have an internal structure, something an
electric dipole moment can reveal.
from top: goronwy tudor jones, univ. of birmingham/photo researchers; bell laboratories, courtesy aip emilio segrÈ visual archives
To envision an electron electric dipole
moment, imagine that the electron has “a
cloud of stuff following it, like the Pig-Pen
character in the old Charlie Brown cartoons,” says DeMille. Blow that electron
cloud up to the size of Earth, and extra
positive charge would appear as a tiny
dent on the north pole while extra negative charge would be a tiny bulge on the
south pole. Given current limits, the size
of that dent or bulge would correspond to
adding or subtracting no more than about
one-thousandth the width of a human
hair from either end of the planet.
The standard model predicts that the
electron’s electric dipole moment is less
than 10–38 in units of electron charge
times centimeters. That’s equivalent
to separating an electron and a similar
charged particle by a distance of 10–38
centimeters, or a hundred trillionth of a
trillionth of a trillionth of a centimeter.
But extensions of the standard model
predict the electric dipole moment to be
bigger, between 10–25 and 10–30. In 2002,
Commins’ team published the most
stringent limit yet: 1. 6 × 10–27. That means
researchers are well within the hunting
grounds where they might find the beast.
Each time experimentalists fail to
detect the dipole moment despite
increasing the sensitivity of their tests,
they tighten the limit on how big it
could be, like lowering the bar in a game
of limbo. When the bar drops, theorists
have to rule out more of their ideas on
how the universe works. “A good theorist
can make a model in an hour, but it takes
us 20 years to destroy it,” Commins says.
Electric past the electron
was at the heart of many scientific
discoveries at the turn of the 20th
century. detecting its electric dipole
moment could once again put the
particle in the spotlight.
1897 j.j. thomson discovers electrons,
calling them “corpuscles,” revealing that
atoms are divisible.
1900 henri becquerel, who discovered
radioactivity, finds that beta particles are
in fact electrons.
1913 robert millikan publishes results
of his famous oil-drop experiments, which
determine the charge of the electron.
1925 samuel goudsmit and george
uhlenbeck propose that an electron has an
intrinsic angular momentum, called spin.
1927 lester germer and clinton davisson
(left to right, above) find that electrons
scatter from the surface of a crystal the
same way X-rays do, proving particles can
act like waves.
1928 paul dirac formulates his electron
equation, which implies the existence of
antielectrons —particles with the same
mass as electrons but opposite charge.
1932 carl anderson discovers the
antielectron, or positron, confirming the
existence of antimatter (electron-positron
pair formation shown above).
The current limit has already ruled
out the simplest version of a popular idea
known as supersymmetry, which tries to
explain the cosmic matter/antimatter
imbalance by suggesting that every particle has an as-yet-unseen “superpartner.”
If researchers can push the limit to 10–29,
that would rule out another extension to
the standard model that tries to solve the
matter problem by postulating multiple
kinds of the particle known as the Higgs
boson, which Europe’s Large Hadron
Collider was designed to detect.
To measure the electron electric
dipole moment, physicists need to watch
an electron closely as they flip on an electric field. They then scrutinize whether
a property known as the particle’s spin
responds differently when the field is
switched on in different orientations,
which would mean the electron pos-sesses an electric dipole moment. Seeing
that difference is the hard part. In particular, because of the deep link between
electricity and magnetism (moving
Detecting an electric dipole moment
would mean electrons, which buzz about
atomic nuclei, have internal structure.