Particle and Nuclear Physics Seminar at J-PARC

(J-PARC 素粒子原子核セミナー)

DATE: 2015 Nov. 12th 15:30-

PLACE:large meeting room, J-PARC research building

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


SPEAKER: Prof. B Lee Roberts
(Boston University)

CONTACT: Prof. Shunzo Kumano,


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, ¥vec{
mu}= g (Qe ¥hbar/2m) ¥vec{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.