targeted branching fractions as low as 10^−13 for μ+ → e+γ to test flavour-violating operators predicted by frameworks including Minimal Supersymmetric Standard Model, Left–Right symmetric model, Little Higgs models, and models invoking heavy Majorana neutrino mediation like the Type I Seesaw. The measurement strategy combined kinematic constraints—positron momentum and photon energy near half the muon mass—with coincidence timing and relative angle measurements inspired by analyses at BaBar, Belle, and LHCb. Backgrounds from radiative muon decay and accidental coincidences were constrained using control samples from runs at varying beam intensities and by comparisons with Monte Carlo generators such as those developed at GEANT4 groups associated with CERN and KEK simulation teams.

Data Analysis and Calibration

Calibration and alignment exploited reference processes and sources, including Michel positron spectra measurements standardized by Paul Scherrer Institute calibration facilities, photon calibration using π0 decays from dedicated pion runs coordinated with accelerator teams at Paul Scherrer Institute, and timing calibration using pulsed lasers and radioactive sources procured with contributions from Institut Laue–Langevin and Brookhaven National Laboratory. Data analysis pipelines incorporated likelihood-based maximum-likelihood fits and multivariate classifiers trained with simulated samples produced by GEANT4 and validated against control samples from TRIUMF and CERN test beams. Systematic uncertainties were evaluated in collaboration with experts from University of California, Los Angeles, University of Pisa, and ETH Zurich using bootstrapping, toy Monte Carlo, and cross-calibration methods developed in part from techniques at ATLAS and CMS experiments.

Results and Publications

MEG published incremental improvements culminating in world-leading limits on the μ+ → e+γ branching ratio, with major results reported in journals and conferences attended by members from Physical Review Letters contributors, European Physical Journal C authors, and presentations at the International Conference on High Energy Physics and Lepton-Photon Conference. Key publications, authored by the collaboration including scientists from INFN, KEK, CERN, Paul Scherrer Institute, University of Tokyo, and Rutherford Appleton Laboratory, set stringent constraints that ruled out or tightly limited parameter space in models such as Supersymmetry with flavour-violating soft terms and certain Grand Unified Theory embeddings. The experimental outcomes guided subsequent proposals and informed the design of successor programs at Paul Scherrer Institute and proposals at J-PARC and Fermilab.

Collaboration and Timeline

The MEG collaboration consisted of institutions from Europe, Asia, and North America, including INFN Sezione di Pisa, University of Zurich, Nagoya University, KEK, CERN, Rutherford Appleton Laboratory, CEA Saclay, Paul Scherrer Institute, Max Planck Institute for Physics, University of Oxford, and University of Tokyo. The project timeline featured design and R&D in the early 2000s, commissioning and early data taking around 2008–2009, full data taking in the 2010s, and final published limits and decommissioning around 2013–2016, after which a follow-up experiment leveraging upgraded technology and lessons learned from MEG was proposed by groups at Paul Scherrer Institute, RIKEN, and TRIUMF.

Category:Particle physics experiments