An improved limit on the muon electric dipole moment
Three independent searches, one answer: no muon EDM, down to a level five times tighter than anyone had reached before.
G. W. Bennett et al., Muon g−2 Collaboration, including S. Giron. Physical Review D 80, 052008 (2009).
This is the collaboration’s final word on the muon’s electric dipole moment from the Brookhaven data. Three separate analyses, using three different detector systems and both positive and negative muons, all looked for the same up-down signature.
All three came back consistent with zero. Combined, they set the tightest limit on a muon EDM up to that time, about five times better than the previous best from CERN.
My part Co-author. The negative-muon analysis in Section VI draws on my dissertation, cited as reference 25.
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The idea
An EDM tilts the muon’s spin precession plane, so decay positrons oscillate up and down at the g−2 frequency, 90° out of phase with the count oscillation. The signal’s phase is the same for positive and negative muons, which helps reject false signals.
Analysis 1: traceback chambers (μ⁺, 1999–2000)
Drift-tube chambers tracked about 9 million positrons back toward the beam and measured their vertical angle. Result: dμ⁺ = (−0.04 ± 1.6) × 10−19 e·cm.
Analysis 2: front scintillators (μ⁺, 2000)
Scintillator strips on the calorimeter faces measured the mean vertical hit position. Result: dμ⁺ = (−0.1 ± 1.4) × 10−19 e·cm.
Analysis 3: position-sensitive detectors (μ⁻, 2001)
Finely segmented hodoscopes on five stations measured vertical position and the g−2 phase versus height, using 975 million decay electrons. Result: dμ⁻ = (−0.1 ± 0.7) × 10−19 e·cm. The paper cites my dissertation for the calibration constant and the simulation scale factors.
Putting them together
Positive and negative results agree, as CPT symmetry requires. Assuming CPT, the weighted average is (−0.1 ± 0.9) × 10−19 e·cm, giving the 95% limit of 1.9 × 10−19 e·cm.
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