We present a new measurement of the positive muon magnetic anomaly, a µ ≡ (gµ − 2)/2, from the Fermilab Muon g −2 Experiment based on data collected in 2019 and 2020. We have analyzed more than four times the number of positrons from muon decay than in our previous result from 2018 data. The systematic error is reduced by more than a factor of two due to better running conditions, a more stable beam, and improved knowledge of the magnetic field weighted by the muon distribution, ω′ p , and of the anomalous precession frequency corrected for beam dynamics effects, ωa. From the ratio ωa/ω ′ p , together with precisely determined external parameters, we determine a µ = 116 592 057(25) × 10 −11 (0.21 ppm). Combining this result with our previous result from the 2018 data, we obtain a µ (FNAL) = 116 592 055(24) × 10 −11 (0.20 ppm). The new experimental world average is aµ(Exp) = 116 592 059(22) × 10 −11 (0.19 ppm), which represents a factor of two improvement in precision.

We present a search at the Jefferson Laboratory for new forces mediated by sub-GeV vector bosons with weak coupling α' to electrons. Such a particle A' can be produced in electron-nucleus fixed-target scattering and then decay to an e + e- pair, producing a narrow resonance in the QED trident spectrum. Using APEX test run data, we searched in the mass range 175-250 MeV, found no evidence for an A'→ e+ e- reaction, and set an upper limit of α'/α ~/= 10(-6). Our findings demonstrate that fixed-target searches can explore a new, wide, and important range of masses and couplings for sub-GeV forces.

The reaction γ + p → K + + Σ + π was used to determine the invariant mass distributions or "line shapes" of the Σ + π − , Σ − π + and Σ 0 π 0 final states, from threshold at 1328 MeV/c 2 through the mass range of the Λ(1405) and the Λ(1520). The measurements were made with the CLAS system at Jefferson Lab using tagged real photons, for center-of-mass energies 1.95 < W < 2.85 GeV. The three mass distributions differ strongly in the vicinity of the I = 0 Λ(1405), indicating the presence of substantial I = 1 strength in the reaction. Background contributions to the data from the Σ 0 (1385) and from K * Σ production were studied and shown to have negligible influence. To separate the isospin amplitudes, Breit-Wigner model fits were made that included channel-coupling distortions due to the NK threshold. A best fit to all the data was obtained after including a phenomenological I = 1, J P = 1/2 − amplitude with a centroid at 1394 ± 20 MeV/c 2 and a second I = 1 amplitude at 1413 ± 10 MeV/c 2 . The centroid of the I = 0 Λ(1405) strength was found at the Σπ threshold, with the observed shape determined largely by channel-coupling, leading to an apparent overall peak near 1405 MeV/c 2 .

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