When it comes to measuring how round the electron is,
physicists hate uncertainty.
In a series of experiments over the last 30 years,
they have established that if the shape
of the electron has any distortion at all,
the anomaly must be smaller than one thousand
trillion trillionth of a millimeter (10 -27mm).
Now, researchers have demonstrated a radical new way
of shrinking the uncertainty even further
Though we usually think of the electron
as a uniform sphere of negative charge,
an EDM or Electric Dipole Moment
would mean that charge is distributed unevenly –
forming one region fractionally more positive
than the particle’s average charge and another
fractionally more negative.
This asymmetry would have far-reaching implications
including implying a fundamental asymmetry
between matter and antimatter
that might explain why the universe
contains far more matter today
than it does antimatter, even though equal amounts
should have been made in the Big Bang.
Because an EDM would cause an electron’s spin axis
to rotate when placed in an electric field,
it should be detectable by sticking an electron
between positive and negative electrodes.
Researchers have probed beams of neutral
atoms for signs that certain electrons are wobbling—
evidence of an EDM.
But the motion of the beam
limits the measurement time.
Now, instead of probing a beam of neutral particles,
they confine molecular ions of hafnium fluoride
in a rotating electric field, which causes the ions
to trace out little circles
rather than speeding by in a beam.
They found they could track electrons’ spin wobble
over the course of 0.7 seconds –
around 1000 times longer than previously possible,
which should open the way to greater sensitivity.
There are, however, drawbacks.
Each ion’s charge tends to disturb the spin
of neighboring ions, which limits the number
that can be measured together.
As a result, researchers were not able to improve on the
best existing measurement of the electron’s sphericity.
Last month they started up a new version
of the experiment with higher electric fields
in order to trap more ions simultaneously,
which, when combined with other “nickel
and dime” improvements,
could boost sensitivity by about a factor of ten
over the next couple of years.
A tiny asymmetry would have far-reaching implications,
because it would contradict the idea that
all physical processes look the same
whether time runs forwards or backwards.
While time reversal would flip an electron’s
magnetic spin, it would leave any EDM unchanged,
inverting the relationship between the two properties.
This breakdown in time-reversal symmetry
would in turn blow a hole in particle physicists’
simplest model of particles and forces
and favor models in which nature contains
many more fundamental particles
than have been seen to date.
