Muon g-2 (experiment)

Muon g-2 (experiment)
Name	Muon g-2 (experiment)
Location	Fermilab, Batavia, Illinois
Established	2017
Collaborators	Fermi National Accelerator Laboratory, Brookhaven National Laboratory, University of Washington, Yale University, Imperial College London, Paul Scherrer Institute

Muon g-2 (experiment) is a precision particle physics experiment measuring the anomalous magnetic moment of the muon. It compares a high-precision experimental determination of the muon's gyromagnetic ratio with predictions from the Standard Model (particle physics), testing contributions from virtual particles and potential effects from physics beyond the Standard Model (particle physics). The collaboration repurposed technologies and data from earlier projects at Brookhaven National Laboratory and integrated expertise from institutions such as Fermi National Accelerator Laboratory, CERN, and the Paul Scherrer Institute.

Background and theoretical motivation

The measurement targets the muon anomalous magnetic moment, aμ ≡ (g−2)/2, which arises from radiative corrections calculated within the Standard Model (particle physics), including contributions from Quantum Electrodynamics, Electroweak theory, and Quantum Chromodynamics. Discrepancies between experiment and theory can signal effects from proposed extensions such as Supersymmetry, Dark matter models, Leptoquarks, or new gauge bosons like a Z' boson. Historical context traces to precision tests performed at CERN and a notable prior experiment at Brookhaven National Laboratory’s Alternating Gradient Synchrotron that reported a tension with theoretical predictions, motivating the successor program hosted at Fermi National Accelerator Laboratory. Theoretical efforts by groups at MIT, University of Washington, Institute for Advanced Study, and CERN contributed to refined hadronic vacuum polarization and hadronic light-by-light scattering estimates, engaging collaborations across Princeton University, Columbia University, and University of Cambridge.

Experimental design and apparatus

The apparatus comprises a superconducting storage ring relocated from Brookhaven National Laboratory to Fermi National Accelerator Laboratory and instrumented with precision magnetic field shimming, inflector magnet, kicker systems, and an array of calorimeters. The ring maintains a highly uniform magnetic field generated by superconducting coils and monitored by a trolley system carrying nuclear magnetic resonance probes calibrated against standards traceable to National Institute of Standards and Technology. Detectors include segmented lead fluoride calorimeters read out by silicon photomultipliers developed with partners such as Yale University and University of Washington, and straw-tube tracking systems informed by designs from Imperial College London and University College London. Engineering and project management drew on practices from Fermi National Accelerator Laboratory’s accelerator division, with commissioning steps coordinated with teams from Brookhaven National Laboratory and Argonne National Laboratory.

Beam production and muon injection

Muon beams originate from high-intensity proton pulses produced by the Fermilab Booster and Recycler complex, striking a target station inspired by efforts at CERN and J-PARC to produce pions that decay to muons. Beamlines and delivery optics were designed with input from SLAC National Accelerator Laboratory and TRIUMF to select forward-decay muons with the appropriate momentum bite. The injection sequence uses a superconducting inflector magnet adapted from the Brookhaven National Laboratory design, followed by fast kicker magnets developed in collaboration with Paul Scherrer Institute experts to place muons onto stable orbits in the storage ring. Precision timing and synchronization systems referenced technologies from Oak Ridge National Laboratory and Lawrence Berkeley National Laboratory ensure accurate phase space control of the injected ensemble.

Data acquisition and analysis methods

Data acquisition systems employ high-bandwidth digitizers, clock distribution synchronized to cesium and rubidium standards, and real-time triggering developed with groups from Fermi National Accelerator Laboratory and CERN. Event reconstruction algorithms compute positron energies and arrival times from calorimeter waveforms; track reconstruction from straw trackers provides decay vertex and phase-space information. Analysis pipelines compare time-dependent decay asymmetries to extract the anomalous precession frequency ωa, while magnetic field mapping yields the average field ⟨B⟩ through weighted integration over muon distributions. Statistical analyses use maximum-likelihood fits and bootstrapping procedures implemented by teams at University of Chicago, University of Illinois Urbana-Champaign, and University of Michigan. Cross-checks utilized independent analyses from subgroups at Yale University, Princeton University, and Imperial College London.

Systematic uncertainties and calibration

Major systematic sources include magnetic field nonuniformity, beam dynamics such as electric field and pitch corrections, detector gain stability, and pileup in calorimeters. Calibration programs used pulsed laser systems tied to traceable standards from National Institute of Standards and Technology and trolley-based nuclear magnetic resonance probes cross-calibrated by teams at Brookhaven National Laboratory. Beam dynamics studies employed tracking data and simulations benchmarked against codes developed at CERN and SLAC National Accelerator Laboratory. Uncertainty budgeting followed protocols informed by prior work at Brookhaven National Laboratory and international collaborations across Princeton University and University of Cambridge, achieving sub-ppm control of key systematic terms.

Results and scientific impact

Initial results reported a measured value of aμ that reinforced a persistent deviation from contemporary Standard Model predictions, intensifying theoretical re-evaluations by groups at MIT, Fermilab, CERN, and Institute for Advanced Study. The tension stimulated renewed lattice QCD computations from teams at Budker Institute of Nuclear Physics, Riken, University of Edinburgh, and Brookhaven National Laboratory, and prompted phenomenological studies across Harvard University, Columbia University, and University of Tokyo exploring implications for Supersymmetry, Dark matter models, and other beyond-Standard-Model scenarios. The experiment influenced planning at major laboratories including CERN and J-PARC for complementary precision tests and constrained parameter space for proposed new particles and interactions discussed at conferences such as ICHEP and meetings of the American Physical Society.

Future upgrades and follow-up experiments

Planned upgrades include increased beam intensity from Fermilab accelerator improvements, enhanced calorimeter segmentation with contributions from Imperial College London and Yale University, and refined magnetic shimming techniques informed by studies at Paul Scherrer Institute. Parallel efforts at J-PARC and proposals for next-generation muon g−2 measurements at CERN seek independent cross-checks using alternative muon production and storage technologies. Continued collaboration with lattice QCD groups at Riken, CERN, and Brookhaven National Laboratory aims to reduce theoretical uncertainties, while combined experimental and theoretical progress will further probe potential indications of new fundamental physics.

Category:Particle physics experiments