(soothing music)
- Actually this is a nice
enjoyable contrast, the back and
forth between the theory theorists
and the experimentalists.
I want to talk about looking
for the everyone's favorite
type of dark matter that
Lawrence no longer believes
is what we're gonna or
maybe never believes
- (audience member mumbles)
- (laughs)
Is what we're gonna find, well,
the thing I wanted to say is
that from an experimental
perspective, the story I'm about
to tell is compelling enough
that a large number of people
have spent well in my case
anyway almost my entire career
looking for this stuff, and so
I wanna back up a little bit
and sort of give my naive
view of why it's compelling.
In fact back way up to gravity,
so here's where's the laser,
here we go, Newton actually
when he first talked about
gravity and orbits he sort
of had this idea that if you
throw a ball it goes a distance
if you throw it harder it
goes further, gravity is
trying to bend it, and so the
balance between it wanting
to go straight so-called
centrifugal force if you will
and gravity results in a point
where you throw it just hard
enough and you get an orbit.
And that orbit is a balance
between gravity and the
tendency of the rock to go straight.
If we come to the solar
system, his prediction
which is that the velocity of
something is falling off if
you get further away from
the sun and there are the
planets, and the measurement
matches his theory,
just Newtonian gravity just beautifully.
And in fact in some sense,
if you measured this curve
when you measure this curve
you've weighed the sun
because the sun is providing
the force that's holding
all the planets together.
And one of the main points
here I wanna make is that
Newton was right, that
gravity has now been tested to
sub, from submillimeters
scales to the size of the solar
system, billions of miles,
that's a vast spacial range
for which one simple
theory works beautifully.
So then the thing is why don't
we go to the next scale up,
and that's a galaxy, a hundred
billion stars, looks like a
whirlpool, it's rotating,
everything is orbiting around
it's own sort of mutual gravity.
And does the gravity work there?
Well we think probably
gravity works, but something
certainly doesn't.
This is a galaxy, it's rotating,
and here's just like in my
last graph, here's the
velocity, sorry the distance
out from the center, and
here's the velocity of things,
and you predict something
like this curve here,
if these objects on the
periphery are behaving just
like the solar system, the
mass of the stars that are
present is what is is
what's driving the rotation.
And in fact the data
famously doesn't agree.
This has been measured now
for some huge number of
galaxies and so there's some
other form of matter we think,
I guess in principle it could
be the gravity is wrong, but
there's lots of reasons we
don't think that's the case,
my laser's fully run out oh there it is.
And so galaxies are full of
some other form of matter
which we say is dark.
And in fact it's not simply
galaxies, if in another talk
and in fact in another
whole session not titled
Particle Physics, we could
go on about the evidence from
bigger scales than galaxies
and cosmology and the microwave
background, and the fact
is that dark matter is
permeates our understanding
of the universe back to the
earliest days of the Big Bang.
And so for instance this
is a view of the Milky Way,
the Milky Way would be a galaxy
and it's embedded in this
sort of larger sort of diffuse
component of mass which is
overall about ten times as
much mass maybe seven times
as much mass as in the stars
and the dust out of normal
things.
Part of that story which I
didn't tell it really is very
obvious from cosmology that
the dark matter is not made
of any normal elements.
It is not made of neutrons
and protons or anything
built up out of neutrons and protons.
So then it's natural to
look to particle physics.
And indeed this is the particle
physicist's view of the
world, this is like the particle
physicist's equivalent of
the periodic table, and there's
these fundamental particles,
they all have different
masses, and there's a few known
forces.
In this set of particles only
these ones here are stable,
we know we can create all
these, they live a short time,
none of these work out
to be the dark matter.
But let's think about that a
little bit, how could we have
the dominant form of mass in
the Milky Way not be normal
matter and we've never detected it?
And that's gets to what we
mean by the size of a particle.
Here for instance is an electron
and it's got an electric
force field, but then at a very
small scale it's got a weak
force field, and the
ratio of these forces is
a hundred thousand and
a hundred thousand-fold
that this force is shorter
range than the electric force.
A neutron for instance has
a strong force which is
about a hundred thousandths the
size of the electrical force
and also a weak force.
And then there's the
neutrino which we know about,
it only has this tiny little
weak force field, which gives
it an incredibly small size,
so if a neutrino comes in
ordinary matter it passes
right through unless it just
happens to strike in
some sense the center.
So in this picture, what
would make sense is the
dark matter for something to
be all through the galaxy and
not have been seen so far,
it's natural to make it
something that has
effectively a very small size.
Or perhaps indeed a force field
that's the weak that's like
it's either like or is the
weak force we know about in
particle physics.
Then there's the Freezeout
Argument which I decided
I didn't have time to
explain, and so very helpfully
Lawrence has explained it to
you, lemme phrase it my way.
Any stable exotic new form
of matter in the plasma
hot bag bang, and what do I mean by that?
I mean if there's a type of,
if there's a particle physics
at a higher scale that we
don't yet know, which would
have created matter that
was interacting in the early
universe if that particle
happened to be about the
weak scale of particle
physics in terms of its size,
then it would be dark matter today.
And this is the compelling
argument some years ago
people called it the Freezeout
Argument, some people now
call it the WIMP Miracle, I
think it's better to call it
the Freezeout Argument, that
makes people look for a certain
type of particle.
You give it a weak force like
a neutrino but a mass of say
about like a gold atom so it's a WIMP,
a weakly interacting massive particle.
And this is still a very
compelling idea, and we
oughta go test it.
How would you test it?
Well, here again is a galaxy
sort of shown in cutaway with a
perversely in white the
dark matter, in a halo,
and all the matter in this
halo is orbiting just like
the rotation of the Milky Way.
And so we know the speed
of these particles.
If here's a detector that's
say, about a meter in dimension,
a hundred billion of these
particles will just will
go through it given the
velocity and density of these
particles that we know from astrophysics.
Where that's about ten
to the sixteenth a year.
And interesting thing about
a neutrino, a neutrino is
so small, how small is it, a
neutrino is so small that if
you wanted to stop it,
you would essentially need
a light year of lead.
You go to the dentist and you
get a lead shield for x-ray
and it's maybe a thirty
second of an inch thick.
If you wanna stop a neutrino
you need a light year.
A light year's about ten
to the sixteenth meters.
So if you combine the idea that
there's ten to the sixteenth
per year, through a meter
like thing, and they have
a probability sort of an
inverse of ten to the sixteenth
meters, then in a meter
dimension detector you might get
about one per year.
And that's the experimental goal.
We wanna build a detector,
I'll explain the detector in a
moment, and we're gonna
see about once a year
one of these particles hitting it.
The problem with that is
that radio activity is about
a hundred billion times higher than that.
So it is like looking
for the proverbial needle
in a haystack.
We have to remove ambient
radio activity from just
uranium and thorium of potassium around us
to an extreme level in order
to look for these interactions.
One piece of radio activity,
what you would think of as
generally as radioactivity
is high energy particles from
outer space, cosmic rays,
these are high energy protons
in particular and other
particles raining down on
the atmosphere creating a
cascade and a shower of
particles on the surface
of the earth.
One particular piece of that
is a muon, and to get away from
muons you have to go deep underground.
There's a set of labs around
the world, one in particular
is in South Dakota in a gold
mine, and that is a tank of
water in which there is a
detector, and being a mile
underground there's very
few of these cosmic rays
striking it.
So now inside of that water
tank we have a detector,
I would like to point there we go,
this is like the cartoon
I just showed you, and
a dark matter particle could
strike it, say it's coming from
below, but the problem
of course I mentioned is
the radioactivity.
This particular detector
is from the LUX Experiment.
And the problem with the
radioactivity is that outside
in the rock you had all
these gamma rays and neutrons
and they were trying to
penetrate, this enclosure we're in
is a water tank, we fill it
with water and that water will
sort of stop all these particles.
Water's not a particularly
great material for that,
but it's cheap and you
can put a lot of it.
Even people are radioactive
at this scale, so this fellow
has to get out of the water
tank before we fill it or
he'll be radioactive for the experiment.
Let's look a little bit
at how the detector works.
There's actually a large
number of groups worldwide
with different detector
ideas as to how to do this.
But I'm gonna talk about
one particular one just to
illustrate how this works.
So this is a so-called
time projection chamber,
where the particle comes
in, strikes material
in the middle here, and the
material in this case is
liquefied xenon or liquefied argon,
and leaves, and there's a
flash of light from the site
where the an atom or
the nucleus of an atom
was struck hard by the WIMP.
You measure the light with light sensors,
photo multiplier tubes,
and also electrons are
drifted upwards.
There's meshes here that
create a strong electric field,
and you pull the electrons
to the surface of the liquid
and you actually pull
them out of the liquid and
then the gas they make
a small discharge like
at the center of a neon lamp,
and you get a localization
there.
And so you basically can see
where the event was and in
this direction from where
that light is, and the time
difference between that
flash and that flash,
based on electron drift
speed gives you the depth.
And so you find out that
there was an interaction
in this detector and you
know exactly where it was.
That's the so-called time
projection part of this.
Here's what the data
actually looks like from
the LUX Experiment.
The first flash of
light can be as small as
two measured photons, this is a photon and
that's a photon.
This is time.
And then a little while later
the electrons get to the
surface and you get a
much bigger flash and
that's measuring the electrons.
Now, if we wanna have
a detector and there's
a hundred billion times
too much radioactivity,
and then we think we've seen dark matter,
how are we ever gonna
prove that we've seen it?
And the way we will in
this standard WIMP paradigm
of dark matter, is that the
dark matter is fundamentally
a little bit different than
almost all backgrounds,
dark matter strikes a nucleus
whereas most backgrounds
are from things that strike an electron.
A gamma ray strikes an
electron in an atom.
And so here's a low energy
electron getting struck by
a gamma ray and the in
blue is where the electron
wandered around and made
charge in light that gave you
signal.
This is a scale that we
cannot possibly resolve in a
meter scale detector,
that's about the width of
a human hair, tenth of a micron.
That'd be a small human hair.
But if I think of a little
box there and I expand it out,
now we're looking at
something that's about
the size of a hundred atoms,
that is the recoil track
if you if a particle which a WIMP will,
strikes a nucleus and you
get a very small track.
These are about the same
energy, very very different
size track, both in a microscopic scale.
The density of this track
sompared to that track leads to
a different signature in the
amount of charge and light
in this class of detector.
So, here on the vertical
scale is a measure of
how much charge to how much light, and
on the horizontal scale is a
measure of essentially energy,
the light signal alone, and
we expose the detector to what
an intense amount of what will
naturally be the radioactive
backgrounds, and each
dot is an event where the
detector was struck and
we populate a region here
kinda between these blue
bands, with a little spillover,
and if we turn around and
expose the detector to
a neutron source, we
populate our region between
these red bands.
Neutrons also look like like
WIMPS on an event per event
basis but neutrons are quite
a bit easier to get rid of and
to recognize otherwise in your detector.
And here's the world's
leading data set looking for
WIMP dark matter.
Some population of events
between the blue bands
I showed you and in this red
region a little spillover from
the blue region, but nothing's
centered on the red region.
So unfortunately to cut
to the chase, we haven't
seen the dark matter yet.
But this is a very promising
way to look for it.
This was essentially a
hundred kilograms of material
in a fraction of a year
and we've seen nothing.
There is a very competitive
set of experiments
looking for this type of dark
matter, one particular one
which myself and a number of
people actually in the room
are part of is was the
LUX experiment and now
a next generation LUX Zeplin,
so just to give a sense of
where things are headed,
this detector is instead of
made of three hundred kilograms of xenon,
it's made with ten tons
of liquefied xenon,
this is now a meter and a half
scale, the previous one was
only a half-meter in scale.
It'll be the largest dark
matter experiment and
three hundred times
more sensitive than LUX.
This is the way we present
our results, this is sort of
a map of our knowledge or
ignorance of dark matter.
We don't know the exact
cross section of dark matter,
essentially the size of it.
We have guidance from
the Freezeout Argument.
The Freezeout Argument
picks something sort of
very vaguely, well, on this plot.
Perhaps not so much at
the bottom of the plot,
and not so much at the
very top of the plot,
but vaguely on this plot.
For various reasons, if you
think it's a weakly interacting
particle, you would expect
it to be related to the
mass scale which gives
us the weak interaction,
and that's more or less on the
horizontal axis of this plot.
If you sort of throw that
out, then we can start going
down lower or very very high.
When you make a measurement,
let's pick this one here,
this was the measurement about
a decade after this whole
idea came up, and the very
first of sort of the modern
detectors, they made a line
like this and what that meant
is anything above this was ruled out.
So a bigger cross section would
have meant more interactions
and that's excluded.
And this line just goes
straight on off, if there are
if the dark matter particles
are heavier, there's fewer
of them and that's why
this line curves up.
A hundred GEV here is
sort of like a heavy atom.
This is the current LUX
experiment so this is
the world's exclusion now.
We know it's not here, these
are various models from
supersymmetry that are
still allowed after the
(mumbles), and this is where LZ is going.
I'm on my last slide.
This is interesting in since
the field started when actually
limits were about here
at this minus five level,
we've advanced a good
four orders of magnitude,
which is really impressive
when the idea first came out
I think of Lawrence's cycles,
we had detectors that had
a fraction of a kilogram and
no background, and we set
limits.
Now we're building detectors
that are in the hundred
kilogram scale and we
haven't seen anything.
We now basically see how
to plow right on down.
Give us money and we're gonna go.
For better or worse
something has now come up.
In the last several years
people thought hard about
where this was headed and
realized that there are
neutrinos from cosmic rays
which are producing signals
which look very much like
the WIMP dark matter.
Unless we can build a detector
which can actually measure
the direction of those
individual tracks, this will be
a floor which we cannot go below.
This will be a practical
within money bounds limit to
what we can do, and we're
getting close to it.
So one thing we should definitely
do is measure as much of
this region as we can, down
to this neutrino floor.
There is no upper end of the
data here, down at lower masses
that's yet a whole other idea,
and the different type of
technology can push in that
direction, although that's
sort of the next world.
So what is my summary?
My summary is that we
know there is dark matter,
and we don't know what it is.
Weakly interacting fundamental
particles are a good guess,
they are a guess, but they're
a good guess from both
cosmology and particle
physics, they certainly have
been a good enough guess to
launch a whole industry of
people trying to do this.
And after much effort, I'd
say twenty years of effort,
we do know how to test this
idea to the limit it can be done
based on this neutrino background.
And we should do that.
And if we're lucky, the
dark matter will be there.
(applause)
(soothing music)
