Leading hadronic contribution to the muon magnetic anomaly from lattice QCD

International audienceAnomalous magnetic moments have guided the evolution of quantum field theory ever since its earliest stages, serving both as a stringent test of the theory at increasingly higher levels of precision and as a possible window to new physics. After decades of perfect agreement, the measured muon magnetic moment (which is known to a precision of about 0.5 parts per million both from theory and experiment) now deviates from the theoretical expectation by around 3.5σ. In order to accentuate or resolve this discrepancy, an experiment at Fermilab is currently underway and is aiming to improve the precision of the measurement to 0.14 ppm. But the theoretical calculation has to be improved as well. The largest source of error are low energy hadronic contributions, that can be evaluated either via a phenomenological approach or ab initio, directly from the standard model Lagrangian, using lattice QCD. We will see how lattice QCD can be used to compute the leading hadronic contribution to the muon magnetic moment: the one induced by hadronic vacuum polarization

Un calcul de la contribution hadronique dominante au moment magnétique anomal du muon en chromodynamique quantique sur réseau

Toute déviation significative par rapport aux prédictions du modèle standard des particules constitue une indication d'une nouvelle physique fondamentale. L'une de ces déviations concerne le moment magnétique anomal du muon, aμ. En avril 2021, une nouvelle mesure de aμ a été publiée par la "Muon g-2 Collaboration" de Fermilab, portant l'écart avec la prédiction de référence du modèle standard à 4,2σ. La contribution la plus incertaine dans la prédiction théorique de aμ est due aux effets de polarisation hadronique du vide (HVP), provenant de la chromodynamique quantique (QCD) à basse énergie. Dans cette thèse, nous présentons un calcul en QCD sur réseau de la contribution HVP d'ordre dominant à aμ. Il s'agit du premier calcul à atteindre une précision proche de celle de la prédiction de référence, basée sur des techniques dispersives et sur des mesures de la section efficace de l'annihilation électron-positron en hadrons. Contrairement à cette dernière, notre prédiction est en accord avec la mesure moyenne mondiale de aμ au niveau de 1,5σ, ce qui suggère qu'une nouvelle physique n'est peut-être pas nécessaire pour expliquer cette mesure. Cependant, cela se fait au prix d'une tension de 2,1σ avec la détermination de la contribution HVP basée sur les données et d'un désaccord de 3,7σ avec le calcul basé sur les données pour une quantité connexe, moins difficile à calculer sur le réseau. Ce dernier désaccord a été confirmé par d'autres calculs indépendants sur le réseau, ce qui indique que les tensions entre le réseau et les approches basées sur les données doivent être mieux comprises pour pouvoir tirer pleinement parti des mesures à venir de l'expérience de Fermilab.Any significant deviation from the predictions of the Standard Model of particle physics represents evidence of new, fundamental physics. One such deviation concerns the anomalous magnetic moment of the muon, aμ.In April 2021, the results of a new measurement of aμ were published by Fermilab's Muon g-2 Collaboration, bringing the discrepancy with the reference Standard Model prediction to 4.2σ. The most uncertain contribution in the theoretical prediction of aμ is due to hadronic vacuum polarization (HVP) effects, coming from low-energy quantum chromodynamics (QCD). In this thesis, we present a lattice QCD calculation of the leading-order HVP contribution to aμ. This is the first computation to reach a precision close to that of the reference prediction, based on dispersive techniques and on measurements of the cross-section for electron-positron annihilation into hadrons. Unlike the data-driven determination, our calculations yield a Standard Model prediction that agrees with the world-average measurement of aμ, at the 1.5σ level, suggesting that new physics may not be required to explain this measurement. However, this comes at the expense of a 2.1σ tension with the data-driven determination of the HVP contribution and a 3.7σ discrepancy with the data-driven computation for a related quantity, less difficult to calculate on the lattice. The latter discrepancy is now being confirmed by other, independent lattice calculations, indicating that the tensions between the lattice and the data-driven approaches, revealed by the calculations described in this thesis, must be better understood to be able to fully leverage the upcoming measurements by the Fermilab experiment

Muon g-2: BMW calculation of the hadronic vacuum polarization contribution

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QED and strong isospin corrections in the hadronic vacuum polarization contribution to the anomalous magnetic moment of the muon

International audienceRecently, the Budapest-Marseille-Wuppertal collaboration achieved sub-percent precision in the evaluation of the lowest-order hadronic vacuum polarization contribution to the muon $g_\mu-2$. At this level of precision, isospin-symmetric QCD is not sufficient. In this contribution we review how QED and strong-isospin-breaking effects have been included in our work. Isospin breaking is implemented by expanding the relevant correlation functions to second order in the electric charge $e$and to first order in $m_u-m_d$. The correction terms are then computed using isospin-symmetric configurations. The choice of this approach allows us to better distribute the available computing resources among the various contributions

JRJC 2019. Book of Proceedings

International audienc

JRJC 2019. Book of Proceedings

International audienc

JRJC 2019. Book of Proceedings

International audienc