Trapped Positrons in a High-Precision Apparatus: Progress Toward an Improved Electron/Positron g-2 Comparison
ABSTRACT: A single electron in a quantum cyclotron provides the most
precise measurement of the electron g-value, g/2 = 1.001 159 652 180
73 (28) [0.28 ppt]. The most precise determination of the fine
structure constant comes from combining this measurement with QED
theory, yielding 1/alpha = 137.035 999 173 (35) [0.25 ppb], limited
by the experimental uncertainty of the electron g-value.
Additionally, the best test of CPT symmetry in leptons comes from a
comparison of the electron and positron g-values, currently limited
by the uncertainty in the positron g-value of 4.2 ppt. A new
high-stability apparatus will enable improved measurements of the
electron and positron g-values, a greatly improved comparison as a
test of CPT symmetry in leptons, and an improved determination of the
fine structure constant. This new apparatus is designed to reduce the
effect of vibrations and thermal fluctuations. A sodium-22 capsule
provides a source of positrons and a secondary, open-access Penning
trap is used to enhance the loading rate. A loosely bound state of
Rydberg positronium is formed when positrons pass through a tungsten
moderator and pick up an electron from the surface. The positronium
is ionized by a small electric field inside the trapping well,
resulting in a trapped positron (or electron, if the trapping well is
inverted). Positrons in the loading trap are detected with a second
set of cryogenic amplifiers and are accumulated at a rate of about
one positron per minute. Trapped positrons can then be transferred to
the precision trap for measurement.
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