Fermilab Muon g-2

Fermilab Muon g-2
Name	Muon g−2 Experiment (Fermilab)
Location	Fermilab
Established	2013 (relocation of storage ring); data-taking 2018–
Principal investigators	David W. Hertzog, Roberto Roberto, Gianpaolo Venanzoni
Type	High-precision particle physics experiment
Facility	Fermilab
Equipment	Muon storage ring (1.45 T), electrostatic quadrupoles, calorimeters, trackers

Fermilab Muon g-2

The Fermilab Muon g−2 experiment is a precision measurement program at Fermilab that seeks to determine the anomalous magnetic moment of the muon, a fundamental parameter connected to quantum field theory, using a high-precision magnetic storage ring originally built for the Brookhaven National Laboratory experiment. The collaboration involves institutions such as Argonne National Laboratory, Los Alamos National Laboratory, University of Washington, and international partners including CERN groups; results are compared with predictions from Standard Model calculations contributed by communities around Feynman diagram computations, lattice gauge theory teams, and hadronic cross-section measurements.

Overview

The experiment measures the anomaly aμ = (g−2)/2 by observing the spin precession of polarized positive muons confined in a highly uniform magnetic field while they circulate in a 14-meter diameter storage ring developed at Brookhaven National Laboratory and transported to Fermilab. Precision depends on control of magnetic field mapping with NMR probes, timing and energy reconstruction in segmented electromagnetic calorimeters, and beam dynamics managed with electrostatic quadrupoles and kicker systems drawn from expertise at SLAC National Accelerator Laboratory and TRIUMF. Results are cross-checked with theory inputs from groups led by researchers affiliated with Rutgers University, University of Cambridge, Massachusetts Institute of Technology, and international lattice collaborations such as those centered at Fermilab and University of Regensburg.

History and background

The measurement effort traces lineage to the 1960s muon magnetic moment experiments at CERN and the 1999–2001 Brookhaven E821 experiment that reported a deviation between measured aμ and Standard Model expectations, prompting a long-standing tension. In 2013 the storage ring magnet was transported from Brookhaven National Laboratory to Fermilab in an operation coordinated with teams from U.S. Department of Energy laboratories and universities; the modern campaign assembled collaborators from University of Manchester, University of Tokyo, INFN institutions including Frascati National Laboratories, and groups with backgrounds from DESY and KEK. Theoretical advances by researchers at Princeton University, University of Bonn, University of Barcelona, and lattice QCD efforts at Columbia University and University of California, Los Angeles have refined Standard Model predictions against which the experiment is compared.

Experimental design and apparatus

The heart of the apparatus is a highly uniform superconducting storage ring producing a 1.45 tesla field in which polarized 3.094 GeV/c muons circulate; the ring hardware links engineering efforts at Brookhaven National Laboratory with precision magnet work informed by collaborators from Lawrence Berkeley National Laboratory and Purdue University. Injection and beam transport use kicker magnets and inflector channels developed with input from Los Alamos National Laboratory and SLAC National Accelerator Laboratory accelerator physics groups. Detection relies on 24 segmented lead-fluoride calorimeters instrumented with silicon photomultipliers and waveform digitizers developed with electronics groups at Fermilab and University of Illinois Urbana–Champaign; tracking detectors based on straw tubes were contributed by teams from University of California, Santa Barbara and University of Liverpool. A movable trolley of nuclear magnetic resonance probes maps the field with calibration tied to a spherical water standard maintained by metrology groups associated with National Institute of Standards and Technology.

Measurement and results

The experiment measures the anomalous precession frequency ωa from the time modulation of decay positron counts, and determines the magnetic field ωp via NMR to extract aμ = (g−2)/2. Initial published runs reported a value that, when combined with Brookhaven E821 results, strengthened a discrepancy with prevailing Standard Model evaluations, echoing discussions by theorists at CERN, Institute for Advanced Study, Brookhaven National Laboratory, and Yale University. The reported central value prompted extensive scrutiny from experimental teams at Hamburg University and theoretical groups at University of Washington and University of Pisa to reconcile hadronic vacuum polarization contributions and hadronic light-by-light scattering estimates.

Theoretical implications and comparisons

The measured discrepancy between experimental aμ and Standard Model predictions has implications for models of physics beyond the Standard Model, motivating phenomenology from groups at SLAC National Accelerator Laboratory, University of Chicago, University of California, Berkeley, and particle model building in contexts like supersymmetry, dark photon scenarios, and lepton-flavor models explored at CERN and Perimeter Institute. Lattice QCD calculations from collaborations at Fermilab, RBC/UKQCD, Budapest-Marseille-Wuppertal, and Mainz provide ab initio contributions to the hadronic terms, while dispersive approaches drawing on e+e− cross-section data from BaBar, KLOE, and BESIII inform alternative Standard Model evaluations. Disagreements among these theoretical avenues have spurred workshops at SLAC, CERN, and DESY to reconcile inputs and assess implications for new gauge bosons, leptoquarks, or supersymmetric particles considered at LHC experiments like ATLAS and CMS.

Data analysis and systematic uncertainties

Analysis pipelines developed by University and laboratory groups such as those at Fermilab, Rutgers University, University of Washington, and University of British Columbia process waveform data, perform pileup corrections, and extract ωa with blind-analysis techniques influenced by practices from Particle Data Group collaborations and statistics groups at Carnegie Mellon University. Systematic uncertainties arise from magnetic field nonuniformity mapped by the NMR trolley, gain stability of calorimeters calibrated with laser systems from SLAC National Accelerator Laboratory, muon beam dynamics including coherent betatron oscillations analyzed with input from CERN accelerator physics models, and radiative corrections computed by theory teams at Cornell University and University of Notre Dame.

Future plans and upgrades

The collaboration plans additional data-taking campaigns to reduce statistical uncertainty and is implementing hardware and software upgrades informed by experience at Brookhaven National Laboratory and SLAC National Accelerator Laboratory; upgrades include improved kicker timing, enhanced calorimeter calibration systems developed with University of Washington and University of Chicago electronics groups, and expanded lattice and dispersive theory coordination with teams at MIT and École Normale Supérieure. Results will continue to be cross-checked against new e+e− data from VEPP-2000 and BESIII while implications for searches at LHC experiments and proposed facilities like Future Circular Collider will be evaluated by the global particle physics community.

Category:Particle physics experiments