Muon g−2 or stress testing the SM

Tuesday 14 Oct 2025, 10:00 → 15:30 Europe/Rome

Aula Gismondi (Università degli Studi di Roma Tor Vergata)

Roberto Frezzotti (Istituto Nazionale di Fisica Nucleare), Giuseppe Di Sciascio (Istituto Nazionale di Fisica Nucleare)

Locandina.pdf

10:00 Welcome

1 Introduction

Speaker: Roberto Frezzotti (Univ. & INFN Roma Tor Vergata)

2 Final Results from the Muon g-2 Experiment at Fermilab

The Muon g-2 experiment at Fermilab has recently published the final measurement of the muon anomalous magnetic moment, reaching an unprecedented precision of 127 parts per billion (ppb), well below the original design goal of 140 ppb. This result serves as a critical reference point for theory, placing a benchmark to probe the Standard Model. The experiment is based on the precise determination of the anomalous spin precession frequency of muons in a magnetic field, extracted from the time distribution of decay positrons recorded by 24 electromagnetic calorimeters placed around a 14 m diameter storage ring. Equally crucial is the precise knowledge of the storage ring magnetic field and of the beam’s spatial and temporal distribution. The result presented in this talk is based on the full dataset collected between 2018 and 2023. I will present the result and discuss the experimental technique, the analysis details and the control of systematic effects that were decisive in achieving the final precision.

Speaker: Matteo Sorbara (Univ. & INFN Roma Tor Vergata)

3 The HVP contribution to the Muon g-2 in the SM from lattice QCD

The Muon g-2 Collaboration has recently measured the anomalous magnetic moment of the muon, $a_\mu$, with an unprecedented precision of about 0.13 ppm. To fully exploit this remarkable achievement, theoretical predictions must reach a comparable level of accuracy. A major challenge arises from the leading hadronic vacuum polarization (HVP) contribution, which needs to be determined with sub-percent precision. In this talk, I will discuss the latest lattice results for this contribution to $a_\mu$ evaluated "ab initio" in the Standard Model (SM) and also give an update on the ongoing calculations carried out by the Extended Twisted Mass Collaboration (ETMC).

Speaker: Giuseppe Gagliardi (Univ. & INFN Roma Tre)

4 Muon g-2 and the quest for new physics

The hadronic vacuum polarization (HVP) contributions to the muon g−2 are the crucial quantities to resolve whether new physics is present or not in the comparison between the Standard Model (SM) prediction and experimental measurements at Fermilab. They are commonly and historically determined via dispersion relations using a vast catalogue of experimentally measured, low-energy e+e−→hadrons cross section data as input. These dispersive estimates result in a SM prediction that exhibits a muon g−2 discrepancy of more than 5σ when compared to experiment. However, recent lattice QCD evaluations of the HVP and a new hadronic cross section measurement from the CMD-3 experiment favor a no-new-physics scenario and, therefore, exhibit a common tension with the previous e+e−→hadrons data. We discuss the current and
future implications of these two scenarios on other observables, such as the electron and tau g−2, the running of α and of the weak mixing angle, and the Muonium hyperfine splitting, that are also sensitive to the HVP contributions in the hope that they may provide independent tests of the current tensions observed in the muon g−2.

Speaker: Paride Paradisi (Univ. & INFN Padova)

12:00 Lunch Break

5 Muon g-2: issues with data driven approaches

After a brief review of the data driven approach for the determination of the hadronic contribution to the vacuum polarization, the focus of the talk will be on the challenges posed by the measurement of the hadronic cross section and possible issues of theoretical origin. Also the spacelike approach, implemented through the MUonE experiment and its future perspectives will be discussed.

Speaker: Fulvio Piccinini (GGI dir. & INFN Pavia)

6 Muon g-2: present and future experiments

The final result on the measurement of the muon gyromagnetic anomaly, recently released by the E989 Collaboration at Fermilab, is a solid textbook reference for theory and experiment alike. It is not, however, the end of the story. The discrepancies in the theoretical approaches between lattice and data-driven calculations, compounded by the tensions in the current experimental determinations of the leading hadronic vacuum polarization contribution, call for a precision observation conducted with a novel approach. The MUonE experiment at CERN tries to address this concern. Furthermore, a muon g-2 measurement affected by different systematic effects and uncertainties with respect to E989 is clearly highly desirable. The Muon g-2/EDM@JPARC project in Japan promises a new precision measurement of the muon gyromagnetic anomaly, exploiting a pure beam of low-energy muons and a compact magnetic environment.
I will discuss the current status of the MUonE experiment at CERN and the perspectives for joining forces with the ongoing effort at JPARC.

Speaker: Giovanni Cantatore (Univ. & INFN Trieste)

Moderated final discussion