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Essentials of the Muon g-2

F. Jegerlehner

TL;DR

The paper surveys the muon g-2 problem, detailing both experimental technique and SM predictions. It emphasizes the dominant role of hadronic vacuum polarization and hadronic light-by-light contributions in the theory error, and it discusses how the measured 0.54 ppm Brookhaven result shows a 3.2σ deviation from the SM, potentially signaling new physics. The work surveys QED and electroweak inputs, outlines current uncertainties, and sketches future experimental and theoretical paths, including possible SUSY interpretations and the need for improved hadronic calculations. Overall, muon g-2 remains a centerpiece for precision tests of the SM and a potential gateway to new physics at the TeV scale.

Abstract

The muon anomalous magnetic moment is one of the most precisely measured quantities in particle physics. Recent high precision measurements (0.54ppm) at Brookhaven reveal a ``discrepancy'' by 3 standard deviations from the electroweak Standard Model which could be a hint for an unknown contribution from physics beyond the Standard Model. This triggered numerous speculations about the possible origin of the ``missing piece''. The remarkable 14-fold improvement of the previous CERN experiment, actually animated a multitude of new theoretical efforts which lead to a substantial improvement of the prediction of a_mu. The dominating uncertainty of the prediction, caused by strong interaction effects, could be reduced substantially, due to new hadronic cross section measurements in electron-positron annihilation at low energies. After an introduction and a brief description of the principle of the experiment, I present a major update and review the status of the theoretical prediction and discuss the role of the hadronic vacuum polarization effects and the hadronic light--by--light scattering contribution. Prospects for the future will be briefly discussed. As, in electroweak precision physics, the muon g-2 shows the largest established deviation between theory and experiment at present, it will remain one of the hot topics for further investigations.

Essentials of the Muon g-2

TL;DR

The paper surveys the muon g-2 problem, detailing both experimental technique and SM predictions. It emphasizes the dominant role of hadronic vacuum polarization and hadronic light-by-light contributions in the theory error, and it discusses how the measured 0.54 ppm Brookhaven result shows a 3.2σ deviation from the SM, potentially signaling new physics. The work surveys QED and electroweak inputs, outlines current uncertainties, and sketches future experimental and theoretical paths, including possible SUSY interpretations and the need for improved hadronic calculations. Overall, muon g-2 remains a centerpiece for precision tests of the SM and a potential gateway to new physics at the TeV scale.

Abstract

The muon anomalous magnetic moment is one of the most precisely measured quantities in particle physics. Recent high precision measurements (0.54ppm) at Brookhaven reveal a ``discrepancy'' by 3 standard deviations from the electroweak Standard Model which could be a hint for an unknown contribution from physics beyond the Standard Model. This triggered numerous speculations about the possible origin of the ``missing piece''. The remarkable 14-fold improvement of the previous CERN experiment, actually animated a multitude of new theoretical efforts which lead to a substantial improvement of the prediction of a_mu. The dominating uncertainty of the prediction, caused by strong interaction effects, could be reduced substantially, due to new hadronic cross section measurements in electron-positron annihilation at low energies. After an introduction and a brief description of the principle of the experiment, I present a major update and review the status of the theoretical prediction and discuss the role of the hadronic vacuum polarization effects and the hadronic light--by--light scattering contribution. Prospects for the future will be briefly discussed. As, in electroweak precision physics, the muon g-2 shows the largest established deviation between theory and experiment at present, it will remain one of the hot topics for further investigations.

Paper Structure

This paper contains 10 sections, 71 equations, 18 figures, 4 tables.

Table of Contents

  1. Lepton magnetic moments
  2. The BNL Muon $g-2$ Experiment
  3. The BNL Muon $g-2$ Experiment
  4. QED Prediction of $a_e$ and the Determination of $\alpha$
  5. Standard Model Prediction for $a_\mu$
  6. QED Contribution
  7. Weak Contributions
  8. Hadronic Contributions
  9. Theory Confronting the Experiment
  10. Prospects

Figures (18)

  • Figure 1: Spin transfer properties in production and decay of the muons (P=parity, C=charge conjugation). $\mu^-$ [$\mu^+$] is produced with positive [negative] helicity $h=\vec{s}\cdot\vec{p}/|\vec{p}|$, decay $e^-$ [$e^+$] have negative [positive] helicity, respectively
  • Figure 2: The schematics of muon injection and storage in the $g-2$ ring
  • Figure 3: Spin precession in the $g-2$ ring ($\sim 12^\circ$/circle)
  • Figure 4: Decay of $\mu^+$ and detection of the emitted $e^+$ (PMT=Photomultiplier)
  • Figure 5: Distribution of counts versus time for the 3.6 billion decays in the 2001 negative muon data-taking period. Courtesy of the E821 collaboration 02BNL04
  • ...and 13 more figures