Statut  Confirmé 
Série  COLLOQUIUMENS 
Domaines  physics 
Date  Mercredi 6 Décembre 2023 
Heure  13:30 
Institut  DPTPHYSENS 
Salle  ConfIV (E244)  Dépt de Physique de l'ENS  24 rue Lhomond 75005 PARIS 
Nom de l'orateur  Lellouch 
Prenom de l'orateur  Laurent 
Addresse email de l'orateur  
Institution de l'orateur  CNRS  Aix Marseille Université 
Titre  The mysterious magnetism of the muon 
Résumé  Nearly two decades ago, physicists at Brookhaven National Laboratory achieved a remarkable precision of 0.54 parts per million in measuring the muon's anomalous magnetic moment, aµ This measurement lead to a persistent discrepancy of over 3 standard deviations (3σ) between the experimental result and the reference Standard Model prediction, hinting at potential undiscovered elementary particles or fundamental forces. On April 7, 2021, Fermilab physicists presented initial results from an ongoing experiment, confirming Brookhaven's measurement and bringing the deviation with the theory prediction to a neardiscovery level of 4.2σ A subsequent publication this past summer further enhanced this discrepancy to 5.1σ exceeding the usual threshold for the discovery of new fundamental physics in the field of particle physics. In the meantime, the BudapestMarseilleWuppertal (BMW) collaboration employed largescale supercomputer simulations to calculate the contribution that most limits the precision of the Standard Model prediction, a contribution traditionally determined via a datadriven approach. This lattice quantum chromodynamics (QCD) calculation paints a different picture, significantly reducing the difference between theory and experiment. However, it introduces a discrepancy with the datadriven result. The presentation will begin with an introduction and a comprehensive discussion of the current experimental and theoretical status of $a_\mu$. A focus will be placed on BMW’s precise lattice QCD calculation and the confirmation by other research groups of parts of the computation. A framework for comparing the primary ingredients used in the lattice QCD and datadriven approaches will also be presented, along with the necessary steps to make a Standard Model prediction that should allow to determine whether the final results from the Fermilab experiment, expected in 2025, indicate the presence of new fundamental physics. 
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