Gilberto Colangelo, Marco Incagli, "The puzzle of muon g-2"
Aula Berti-Pichat 2B (secondo piano)
INFN
The muon magnetic moment anomaly, aμ = (g − 2)/2 is one of the most precisely measured quantities as well as one of the most precisely calculable in the Standard Model, that’s why it constitutes a stringent test of the theory. For the last twenty years the BNL measurement has been pointing to a significant discrepancy from the theory prediction.
The Muon g-2 experiment at Fermilab aims to measure aμ with a final accuracy of 140 parts per billion. The experiment’s first result from the Run 1 was published in 2021 and confirmed the BNL measurement with a similar sensitivity. The new result will be presented, based on Run 2 and 3 data collected in 2019-20, which amount to a factor of four more data, thus entering a new sensitivity regime to g-2. The improvement in the accuracy of aμ measurement will be discussed, as well as the future prospects for the experiment.
The comparison with the Standard Model predictions will be discussed. After a brief introduction on the QED and electroweak contributions, the two main hadronic contributions will be addressed: hadronic vacuum polarization (HVP) and hadronic light-by-light (HLbL). Both of them can be calculated either with a dispersive, data-driven approach, or on the lattice. The two approaches provide compatible results for the HLbL contribution, which is currently known with a ~20% uncertainty, with a final goal of 10%. For the HVP, the precision requirements are much higher, with a final goal below 0.5%. For this contribution there is a discrepancy between the dispersive approach based on e+e- experiments and the lattice calculation by the BMW collaboration. Moreover, a recent, yet unpublished measurement of the e+e- --> hadrons cross section by the CMD-3 collaboration, disagrees with all previous measurements at the level of 2.5-5 sigmas, whereas it agrees with the BMW result. At the end the future prospects will be discussed, including the role that the MUonE experiment could play for a clarification of this situation.
