Muon g-2 experiment

Muon g-2 experiment. The Muon g-2 experiment is a high-precision particle physics investigation measuring the anomalous magnetic dipole moment of the muon. This fundamental property tests the predictive power of the Standard Model of particle physics, with any discrepancy hinting at new particles or forces. The current effort at Fermilab follows a seminal precursor at Brookhaven National Laboratory, with both collaborations involving hundreds of scientists from institutions worldwide.

Overview

The primary goal is to measure the muon's anomalous magnetic moment, denoted as *a*_μ, with unprecedented accuracy. This value is exquisitely sensitive to contributions from all known particles, including virtual ones predicted by quantum field theory. A persistent deviation from the Standard Model calculation would provide compelling evidence for physics beyond the Standard Model, such as supersymmetry or other exotic phenomena. The experiment represents a decades-long quest, bridging the work of pioneers like Polykarp Kusch and the modern computational might of lattice QCD theorists.

Theoretical background

The magnetic moment of a point-like spin-½ particle like the electron or muon is predicted by the Dirac equation to be exactly 2. Quantum corrections from virtual particles in quantum electrodynamics (QED) create a small deviation, the anomalous magnetic moment. For the muon, contributions from the strong nuclear force, described by quantum chromodynamics (QCD), and the weak nuclear force are also significant. Theoretical calculations, which integrate predictions from the Standard Model using data from experiments like those at CERN and SLAC, yield a precise expected value for *a*_μ. The tension between this prediction and experimental results constitutes the "muon g-2 anomaly."

Experimental setup

At the heart of the Fermilab experiment is a 14-meter-diameter superconducting storage ring, originally built for the earlier Brookhaven effort and relocated to Illinois. Polarized muons from the Fermilab accelerator complex are injected into this ring, which maintains a highly uniform magnetic field. As muons decay, producing positrons, an array of 24 calorimeters and tracking detectors measures the decay asymmetry. Key challenges include controlling the magnetic field uniformity to a few parts per billion and precisely mapping it using nuclear magnetic resonance probes calibrated against the magnetic moment of the proton.

Results and significance

The first results from the Fermilab collaboration, announced in 2021, confirmed and refined the earlier discrepancy found at Brookhaven. The combined experimental average shows a statistically significant deviation from the leading Standard Model prediction, with a significance exceeding 4.2 standard deviations. This announcement, coordinated with a major conference at Fermilab, sent ripples through the global physics community. While some theoretical calculations using lattice QCD from groups like the BMW collaboration show less tension, the experimental result remains a potent beacon for new physics, challenging models developed at institutions like KEK and DESY.

Future experiments

The Fermilab experiment continues to acquire data, aiming to halve the experimental uncertainty by 2025. Concurrently, the J-PARC facility in Japan is preparing a complementary experiment, E34, using a novel method with ultra-cold muons to measure *a*_μ independently. These parallel efforts, alongside advances in lattice QCD calculations by groups worldwide, are critical for resolving whether the anomaly signifies a breakthrough. The ultimate findings could guide the research programs of next-generation colliders, such as the proposed International Linear Collider or the Future Circular Collider at CERN.

Category:Particle physics experiments