MEG II

MEG II
Name	MEG II
Location	Paul Scherrer Institute
Institution	Paul Scherrer Institute
Collaborators	University of Zurich, ETH Zurich, CERN, INFN, KEK, RIKEN, University of Tokyo, Osaka University, University of Cambridge
Status	Active
Start date	2014

MEG II MEG II is a particle physics experiment located at the Paul Scherrer Institute designed to search for charged lepton flavor violation via the rare decay μ+ → e+γ. The experiment is an upgraded successor to a previous effort carried out at the Paul Scherrer Institute in collaboration with institutions such as CERN, INFN, KEK, and RIKEN. MEG II brings together international groups from universities and laboratories including University of Zurich, ETH Zurich, University of Tokyo, Osaka University, and University of Cambridge to improve sensitivity to physics beyond the Standard Model.

Overview

MEG II builds on the earlier experiment hosted at the Paul Scherrer Institute and benefits from accelerator and detector developments influenced by projects at CERN and KEK. The experiment targets charged lepton flavor violation, a process not observed in the Standard Model with neutrino oscillations but predicted by many extensions like Supersymmetry, Grand Unified Theory, and Seesaw mechanism-based models. MEG II uses a high-intensity continuous muon beam derived from proton accelerators at facilities such as PSI High Intensity Proton Accelerator and is configured to reduce backgrounds studied in experiments like BaBar, Belle II, and MEG. The collaboration interacts with theoretical groups working on Lepton flavor violation, Neutrino mass models, and Flavor symmetries.

Scientific goals

The principal goal is to push the branching ratio sensitivity for μ+ → e+γ by an order of magnitude compared to the predecessor, constraining or discovering contributions from new physics scenarios such as Supersymmetry, Left–Right symmetry, Little Higgs models, Extra dimensions, Z' bosons, and Leptoquarks. MEG II aims to test predictions from Minimal Supersymmetric Standard Model, SO(10) Grand Unified Theory, and models incorporating the Type-I Seesaw. Secondary goals include precise studies of radiative muon decays, inputs relevant to interpretations of signals at LHC, ATLAS, CMS, and complementary constraints from experiments like Mu2e, COMET, Belle II, BaBar, and LSND. Results inform theoretical frameworks including Effective Field Theory analyses, global fits with data from Planck, Fermi Large Area Telescope, IceCube, and implications for Baryogenesis and Leptogenesis.

Experimental apparatus

The apparatus is centered on a superconducting magnet system derived conceptually from designs used in MEG and informed by magnet technology developments at CERN and KEK. A high-intensity surface muon beamline delivers muons from a target station resembling configurations from PSI beamlines. The design integrates photon detection inspired by calorimeter developments at Borexino, BaBar, and KOTO, and tracking systems influenced by Belle II, LHCb, and ATLAS trackers. Shielding and background mitigation strategies take cues from neutrino experiments such as Super-Kamiokande and SNO, while cryogenic and DAQ infrastructure share techniques from ALICE and CMS upgrades.

Detector systems

Key detector systems include a liquid xenon photon detector conceptually related to calorimetry at MEG and applications at XENON1T; a high-resolution positron spectrometer with a drift-chamber system influenced by BABAR DCH and LHCb VELO; and timing counters inspired by Belle TOF and CMS HCAL technology. Ancillary systems for alignment and calibration use methods from KLOE, NA62, and T2K. The detector suite incorporates photosensors and silicon photomultipliers whose development parallels efforts at J-PARC, KEK, PNPI, and DESY. Background rejection leverages veto systems akin to those in Mu2e and COMET.

Data acquisition and analysis

The data acquisition system uses waveform digitization and trigger schemes refined in experiments like MEG, LHCb, Belle II, and ATLAS upgrades. Real-time processing employs FPGA and GPU technologies comparable to implementations at CMS, ALICE, and CERN experiments. Analysis pipelines integrate simulation frameworks and statistical tools common to HEPData analyses and global fits used by groups participating in PDG reviews. Systematic studies incorporate calibration strategies from KOTO, NA62, and BESIII and combine likelihood methods frequent in ATLAS and CMS searches. Cross-checks and blind analysis protocols echo standards set by collaborations such as LSND and MiniBooNE.

Collaboration and organization

The collaboration comprises institutes including Paul Scherrer Institute, INFN, CERN, KEK, RIKEN, University of Tokyo, Osaka University, University of Zurich, ETH Zurich, University of Cambridge, University of Bologna, Ecole Polytechnique, Imperial College London, University of Manchester, TRIUMF, IHEP Beijing, Seoul National University, University of Melbourne, University of Sydney, University of California Berkeley, Harvard University, Princeton University, Yale University, University of Chicago, Columbia University, Stanford University, MIT, Caltech, Max Planck Institute for Physics, DESY, FNAL, JINR, PNPI, CEA Saclay, LAL Orsay, IPN Orsay, University of Padova, University of Pisa, University of Milan, University of Naples Federico II, University of Rome La Sapienza, University of Geneva, University of Lausanne, University of Antwerp, and Ghent University. Governance follows models used by CERN collaborations with spokespersons, institutional board structures, and physics and technical boards similar to those at ATLAS, CMS, LHCb, and ALICE.

Results and future prospects

MEG II aims to set new upper limits or potentially observe μ+ → e+γ, with sensitivity improvements guiding interpretations relevant to LHC searches in ATLAS and CMS and complementary muon experiments such as Mu2e and COMET. Early runs provide calibration data and background measurements analogous to commissioning phases at Belle II and LHC upgrades. Future prospects include synergy with next-generation facilities at CERN, KEK, PSI, and potential inputs to global analyses involving Planck, Fermi LAT, IceCube, and DESI. Depending on outcomes, results will shape research directions in Supersymmetry model building, Grand Unified Theory constraints, and proposals for successor experiments at major laboratories.

Category:Particle physics experiments