i first heard about it on the bbc news
going home um on
five o'clock news
and they talked about this new discovery
which is going to change the world of
particle physics and how we understand
the universe and i'm going oh
and they talked about the uh
how the electron is a perfect sphere and
i'm thinking well what experiment is
this is
and gradually as they talked a little
bit more i realized it's i knew what the
experiment was and i even know the
people who have done it they were my
colleagues in sussex and so uh
i know johnny hudson and ed heinz very
well and ben sauer
and so it was really great to hear about
the experiment i'd never thought about
it as being the electron is a sphere i
thought about it as the electron dipole
moment somehow don't think if they'd
have talked about the electric
dipole moment of the electron that might
not have made the bbc news
i i can't give you the heavy
experimental details because it's too
complicated for a theorist like myself
but i can tell you some of the
background
first of all why say the electron is a
sphere when we we think of the electron
actually as a fundamental particle it's
one of the building blocks of of
the standard model
and normally you think of that as a
point-like object
so what does it mean to say it's a
sphere
and i think what they're really talking
about is the charge the electron's got a
charge
an electric charge and and they're
talking about the distribution of that
charge they've measured it to be almost
spherical
and in fact to show the precision of
their experiment they've demonstrated
that that distribution if if they could
scale it up to the size of the solar
system
then it would be a sphere to within a
hair's breath they could measure a
deviation of a hair's width
so a sphere the size of the solar system
they could measure deviation of that
charge distribution of a hair's width it
turns out that the that distribution of
the charge around the electron
is is really important in understanding
the nature of the matter antimatter is
symmetry the fact that we live in a
universe
made up mainly of mata very little
antimatter around and yet we believe in
the early universe they were created in
equal amounts so what has happened
and the
by finding this a deviation of of this
charge from this the perfectly spherical
charge
you we would hopefully gain some
information about this origin of this
matter antimatter asymmetry so let me
step back before i go into those details
and perhaps just talk a little bit about
the what what it means to be not
spherical
so so a spherical distribution of charge
basically the way i think about it
anyway is that if you happen to have a a
detector that could pick up charge could
measure charge then if my electron is
here
then the detector would pick up an equal
amount of charge on this side as it
would on this side as it would up and
down and all the way around it would be
perfectly spherical it wouldn't see any
preferential distribution of charge
that's called a monopole by the way
where it where it's equally distributed
like that a spherical distribution
now the next thing you can do you can if
you want to distort this charge is the
next simplest one is called a dipole
and
we're used to dipoles at school we're
used to dipoles magnetic dipoles a bar
magnet
right has a south pole and a north pole
and we're used to that in magnetic
cases but it also exists in the electric
case if i had a positive electric charge
and a negative electric charge
then the dipole would be and put them
close to one another the dipole would be
a line that goes from the negative to
the positive charge right so this is the
electron's only got a negative charge
so it turns out that you don't have to
have a positive and a negative charge to
have a have a dipole you just need to
have a distribution of the charge which
is not spherical so if for example i i
distorted this sphere so i made it sort
of maybe more like a a um a raindrop
okay which has got a bull a bulgis end
at one bit then a narrow end at the
other so there's more charge over here
and less charge over here then i would
have a dipole
going from from one one part to the
other part
that's what they're looking for that's
what this experiment is looking for with
the electron they're looking for this
asymmetry in the charge
and things that cause this asymmetry are
the interaction of the electron with the
other particles
now in the standard model of particle
physics the one that describes the
universe as well as we know it
the we there is an asymmetry it's
induced by the interaction between the
electrons and the quarks actually
but it's so small it's expected to be
unobservable so the fact that it hasn't
been seen is no surprise
but we know that the standard model of
particle physics can't describe it
doesn't describe everything
so there there are extra modifications
called beyond the standard model which
people have put forward there are lots
of variants
and one of the
goals of places like the lhc is to test
these models
and what's wonderful one of the
wonderful things about this experiment
is a it's also testing these models but
b it's in a lab
much lower energies just in a lab down
at imperial college with uh ed heinz and
his group
and what they're looking for then is the
interaction between some of these new
particles in particular supersymmetric
particles they're called they're the
partners of the particles that we that
we know exist
they're looking for the possible
interaction between those particles and
the electron
and and the theories predict that in
some of those cases you expect them to
induce this
asymmetry
and what they've managed to demonstrate
is through you know months and months in
fact years of working on this
is that there is no asymmetry they've
they've even though they've got their
sensitivity down down and down and down
the electron distribution of the charge
still seems perfectly spherical they're
seeing no deviation
that then means
that those models which predict there
should be a deviation that's already in
excess of what has been seen are ruled
out it doesn't rule out supersymmetry
supersymmetry is a vast subject and
there are lots of regions of parameters
in supersymmetry which you can still go
to which would be perfectly consistent
with the observation of of no distortion
but it's severely constraining it
what the experiment is is so you've got
to create this dipole you've got you're
trying to test for this dipole moment
well basically they put electrons in an
electric field
but if you were to simply put an
electron in an electric field you've
created a little accelerator it's your
cathode ray tube it's your television it
shoots out
and you've lost it so what they do is
they put they look at molecules
and molecules of
uterbium fluoride
okay
fluoride of fluorine i've heard of
incredibly reactive apparently the
uterbium is heavy a heavy an atom of
uterbium is very heavy
we need martin
an atom of fluorine is very light
so it itself already has remember what i
talked about the
the distribution of the the electric it
produces the electric dipole moment in
its own right
and so the electrons are already
enhanced by just being in the presence
of this
these molecules so they put these
molecules in then they apply an electric
field
that shifts the energy level of the
electrons
then they switch the electric field
and the energy levels shift again
now the amount of shifting of these
of the energy of the electrons actually
depends upon the dipole moment
so the bigger the shift of the electrons
the bigger the dipole moment
so that's what they're looking for
they're looking to measure the shift
and they can double it up by reversing
the field so that it goes from positive
to negative so it increases the range
and that's what they've been looking for
they've been they've been looking for
that shift and they've seen no evidence
of that shift
in in their um analysis
they have they look they have all of
these data all this data
and of course
it's all done on the computer as well
there's analyzing it all on the computer
so there is a box there or a screen
which
will constantly be giving them the
electric dipole moment
but they cover it up
right they've had it covered up because
they don't want to be influenced
by the by the
kind as they're trying to reduce all
their errors they don't want to sort of
be influenced and have a quick look and
say oh if we just tweak that a bit more
we'll get this down
and and it's very important you do that
that
you it must have been so difficult for
them not to go and have a quick peek to
see what they were seeing and what they
do is they eventually
reach a stage where they think that
they've basically accounted for
everything they can possibly account for
over that you know in a sensible time
period
and uh
they pull the screen off they pull the
cover off the screen and they look and i
i saw ed heinz yesterday i was on a
panel with him and i asked him what it
was like and he said
they pulled it off they had a meeting
they said is it time first time round
there was disagreement people thought
they could still work on a few more
systematics and eventually the group
said yes let's do it pulled it off saw
the result it was there
opened a bottle of champagne
and
the world got to hear about it