Muon g-2 (Fermilab)

Muon g-2 (Fermilab)
Name	Muon g-2 (Fermilab)
Location	Fermilab
Established	2017

Muon g-2 (Fermilab) is a precision particle physics experiment at Fermilab measuring the anomalous magnetic moment of the muon to test predictions of the Standard Model and search for signs of new physics beyond known frameworks such as Supersymmetry, Dark matter, and Quantum chromodynamics. The experiment follows and improves upon measurements made at Brookhaven National Laboratory and interfaces with theoretical work from groups at CERN, DESY, SLAC National Accelerator Laboratory, and university collaborations including Harvard University, University of Chicago, and University of Washington. Results are relevant to interpretations by theorists associated with the Particle Data Group, the FNAL community, and global analyses including efforts at LHC experiments such as ATLAS and CMS.

Background and theoretical motivation

The anomalous magnetic moment aμ of the muon is defined relative to the Dirac g-factor from Paul Dirac, a cornerstone of Quantum electrodynamics research driven historically by figures such as Julian Schwinger, Richard Feynman, and Sin-Itiro Tomonaga. Precision tests juxtapose experimental determinations with theoretical calculations incorporating contributions from QED, electroweak effects described by Sheldon Glashow, Abdus Salam, and Steven Weinberg, and nonperturbative hadronic vacuum polarization and hadronic light-by-light scattering evaluated by lattice groups at institutions like Brookhaven National Laboratory, Riken, and University of Mainz. Discrepancies could indicate physics from candidates such as Supersymmetry, Axion, Z' boson, or Leptoquark models, and tie into anomalies studied at LHCb, Belle II, and IceCube.

Experimental apparatus and beamline

The apparatus relocates and upgrades a superconducting storage magnet originally used at Brookhaven National Laboratory to a new muon beamline at Fermilab and integrates hardware from vendors and labs including Argonne National Laboratory, Los Alamos National Laboratory, and TRIUMF. The storage ring, a precision-engineered C-shaped yoke and coil system, operates with field mapping provided by trolley probes developed in collaboration with University of Liverpool and University of Kentucky, while the beamline uses precession-control components from Main Injector and Recycler sections at Fermilab. Muon injection and kicker systems were developed with groups at University of Cambridge, Rutgers University, and Yale University, and the calorimeter and tracker systems incorporate silicon photomultipliers from suppliers linked to Stanford University and University of California, Berkeley.

Measurement methodology and data analysis

The experiment measures the anomalous precession frequency ωa of stored polarized muons and the magnetic field frequency ωp through NMR probes, comparing ωa/ωp to obtain aμ following procedures standardized by teams at Brookhaven National Laboratory, CERN, and National Institute of Standards and Technology. Event reconstruction uses calorimeter timing and tracker hits developed with software frameworks influenced by ROOT (software), with analysis code contributed by groups at Massachusetts Institute of Technology, Princeton University, and University of Michigan. Blind-analysis techniques and statistical treatments, including frequentist and Bayesian approaches discussed at workshops hosted by IHEP, DESY, and Perimeter Institute, were applied to mitigate bias, while cross-checks referenced Monte Carlo simulations from GEANT4 and beam dynamics studies from K. Peach-style formalisms.

Results and comparison with theory

Initial results released by the collaboration showed an aμ value deviating from the Standard Model expectation computed by groups including the Muon g-2 Theory Initiative, with theoretical inputs from lattice calculations at BMW Collaboration and dispersive evaluations from teams at CMD-3, SND, and BaBar. The combined experimental average, compared with predictions used by the Particle Data Group and contrasted against analyses discussed at meetings of the American Physical Society and European Physical Society, generated intense scrutiny and follow-up work from theorists at Princeton University, University of Bern, and CERN. Interpretations have been debated in the context of models proposed by researchers affiliated with Harvard University, University of Chicago, and Caltech.

Systematic uncertainties and error analysis

Systematic effects were evaluated for magnetic field uniformity measured by NMR probes calibrated against standards at National Institute of Standards and Technology and for beam dynamics uncertainties modeled with inputs from Fermilab Main Injector operations and tracker alignment from collaborations with University of Manchester and Columbia University. Detector effects such as pileup, gain stability, and calorimeter response were studied using test-beam results coordinated with SLAC National Accelerator Laboratory and CERN SPS groups, while theoretical uncertainty budgets for hadronic contributions drew on results from Lattice QCD consortia including RBC and UKQCD and ETM Collaboration.

Collaborations and timeline

The Muon g-2 collaboration comprises institutions including Fermilab, Brookhaven National Laboratory, University of Washington, University of Liverpool, University of Colorado Boulder, University of Kentucky, Rutgers University, and international partners from Japan, Italy, Germany, and United Kingdom. Key milestones include transport of the storage ring from Brookhaven National Laboratory to Fermilab in 2013, commissioning phases in 2017–2018, first physics runs reported in 2021, and subsequent data releases and combinations with legacy results coordinated with the Muon g-2 Theory Initiative and presentations at conferences such as the International Conference on High Energy Physics and Rencontres de Moriond.

Impact and future prospects

The experiment’s findings have driven renewed theoretical work at institutions like CERN, DESY, and Perimeter Institute and influenced search strategies at ATLAS, CMS, and LHCb. Planned upgrades and continued runs at Fermilab aim to reduce statistical and systematic uncertainties, while complementary measurements at facilities such as J-PARC and lattice improvements from Riken and EPFL are expected to refine comparisons; broader implications touch model-building efforts at Princeton University, Harvard University, and Caltech and may inform future projects at Future Circular Collider and next-generation precision experiments.

Category:Particle physics experiments