Muon (g-2) at Fermilab: Preparing for Takeoff

概要

Measurements of the magnetic moments of the electron and muon were intertwined with the development of the “modern physics” of the 20th century. The measurements are expressed in terms of the g-value, the proportionality constant between the magnetic moment and the spin, μ = g (Qe ℏ / 2m) s.

For leptons the factor g is greater than the Dirac value of 2 because of radiative corrections. Thus g has an anomalous part, g =2(1+ a), or equivalently a = (g-2)/2. The anomaly is dominated by the lowest-order (Schwinger) term, a = α/2π ≃ 0.00116. For the muon it is necessary to include contributions from QED, the strong interaction and the electroweak gauge bosons.

To measure the anomaly, polarized muons are injected into a precision magnetic storage ring with average field uniformity of one part per million (ppm). The frequency that the spin turns relative to the momentum is measured, along with the magnetic field felt by the muon beam. The muon anomaly measured at Brookhaven appears to be larger than the Standard-Model value by more than three standard deviations.

To clarify whether this is evidence for New Physics or not, Fermilab E989 will reduce the total experimental error from 540 parts per billion (ppb) to 140 ppb. This requires reducing the total systematic error on the muon spin rotation frequency, and the systematic error on the magnetic field, each to a precision of 70 ppb.

This measurement also requires an increase in statistics over the BNL experiment by a factor of 21, for a total of 2 × 10¹¹ analyzed events. The precision storage ring has been moved from Brookhaven to Fermilab, and is now operational. The magnet shimming has now begun, and detector installation is scheduled in mid-2016. Data collection should begin in early 2017.

I will explain the technique, and the large number of improvements that are being employed in the new experiment, and show pictures of relocation of the 14 m diameter superconducting coils, and the reconstruction at Fermilab.