Scientists come from around the world to
work at Sanford Underground Research
Facility. During the month of October, we
asked what you wanted to know about dark
matter, building a detector and working
underground—and we had a few questions
of our own. On Dark Matter Day,
researchers with the LUX-ZEPLIN dark
matter experiment have the answers. How
do you know that dark matter exists?
So you look at a spiral galaxy like our
Milky Way, and you look at the outer
stars of the Milky Way, so we all know
this one moves around and if you look at
the further ones out and if you look at
the speed of those outer stars in the
galaxy then you realize that they should
actually fly away because the matter in
the galaxy is not enough to keep those outer
stars in check. So you need to add more
matter to the galaxy to actually be able
to keep those outer stars in the spiral
galaxies. Otherwise, it would just like
dissolve itself. That's why we're
thinking if the dark matter particle
that all over. It's kind of like a halo
around a galaxy and it keeps those it
has this gravitational mass that keeps
those outer stars in check. What other
experiments are looking for dark matter?
So there are many experiments looking
forward and in many different ways so
that other direct detection experiments
like our experiment where we build an
earth-based detector and wait for dark
matter particles to interact with our
detector. But then they also colliders
like the Large Hadron Collider which
think that they can maybe produce dark
matter particles and detect them in this
way. And the third way is astronomers and
cosmologists looking at the sky and
seeing—because we know dark matter is
out there—seeing whether it interacts in
some sort of way out there and whether
we see an excess of certain types of
events and so there are really many
different ways of looking for dark
matter and it's really important that we
look in every possible way
because we might get some information
and they get a different kind of
information and we can compare
and compliment the picture. What new
science and applications could arise
from dark matter studies? There are
technologies that we use in building
a dark matter detecter that have
applications elsewhere. So things like
the cryogenic systems that we use that's
kind of used also in medical physics,
those kind of fields. And of course the
advancements that we make in things
like computing and analysis and software
all of those things have some
applications in other areas of science
and in wider industries. So while not
directly coming from the science that we
do, those are some knock-on effects that
might have some applications in in other
fields and industries. What is the
hardest part of designing a dark matter
detector? The most special thing is that
we need materials that have very low
radioactivity and so the radioactivity
creates a background for the experiment
that makes it harder to see dark matter. So to give you some kind of
understanding, your body produces about
4,000 radioactive events in a second, and
the materials that we're looking for are
going to be many orders of magnitude
less than that. So our 2,000 kilogram
cryostat we'd like to produce less than
one neutron in five years. The hardest
part about designing a detector or
anything really is actually making it so
you can put it together. You can design
everything you want but you know you
have to actually be able to bring it in
through the door into your water tank or
your a lab or anything else so if you
can't actually put it together it
doesn't really matter how good it is
because you won't be able to use it.
Why did the detector have to be built in
a clean room? So the signals we're
looking for very very rare and all
around us we have radioactivity in dust
for example. So we have to make sure for
one, to build the detector out of
materials which are non-radioactive and
to also not have any dust in the
detector because dust it has
radioactivity in it. So being in a
cleanroom means that you can control
your dust levels and that you can make
sure that the detector stays clean as
possible. What is the most challenging
thing about working in a clean room? So
first of all you wear all this gear like
you're wearing the mouth guard and you
sometimes have to wear glasses as well
and you have a covered from head to toe
so you first have to get comfortable
with working in that and then we're used
to touching things. Not everything in a
cleanroom is equally clean so you have
to make sure you only if you have
something a little dirtier not to touch
the clean things afterwards to change
your gloves all the time. So to become
aware of that, that's really I think
thing to get really used to beginning
I'd definitely rather work underground
because that you can at least have your
coffee out there in the Davis Campus. Coffee is frowned upon and when you're in
a clean suit. What does WIMP stand for? It needs to be a massive particle and
then we know it doesn't really interact
with other matter or with itself so it
needs to be weakly interacting. And now
you just create this generic name called
weakly interacting massive particle
that's a WIMP.
Does the "weakly" in WIMP mean the
proposed particle interacts rarely with
other matter or that interacts via the
weak force? Since we don't know
exactly how it interacts with normal
matter we know how it should act react
gravitationally because this is how we
sort of proposed it, but we haven't
really detected it and don't know
exactly how it interacts with normal
matter. It needs to be acting weakly—
weakly meaning it needs to act as rarely
as the neutrino interacts which is
on the weak scale. So that's why it's
weakly interacting. Can the latest
discoveries related to gravitational
waves tell us anything about dark matter?
So the fields are quite different
although they both kind of involve
gravity. One way in which gravitational
waves can help us say something about
dark matter is kind of specific to
certain dark matter candidates. So one of
those is primordial  black holes which
are black holes that were thought to be
produced in the early universe and
gravitational waves are a very nice way
to probe the interaction of large cosmic
bodies colliding with each other and
they produce these ripples in space-time
which we call gravitational waves so if primordial black holes do make up
some component of dark matter then
gravitational waves would be a way in
which we could study that. What is the
best thing about working in Lead, South
Dakota?
Well I think you know I could say the
weather but that would be a lie. I think
that the the people here are really good
It's a very friendly place
you walk into a store or a restaurant
and people are generally interested in
what you're doing. When people ask you
how are you they expect an answer. The
people. It's a really great community and
everyone I've met really likes the Lab
out there and what we're doing and
everyone's just really enthusiastic. It's
great. It's really nice to be able to
enjoy like the local area with you know
essentially your friends and your
colleagues. We like all of the kind of
local businesses around so you know if
it's came like coffee at Lotus Up or
going for food Sled Haus or singing
karaoke at the Greenfields on a Friday
night—it's really nice to kind of be in
that space and share it with local
people too that we that we meet.
