anomalous magnetic moment of the muon

anomalous magnetic moment of the muon
Name	anomalous magnetic moment of the muon
Quantity	magnetic moment anomaly
SI units	dimensionless
Discovered	1947
Discovered by	Isidor Isaac Rabi

anomalous magnetic moment of the muon The anomalous magnetic moment of the muon is a precision observable quantifying the deviation of the muon's magnetic dipole moment from the Dirac value, central to tests of the Standard Model and probes of physics associated with CERN, Fermilab, and Brookhaven National Laboratory. High-precision determinations involve collaborations among institutions such as Muon g−2 Experiment (E821), Muong-2 (E989), and theoretical groups from Princeton University, Cornell University, and SLAC National Accelerator Laboratory. Discrepancies between measurement and theory have driven interest from physicists at Perimeter Institute, DESY, and KEK.

Introduction

The muon's magnetic moment was first studied in experiments influenced by techniques developed by Isidor Isaac Rabi and later refined in apparatus at Brookhaven National Laboratory and Fermilab. The anomaly a_μ = (g−2)/2 captures radiative corrections computed in frameworks associated with Richard Feynman, Julian Schwinger, and Sin-Itiro Tomonaga. Historical experimental milestones include results from the CERN muon storage ring program and the E821 experiment led by teams including David Hertzog and William Marciano. Contemporary interest surged after the 2021 Muon g–2 collaboration result at Fermilab suggested a tension with theoretical predictions from groups led by Tomas Blum, Andrzej Czarnecki, and Aleksey Starodubtsev.

Theoretical Background

The theoretical computation of a_μ synthesizes contributions calculated within the Quantum Electrodynamics framework developed by Richard Feynman and Julian Schwinger, electroweak effects associated with Sheldon Glashow, Steven Weinberg, and Abdus Salam, and hadronic effects studied by groups at CERN, Brookhaven, and Lattice Quantum Chromodynamics collaborations such as RBC/UKQCD and BMW Collaboration. Perturbative expansions rely on techniques pioneered by Gerard 't Hooft and Kenneth Wilson, while nonperturbative hadronic vacuum polarization and hadronic light-by-light scattering demand numerical simulations inspired by work from Martin Lüscher and Kenneth G. Wilson. Theoretical uncertainty budgets are influenced by calculations published by teams at MIT, University of Washington, and University of Mainz.

Experimental Measurements

Precision measurement of a_μ uses storage rings and magnetic field metrology developed at Brookhaven National Laboratory and Fermilab, with detector systems designed by collaborations including Muon g−2 Collaboration and institutions such as University of Chicago, University of Virginia, and University of Liverpool. Key experimental techniques trace to methods invented by Isidor Isaac Rabi and refined by Nicholas Bloembergen and Louis Alvarez. The 2006 results from E821 and the 2021 and 2023 results from Muong-2 (E989) employed magnetic field mapping referenced to standards at National Institute of Standards and Technology and exploited data analysis frameworks developed in part by researchers at Columbia University, University of Tokyo, and University of California, Berkeley. Systematic effects are constrained using auxiliary measurements from TRIUMF and beam dynamics studies at Paul Scherrer Institute.

Standard Model Contributions

Within the Standard Model, a_μ receives dominant contributions from Quantum Electrodynamics loops computed through orders connected to work by Julian Schwinger and higher-order expansions by T. Kinoshita and collaborators at Rutgers University and University of Tokyo. Electroweak contributions are small but calculable following formalisms associated with Sheldon Glashow and Steven Weinberg. Hadronic vacuum polarization is evaluated using dispersive integrals tied to experimental cross sections measured by BaBar, KLOE, and CMD-3, while hadronic light-by-light contributions are addressed by lattice efforts from BMW Collaboration, RBC/UKQCD, and analytic models developed by J. Bijnens and Marc Knecht. Tensions between dispersive-data-driven evaluations and lattice results have involved comparisons among groups at IHEP, INFN, and University of Mainz.

Beyond Standard Model Interpretations

Possible explanations for any discrepancy in a_μ invoke extensions proposed by theorists at CERN, Institute for Advanced Study, and Perimeter Institute, including supersymmetric frameworks advanced by Howard Georgi and Sergio Ferrara, dark photon models inspired by searches at BaBar and Belle II, and leptoquark scenarios developed in contexts studied by Brookhaven National Laboratory and Fermilab. Models invoking extra Higgs bosons connect to work by Graham G. Ross and Howard Haber, while explanations via heavy neutral leptons reference studies at CERN and KEK. Global fits combining a_μ with results from Large Hadron Collider experiments (for example teams at ATLAS and CMS) and flavor experiments at LHCb and Belle II constrain model parameter space.

Ongoing and Future Experiments

Current and planned efforts to refine a_μ involve Muong-2 (E989) continued runs at Fermilab, a proposed follow-up at J-PARC in Japan with collaboration from KEK and RIKEN, and supportive measurements from projects at TRIUMF and Paul Scherrer Institute. The interplay with lattice computations from BMW Collaboration, RBC/UKQCD, and theory groups at CERN and MIT will be critical; coordinated efforts include workshops hosted by Perimeter Institute and Institute for Advanced Study. Complementary probes at Large Hadron Collider experiments including ATLAS and CMS and intensity-frontier searches at Belle II will test new-physics explanations if the anomaly persists.

Category:Muon physics