Muon magnetic anomaly: experimental status and prospects

Abstract

The past five years have brought significant new results on the muon magnetic anomaly, $a_μ= (g_μ- 2)/2$, and on the hadronic vacuum polarization (HVP) contribution dominating the uncertainty $Δa_μ$. Serious tension has emerged between the experimental and standard model(SM) values for $a_μ$, as well as between the SM and the first precise lattice QCD values. We review the current experimental and theoretical status of $a_μ$, along with the prospects for new results,focusing on MUonE,a new experiment at CERN, aiming to evaluate the leading order contribution to a(HVP) in a direct measurement of muonic Bhabha scattering.

Figures (8)

• Figure 1: Feynman diagrams of representative SM contributions to $a_{\mu}$. From left to right: first-order QED and weak processes, leading-order hadronic (H) vacuum polarization and hadronic light-by-light contributions.
• Figure 2: (a) Evolution of the measured values for $a_\mu$ in experiments at CERN and BNL, compared to the SM prediction, as of 2006, the time of the final BNL E821 results. (b) Layout of the Fermilab E989 storage ring that shares its design and most components with BNL E821, though with critical additions and upgrades. ESQ denotes the electrostatic quadrupole beam focusing system comprising quadrupoles Q1--Q4, IBMS is the Injector beam monitoring system comprising scintillating fiber detectors, T0 is a thin plastic scintillation detector providing the time profile of arriving muon bunches, K1--K3 are three pulsed kicker plates needed to place the muon into a stable stored orbit, T1 and T2 are in-vacuo straw-tube tracking detectors, and, outside the vacuum, electromagnetic calorimeter stations, numbered 1--24, to detect the decay positrons.
• Figure 3: (a) Fourier transform of the residuals from a basic fit not including three beam dynamics parameters (red dashed line), and from the full fit (black line). The peaks correspond to the missing betatron frequencies and muon losses. Data are from the Run-3a data set. Inset: corresponding asymmetry-weighted $e^+$ time spectrum (black line) with the full fit function (red line) overlaid. (b) Azimuthally averaged magnetic field contours overlaid on the time- and azimuthally averaged muon distribution for the Run-3b data set. For more details see Refs.Muong-2:2023cdqMuong-2:2024hpx
• Figure 4: (a) Plot of the Run-1 and Run-2/3 E989 results for $a_\mu$, alongside the E821 final result, as well as an overall average. (b) FNAL E989 combined $a_\mu$ result to date, and the experimental world average (2023) are compared with three SM calculations. See text for discussion.
• Figure 5: The leading order Feynman diagram of interest in MUonE (left). The relation between the $\mu$ and the $e$ scattering angles for 150 GeV incident muon beam momentum (right). The blue triangles indicate the reference values of the variable $x$ and the electron energy, while green lines indicate the approximate boundaries of the three kinematic regions of interest in the experiment.