Contents

# Contents

## Idea

In perturbative quantum field theory, the magnetic moment of particles as predicted by classical field theory may receive corrections due to quantum effects. Such corrections are also called quantum anomalies, and hence one speaks of the anomalous magnetic moments, traditionally denoted by “$g-2$”.

The archetypical example is the anomalous magnetic moment of the electron in quantum electrodynamics, which is famous as the pQFT-prediction that matches experiment to an accuracy of about $10^{-12}$ (e.g.Scharf 95, (3.10.20)).

Similarly there is the anomalous magenetic moment $g_\mu - 2$ of the muon and the other leptons.

## Anomalies

In fact, the anomalous magnetic moment of the muon $g_\mu - 2$ has become notorious for apparently showing a noticeable discrepancy between theoretic prediction from the standard model of particle physics and its value as determined in experiment. The discrepancy is now found to have statistical significance around 3.5σ (DHMZ 17) or 4σ (Jegerlehner 18a). Recent measurements may even show a possible deviation around 2.5 σ for the electron’s anomalous magnetic moment (see Falkowski 18). Details depend on understanding of non-perturbative effects (Jegerlehner 18b, section 2).

If these experimental “anomalies” (in the sense of phenomenology) in the anomalous magnetic moment $g_\mu - 2$ (and possibly even in $g_e -2$) are real (the established rule of thumb is that deviations are established once their statistical significance reaches 5σ, see here), they point to “new physics” beyond the standard model of particle physics. See also at flavour anomaly.

In fact Lyons 13b argued that the detection-threshold of the statistical significance of anomalies here should be just 4σ, which would mean that they should already count as being detected:

table taken from Lyons 13b, p. 4

Possible explanations for the anomomalies in the anomalous magnetic moments is the existence of leptoquarks (Bauer-Neubert 15, CCDM 16, Falkowski 17, Müller 18), which at the same time are a candidate for explaining the flavour anomalies (see also Chiang-Okada 17).

## Contributions

### QED contributions

(…) for the electron see e.g. Scharf 95, section 3.10 (…)

### Quantum gravity contributions

The further corrections of 1-loop perturbative quantum gravity to the anomalous magnetic moment of the electron and the muon have been computed in (Berends-Gastman 75) and found to be finite without need for renormalization. These Feynman diagrams contribute:

### Axion contributions

Possible contributions to and xconstraints on $g_{lep}-2$ from hypothetical axions are discussed in ACGM 08, MMPP 16, BNT 17

## References

### General

Basic discussion:

• Othmar Steinmann, What is the Magnetic Moment of the Electron?, Commun.Math.Phys. 237 (2003) 181-201 (arXiv:hep-ph/0211187)

• Kirill Melnikov, Arkady Vainshtein, Theory of the Muon Anomalous Magnetic Moment, Springer Tracts in Modern Physics 216, 2006

• Friedrich Jegerlehner, The Anomalous Magnetic Moment of the Muon, Springer Tracts in Modern Physics 226, Springer-Verlag Berlin Heidelberg, 2008

Discussion of detection-threshold for the statistical significance of anomalies:

### Experiment and deviation

Discussion of precision experiment and possible deviation from theory:

• Michel Davier, Andreas Hoecker, Bogdan Malaescu, Zhiqing Zhang, Reevaluation of the hadronic vacuum polarisation contributions to the Standard Model predictions of the muon g-2 and alpha(mZ) using newest hadronic cross-section data, Eur. Phys. J. C (2017) 77: 827 (arXiv:1706.09436)

• J. L. Holzbauer on behalf of the Muon g-2 collaboration, The Muon g-2 Experiment Overview and Status, Proceedings for The 19th International Workshop on Neutrinos from Accelerators (NUFACT 2017) (arXiv:1712.05980)

• Fred Jegerlehner, The Muon g-2 in Progress, Acta Physica Polonica 2018 (doi:10.5506/APhysPolB.49.1157, arXiv:1804.07409)

• Fred Jegerlehner, The Role of Mesons in Muon $g-2$ (arXiv:1809.07413)

Possible explanation of the anomaly in the anomalous magnetic moments in terms of leptoquarks:

• Martin Bauer, Matthias Neubert, One Leptoquark to Rule Them All: A Minimal Explanation for $R_{D^{(\ast)}}$, $R_K$ and $(g-2)_\mu$, Phys. Rev. Lett. 116, 141802 (2016) (arXiv:1511.01900)

• Estefania Coluccio Leskow, Andreas Crivellin, Giancarlo D’Ambrosio, Dario Müller, $(g-2)_\mu$, Lepton Flavour Violation and Z Decays with Leptoquarks: Correlations and Future Prospects, Phys. Rev. D 95, 055018 (2017) (arXiv:1612.06858)

• {BiswasShaw19} Anirban Biswas, Avirup Shaw, Reconciling dark matter, $R_{K^{(\ast)}}$ anomalies and $(g-2)_\mu$ in an $L_\mu-L_\tau$ scenario (arXiv:1903.08745)

• Adam Falkowski, Leptoquarks strike back, November 2017

• Cheng-Wei Chiang, Hiroshi Okada, A simple model for explaining muon-related anomalies and dark matter (arXiv:1711.07365)

• Dario Müller, Leptoquarks in Flavour Physics, EPJ Web of Conferences 179, 01015 (2018) (arXiv:1801.03380)

• Junichiro Kawamura, Stuart Raby, Andreas Trautner, Complete Vector-like Fourth Family and new $U(1)'$ for Muon Anomalies (arXiv:1906.11297)

### QED contributions

The computation of the anomalous magnetic dipole moment of the electron in QED is spelled out (via causal perturbation theory) in

### Gravity contributions

Corrections at 1-loop from quantum gravity are discussed in

• F. A. Berends, R. Gastmans, Quantum gravity and the electron and muon anomalous magnetic moment, Phys. Lett. B55 Issue 3 Feb 1975 311-312 (doi:10.1016/0370-2693(75)90608-5)

This discussion is adapted to supergravity in

• F. del Aguila, A. Culatti, R. Munoz-Tapia, M. Perez-Victoria, Supergravity corrections to $(g-2)_l$ in differential renormalization, Nuclear Physics B 504 (1997) 532-550 (arXiv:hep-ph/9702342)

### Axion contributions

Contribution of hypothetical axions to the anomalous magnetic moment of the electron and muon in QED:

• Yannis Semertzidis, Magnetic and Electric Dipole Moments in Storage Rings, chapter 6 of Markus Kuster, Georg Raffelt, Berta Beltrán (eds.), Axions: Theory, cosmology, and Experimental Searches, Lect. Notes Phys. 741 (Springer, Berlin Heidelberg 2008) (doi:10.1007/978-3-540-73518-2_2)

• Roberta Armillis, Claudio Coriano, Marco Guzzi, Simone Morelli, Axions and Anomaly-Mediated Interactions: The Green-Schwarz and Wess-Zumino Vertices at Higher Orders and g-2 of the muon, JHEP 0810:034,2008 (arXiv:0808.1882)

• W.J. Marciano, A. Masiero, P. Paradisi, M. Passera, Contributions of axion-like particles to lepton dipole moments, Phys. Rev. D 94, 115033 (2016) (arXiv:1607.01022)

• Martin Bauer, Matthias Neubert, Andrea Thamm, Collider Probes of Axion-Like Particles, J. High Energ. Phys. (2017) 2017: 44. (arXiv:1708.00443, doi:10.1007/JHEP12(2017)044)

The basic relevant Feynman diagrams are worked out here:

Last revised on June 28, 2019 at 06:36:18. See the history of this page for a list of all contributions to it.