The fine-structure constant or electromagnetic coupling constant,
αe, is a dimensionless ratio which unites many
physics subfields. Although known precisely via experiments in each subfield,
there is disagreement within and between subfields. In particular,
precise values obtained via electron ge - 2 experiments
which depend heavily on QED calculations have not always
been in agreement with those obtained via muon gμ - 2 experiments.
Also, solid state measurements (quantum hall effect and AC Josephson effect)
often disagree with neutronic
h/mn measurements.
αe is often said to vary with energy but
the question remains as to whether or not its low energy value is
stable now or has been stable over the history of the universe.
Improved precision helps resolve these issues as they
relate to physics, possibly beyond the standard model.
The Optical Frequency Measurements group in the Time and Frequency
Division at the National Institute of Science and Technology
(NIST, Boulder, CO) developed and maintains a femtosecond
laser frequency comb which is calibrated with respect to
the cesium fountain clock implementation of the second.
A single frequency component of the femtosecond laser comb is used
together with a solid state diode laser and cesium thermal beam to precisely
measure the cesium D1 F∈{3,4} transition frequencies.
The value of
fD1centroid=335 116 048 748.1(2.4) kHz
obtained for the transition centroid
is over fifteen times more precise than the most recent previous measurement.
A precise value for the cesium D1
hyperfine splitting fHFe=1 167 723.6(4.7) kHz
is reported as well. This value is also over fifteen times
more precise than the most recent previous measurement.
These new neutral 133Cs 6s2S½→6p2P½
transition (D1) frequencies,
when combined with the 2002 CODATA values of the Rydberg,
proton/electron mass ratio, cesium atomic mass, and cesium recoil frequency,
provide an almost QED-free value of alpha:
αe=1/137.036 0000(11) or 7.7 ppb.
This value for αe is comparable in precision with
these other measurements. When this value is combined with the other
measurements used to calculate the 2002 CODATA recommended value, an
improved value of αe=1/137.035 999 08(46)
is obtained.
