A Sparkle in the Dark: The Outlandish Quest for Dark Matter

Public lecture presented by Maria Elena Monzani



thanks everybody for coming it's an honor to be at this lecture and some of you traveled for from very far so I'm

very happy you're here most people in the audience have probably heard about that matter before

I hope I will tell you something new specifically I'll talk about some of the

efforts we're doing to find the Dark Matter particle and I would like to give

you an idea why that's difficult and challenging and how we're getting around some of the challenges I was advised to

say something about how I ended up doing these so I'm gonna give you a brief

brief brief in the overview where I come from literally this is my hometown I

grew up in the mountains in Italy we had a river and we had a number of mountains this is in the Alps were famous for

sparkling water which you know it's not that matter but you know that works

anyways it will not surprise you to hear that I was a bit of a nerdy child

growing up those were the years of the those was was the heyday of the Voyager

program one of my first memories was that everybody was talking about the planets everybody even on TV even on

Italian TV or at least the two channels that had reception in the mountains I am

know it my family members so that somebody bought me a science book for children and my book had that drawing

pretty much like this one Sun and planets extremely off scale the book

explained that they did the thing was now possible to draw in a book because you know distances are too big I think I

stared at the drawing for hours on end for like two years three years everyday

something like that then in 1986 something a bit dramatic happened I was

in school at the time and that was the Chernobyl accident that was that became famous again last year because of the TV

show but most of you in the audience remember directly that was kind of a big deal in Italy so that was the explosion

of a nuclear reactor in Ukraine in the former Soviet Union there was a huge

amount of emitted radiation there was active cloud the travel to Europe and

some of the particles were trapped on the Alps and reached the place where I live probably not my house but the general

area and it was a public health crisis so at that point we were up to four TV channels in my mountain region and

everybody was talking about atomic energy so I wanted to know what the atoms were and for the first time I

looked at the different page of my science book and found the drawing kind of like this one this is a drawing of

the carbon atom B has a nucleus and six electrons it kind of looks like the one

above and this is what we call the atomic model of the atom it's not like

accurate it doesn't have quantum physics then again it's very difficult to draw

quantum mechanics especially for a children book the fact that these two

objects kind of look the same way was not lost on me and it was actually like

a mind-blowing discovery for me as a secondary grader the smallest thing I

knew the atom and the largest thing they knew the solar system had the same structure I'm not going to tell you

everything about my childhood although I'm sure you would love to hear about it I'm going to jump to straight 20 years

this is um me with my postdoctoral advisor and the first that matter

detector that a wart on and in the back is my collie trying to do real work white ladies do a photo shoot why am i

showing you this this object is what we call a particle detector it's a technology developed to

study elementary particle physics which are the smallest by definition the smallest object we can study in nature

but we use that to study that matter that matter is a problem is a cosmological problem that means the

problem of the whole universe so we use this thing to study the universe which

by definition is the largest object we start with and studied nature so I ended up doing as Michael said for most of my

career what I wanted to do in second grade and I don't know if that makes me a visionary or just very stubborn but

that's what happened so this is the introduction to tell you I'm going to talk tonight about how we use techniques

from etiquette physics to study the universe basically I'm gonna start by asking what

is that matter and how we know it exists so the first the smallest cosmological

scale where we can observe the effects of that matter are galaxies but to

explain you galaxies I will go back to my favorite thing which is the solar system this is what we call the rotation

curve in this graph we have the mean distance from from Sun of a given object

of the solar system and this is the orbital speed this is the velocity with which the object rotates around the Sun

as you know one orbit of of the earth is

what we call one year the orbit of Neptune is almost 200 times bigger the

orbit of mercury is you know a factor of 10 smaller this is the way it's supposed

to go if you have a system like the planetary system where all the mass is located in the center how does that look

like for galaxies we can measure that it's been done starting from the 60s and

they will tell you a few words about the person who pioneered these kind of studies so basically we can take one

galaxy this is a photo of the triangle galaxy or m33

this is the distance of any star that we observe in the galaxy from the center of

the galaxy we change the scale quite dramatically and we can see that you

know we can do the same calculation of where the math says and how fast if the objects are supposed to move around the

center of the galaxy and we get a curve like this one which is very very similar to what we saw for the solar system

because you know it's kind of bright in the center we can imagine that all the matter is there so the measurement was

made they made if they took different galaxies they started with Andromeda starting in the 60s and what they found

was this which is very different so basically objects far away from the

center of the galaxy are going faster than ones closer to the center and even

like out here will you bear see any stars they're going faster and faster and faster so I'm not gonna tell

you the amount of these but if you do the ratio between those two you between

those two curve it tells that you're missing 90 percent of the mass in the system it doesn't come from the stars we

don't see it in this in this photograph the main person who dis dis original

measurements is Vera Rubin she died three years ago she started doing this in the 60s and for decades people

thought she must not be very good at science this must be wrong this year breaking news and we're

building a big the biggest telescope that will be built in the next decade has been renamed after and it will be

known as the Vera Rubin observatory so she got some delayed recognition okay so

the Alex's are missing some matter from stars so if we could look at the Milky

Way with the dark matter eyes we would see something like this a bright flat

circle or spiral where all the stars live and a much larger halo with I

picture dark matter is like a foamy object a foam is that substance that kind of holds it together obviously

these kind of analogies is imperfect so don't hold me to it but the Milky Way is

estimated to have about 10 times as much dark matter as regular matter and it will kind of look like this if we could

actually see that matter I did not define that matter for now that matter

means matter is something that has a gravitational effect we can see the gravitational effect by the way

planets and stars move around and this the world matter and dark means it

doesn't shine it doesn't make stars so then we can increase the scale a little

bit and talk about galaxy clusters this is a photo of a very nice galaxy cluster and I'm going to come back to this photo

so galaxy clusters have hundreds of thousands of galaxies and we can study

them with a very very cool phenomenon which is gravitational lensing so um you probably heard if not I will

tell you for the first time which is exciting if you have a massive object somewhere in the universe the mass in

the object will deform the space-time to such an extent that the shortest path

for the light from like point A to B may not be the straight path which is what

we normally see with light but will be a curve around curved the path that you

know follows the values of the deformation in space-time I wish there was a easier way to explain this nobody

understands general relativity but we can use it so this is a what we call the

picture you saw a minute ago was an Einstein ring because you know that's the guy who figured this out the

interesting bit is okay suppose I have a galaxy cluster and suppose I'm here on

earth looking at stuff outside if there is a galaxy behind the galaxy cluster

its light will be curved and it will hit me in a weird pattern basically this

curvature the radius radius of curvature will tell me what is the mass the total

mass of the object in here inside of the curve of the galaxy cluster in the middle so specifically going back to our

smiley face there are two clusters here a galaxy way far behind it and this is

the image of the galaxy behind it that is produces what we call an Einstein ring from the angle of these we can

figure out the mass in here and again the mass of the galaxy cluster is about ten times bigger than what we can

reconstruct from the light that's shining there so then we can make one

scale a step one more you know go up one more in size and we go to the whole

universe how much that matter is there in the whole universe and we know that - we found out exactly in 2001 I would say

how do we find out I don't know if people in the audience has seen a

drawing like this one before this is a sketch of the evolution of the universe from the Big Bang to the present day so

the Big Bang is the universe blew up for a fraction of a second and it expanded a lot faster than the

speed of light it cooled down it cooled down it cooled down and you know diseases here and at some point about

400,000 years after the Big Bang it cooled down enough for new neutral atoms

to form hydrogen hydrogen atoms when that happened light was able to escape from the hot and dense universe for the

first time and it kind of made a flash of light and we can still see they

delight from that event we call that the Cosmic Microwave Background it cooled down dramatically since at the moment

this was visible light now we can detect it in a microwave so if you look at the

universe behind and star and galaxy and galaxy cluster you will see a faint glow

and that's the left over from the Big Bang basically from the first light from the Big Bang this is what it looks like

this is a baby picture of the whole universe this was taken by the Planck satellite in 2013 is the oldest line in

the universe and it's been traveling for 13.8 billion years to reach us

this picture is cool but not very informative we have to like change the coordinates a little bit to figure out

the size of these small thoughts and the biggest regions sorry I didn't tell you what the picture shows there are regions

here that are slightly colder and slightly warmer not by much like fat or one in a hundred thousand and the

intensity of these of these objects that are slightly warmer slightly colder and the size of this of these small points

tell us basically everything about the early universe specifically they tell us what the universe is made of and

interestingly enough the stuff that were made of they told them don't element

occurred mentally particles that we know are less than 5% of the total then there is a 30% of that matter and then there

is a 70% of dark energy I'm not going to explain what these two are but basically dark means it doesn't

interact with light matter means it pulls stuff together through gravity dark energy means it pushed stuff apart

to through gravity another to represent this plot which was found

outside newspaper stand in England when this data was first was first published

is 96 percent of the universe is missing there was another big big event I was I

was in I wasn't during doing my Master's when it happened and all of my undergrad have been told by my professors that you

know physics is almost there we got it we made very precise measurements were missing like a couple of details and if

the standard model of elementary particle is very precise and very exhaustive and it was kind of depressing

to go into the field with that attitude because you know what am I going to do for job I need to find a real job which

was not like my preferred option and then and then one of the things we were

missing is the whole density of the universe there was a big question we answered that question with a 2%

precision and we didn't know the order of magnitude it was like a night date change within weeks oh and by the way we

don't know what it's made of so if you find people my age or younger who work on that matter this this this has become

really really really popular in the last 20 years ok fine but what is that matter I'm

going to start with what it is not so this is the famous standard model of

elementary particles by the way in 2001 they hadn't found the Higgs boson when they told me we have everything already

so this is a new new plot because it has the exposed I'm not going I could talk about this

drawing for days but I choose not to maybe it will be presented at a later

public talk but I'm going to compare this drawing this list of all the particles we know with the properties of

that matter ok that matter interacts gravitationally second feature it's not

short lived short most particles in this drawing decay within a fraction of a second they're made and and decay etc

etcetera we know that that matter is not short-lived because it holds the galaxies together so you cannot do that

if your particle decays you know 10 to the minus 25 second so you have to have stable

particles so that gets rid of two-thirds of the particle then it cannot be what

in um in cosmology we call hot or in particle physics we call light what I

call slippery particles such as neutrinos just go through the galaxy and never stop you cannot make a galaxy with

a bunch of slippery particles so unfortunately we have to take out neutrinos from this graph and then we're

left with electrons and port up-and-down so electron proton neutrons which is

what we're made of and so we know it's not that one because that interacts with light so we got rid of all the known

particles which is very excited because these means there has to be a new

particle which is why most of us are in the field and I call this job security

we found an effect that is not explained by something that we've discovered before

all right so what do we know about that matter there is a lot of it about 6 to

110 to 115 to one where you look depending where you look and it was

formed during the Big Bang why do in the Big Bang well when do you have enough energy to make a lot a lot

of anything that's a good time there was almost infinite energy during the Big Bang whole thing if you rewind the photo

I showed you go back and rewind the tape of the Big Bang we sort of see it interacting with regular matter or at

least exchanging energy of some form irregular matter they were actually changing type one into the other and

back and forth so that was interesting okay so then can we make it interact

with regular matter again because then if we can do that again if we can

reproduce something similar to the condition of the early universe then we might be able to figure out what it is

and what it's made of so there are three ways where you can reproduce the

conditions of the early universe in the hope of detecting the dark matter particle one is encounters in the lab

this is one of the studies that's been done at CERN in Geneva the collider you basically smash two particles

together in this case protons they annihilate they transform into pure energy and then the pure energy pulls

all sorts of other particles from the vacuum from the void sorry the quantum

void and one of these particles can be that matter practical so this is a photo

of the LHC accelerator this is the vacuum tube where the beam goes and this is the photo of one of the detectors the

CMS detector spoiler alert I'm telling you all the ways in which we can detect

that matter we haven't found it yet in case I I hadn't said that before so we

haven't found it that way yet second way you can do the reverse you can have to dark matter particles collide and

generate either energy or other particles this is done with like telescopes from ground and this is the

magic telescope I think our telescopes in space and this is the Fermi Space Telescope

which is one of the projects I work on and it's in orbit above the earth and it

hasn't found that matter yet although there is controversy about some you know the data that his firm is observing from

the center of the galaxy a third way is what we call scattering or going bump

underground so you create a target made of regular matter and you hit hope the

one one dark matter particle will hit your target of matter of regular matter

most of the that matter will go through will not interact with your target maybe one particle in a billion of a billion

billions will interact this is the photo one of those detectors of these

techniques that you use for to find it to find that matter in this with this method and this is the dark side

detector which is one of the five that matter projects I worked on so I'm going

to focus on this last technique which is called what we call direct detection that is detecting that matter with

interaction of nuclei on earth this is a photo of the Lux detector which was the

Commission couple years ago principle of the death detection basically you have that mother

coming in it will bump a nucleus or an or an electron but if it's an electron it's not dark matter so this is the

recipe for that matter directed actor build a massive tank or a massive Tower

of nuclei nuclei because to complicate I'll come back to this high bit deep

underground and then wait for a Dark Matter particle to hit one of your nuclei and then you want to look for

tiny vibration from nuclei there have been hit by a dark matter particle I said something upsetting on this slide

and you should like raise your hand why did I say on the ground isn't it weird

that I want to hide my detector underground if I'm looking for something that comes from space so this is a photo

of the atmosphere from the International Space Station the atmosphere is extremely bright in in particles with

our eyes don't see particle but if we have a particle detector it's it's really bright this specifically it's in

aurora borealis from space so a lot of particles come from the Sun interact with the atmosphere and glow particles

normally don't load but they're all over the place at all times and I would like to make a live demonstration if this is

successful please bear with me this is a Geiger counter it's the most rudimental

particle detector do you hear it can you

guys hear it it's not doing what I wanted to be doing which is one particle per second but you know bit irregular

these stuff comes from the Sun basically it interacts with the atmosphere and we

can pick it up we don't normally see it because our eyes only see photons that

is light not particles but that's what it does specifically the particles we

just heard they're called muons there is stuff that comes from the Sun from the

galaxies from outside the galaxy it interacts with the atmosphere it makes other particles are the particles they

stop the reaches the surface is this particle called muon is like they have it

of the electron we don't need to know but it tastes but there is a lot of them on the surface we're continually hit but

continuously hit by those particles but if you put your experiment under a mountain those things will be absorbed

by the mountain and then whatever makes it you know couple miles and at the mountain will be not immune so the thing

you're looking for that matter is very elusive so it doesn't care if there is a mountain it doesn't care if there is a

planet it just goes through this is what where we put NZ our experiment which I'm

gonna tell you about it's buried a mile underground in former Homestake mine in

lead South Dakota if you've seen the advert Evo that's where it happened the

Commission demand was decommissioned ten years ago and that's and now it's a science lab like I said I'm gonna talk

about this experiment the name is LZ or Lux Zeppelin it's an acronym of acronyms

it's a very bad name I didn't pick it please don't hold this against me so the

thing we heard earlier like one per square centimetre per second approximately is what we call mules and

that's the flux at the Earth's surface so one per square centimetre per second this drawing this plot is it is a chart

of all the different underground laboratories in the world we because

they have different rock composition we make we convert everything into water and this is how deep they are in water

so this is one millionth of the surface flux and the surf experiment I forgot to

put the label I'm so sorry surf the day home state that the lab is up here and this day the flux is about

15 per square centimeter per cell per century so it's a factor 200 million reduction so low enough that it did the

residual muons will not bother us this is what the site looks like they started

digging from the surface then they realized that the goal was elsewhere so they built a network of tunnels it goes

as deep as six miles underground this is what we call the headframe which is where the cable runs and the

cable you know a few miles long and he moves things and people and golden particle detectors up

and down this is the headframe this is a gold lamé ingot from from the mine from

a few years ago they switched to gold in the 90s and it shut it down in the mid 2000s and it's a very very interesting

place to visit growing up in Europe I had an idea what America would be like then I came here and it was like you

know it's cities and stuff but if you want to find the Wild West it's still there in like the middle of South Dakota

the people who have been there are nodding vigorously yes there is the Buffalo the Prairie all of that and you

know the Western type of things so I'm gonna give you a little bit of

physics now how does the detector work they lack discipline detector it's a

bucket of liquids II know I'm gonna tell you in a second why liquid xenon it has

we're basically I'm giving you giving away the lid we're basically in

detecting the matter true light so it's a bucket of liquid xenon there is a particle coming here it bumps one of the

atoms these this bump make some lightens and electrons we apply an electric field

top and bottom and we extract the electrons and then the photo sensors observe the light the first one a second

one I have a movie if you haven't heard me the first time I'll have a movie and then the particle goes down so bucket of

liquid xenon movies now playing panic okay this is my Dark Matter particle it

bumps on the lid on the xenon and makes light and light is detected up and down

by the photo sensor arrays that are on the two sides then we apply an electric field and some of the energy is released

in the form of electrons the electrons travel travel travel travel go to the top on the top there is a thin layer of

gas intense electric field and they make more light so that's how it works

we have two lights coming out of the interaction and I'm going to come back to that at the very end of this talk so

why is xenon and why a liquid so this is a incandescent tube it is charge tube

filled with xenon gas you see here it's um purple blueish most of the xenon

light is actually in the ultraviolet and we cannot see it but there is Center can be seen so it's used for example to make

incandescent light it's used for fancy halogen lights if you have an expensive car it is likely that your head your

light headlight will be will have some xenon in them so this is like probably

the most difficult concept I'm gonna say tonight so xenon is really good for that matter because of what we call kinematic

matching so if you are studying an object and you cannot look at it with

your eyes you want to throw stuff at it but the important thing is that is that the thing you throw at it has to be

pretty much the same size so if I want to study an object like this and they cannot use my eyes I'm eating oh I'm

gonna throw a cannonball at it well you will destroy it destroy the object but not really studied but if you're three

years old you know exactly what to do which is this especially if your parents never told you not to touch things this

is very important the fingers of the tree result of the same size as the flower and so then it's a good kinematic

matching to the object you're studying so we think that the zero nucleus has roughly the same size of a large class

of dark matter particles that we think is likely a good model for what we see

in the universe then it has a lot of nucleus per atom nucleons is a word that

says protons plus neutrons it has about 130 this is a drawing representation

planetary model incorrect please forgive me of the xenon atom we have a hundred thirty points in here and then 54

electrons we think that the Dark Matter interacts with regular matter with via

the nuclear weak interaction nuclear says it all interacts with the nucleus

not with the electrons and weak says this is very very infrequent which explains why we hardly ever see it

other thing that I already said it makes light and the light escapes this is a drawing representation or will the noble

ignoble liquids there are a number of noble elements in the periodic table I would like to talk about this for days

but we don't have this each one has a different color helium name etc there they're all like they make light and

they are transparent to their own light which means they make like but then you have a chance of detecting it with the

photo sensor then these two things it's very dense and can do background

rejection those a kind of technical and they're the last two things I'm gonna tell you in this talk and that it's kind

of cool that you have a liquid because if it's a liquid you can take it out of the detector and purify it and filter and put it back and we purify these

things continuously and this one is my favorite which is it's relatively easy to make a larger detector isn't scary

scare quotes is not that easy but you know first approximation if you want to go from a 10 kilogram detector to a

10,000 kilogram detector you just make a bigger bucket if you have to make it with solid-state detectors with a

crystal you have to build 10,000 kristance instead of 10 which is kind of challenging so this is the construction

of the first detector that I showed you earlier the Tang kilogram detector in 2006 so then scaling up by a factor of a

thousand took about 13 years which is quite remarkable in any scientific Tang

technique this is a fan graph that I

like to show most of you will be familiar with Moore's law which says computing power doubles every year than

one - every two years this is why faster than that this is where I started my

career from and this is where we're gonna be next year which is a hundred times hundred thousand times improvement

a thousand times come from the mass the extra 100 comes from making the

experiment better and that his water will tell you from the rest of this talk or in the rest of this talk time is it

805 but probably okay so I showed your first step is to build a massive tank of

nuclei and the tell you the recipe first of all whatever you do take a lot of pictures

so you can show your friends and colleagues and family first element is

photo sensors like I said we need to detect the light these things are very big in the photo but they're about three

inches across they are very weird they work in liquid there are electrical devices they work a

high voltage but they work in liquid and they working called so leak Zenon is only liquid at approximately

negative 200 Fahrenheit we had it we took 20 years to like developed the technique to detect like in light in

liquid and cold and not be radioactive etc a company did it we just bought it from them and we bought a lot of them

like a lot of them little bit more little bit more twice as many so like

five hundred of these okay this is your photo sensors that you saw earlier in

the drawing then we need to build the actual bucket the actual bucket we build modularly

we started here we like a four inch object and obviously you want to make it of highly reflective material because

the goal here is to detect the light from the interactions um like I said moderately we start models of this of

this object this is Teflon PTFE Teflon is very reflective as you can see

because it appears white it's very inflective to regular light but it's freakishly highly reflective to

ultraviolet light especially in liquid especially in xenon we don't exactly know how that happens but the

reflectivity that you see from this picture is seventy to eighty percent in ultraviolet light in liquid xenon it's

like 98 or 99 percent we don't understand the atomic physics or either

happens but that's like one of the big reasons why the entire field works that this thing is freakishly reflective then

you start stacking things and then a little bit more and then you you're happy every time you go up a little bit

more and then you go up a little bit more and almost almost there okay then I

told you earlier that we have to apply an electric field and you do that by building electrodes so the electrodes

were built here that's la they were woven and this object is a loom is a literally

loom our engineer went to the weaving Club of Sunnyvale to learn how to build a loom and we've we we've moved a solid

weave weave at a mesh of stainless steel these wives are about hundred micron

hitch and they're spaced like five millimeters so you you don't see the wires there but if you put a bright

object in the background you can see them they're like hundred micron each and it's like you know a mesh of

stainless steel make four of these ship them to South Dakota build the box that

keeps everything straight and everything aligned because it's a hundred micron thick thin and thick and can be the

phone you send your favorite students and your favorite postdocs to unpack it or just no favorite they wants you

kidding I want to say mean things about these guys but they're they're pretty great so then you unpack it bring a few

more colleagues and then you put the whole thing together so first you put the grids what we call degrees the

electrodes on top of the photosensors array again you cannot really see it but

if you zoom here on one of the photo sensors you see the thing on top and then excitement and then you put the

photo Center solar array on top and on the bottom of the packet and we have video these took you know maybe a little

over a day we have a crane and you know this gets lifted with the crane and it

gets put on top and take pictures that's very important and also a video and

Cedric said all right then you so we cover the tin foil during assembly so

that we don't get it dirty or damaged it accidentally take out the foil and take more pictures that was a big day for

everybody in photography so you see here the bucket and photo sensor array on top

and bottom lots of cables and then you expect it with ultraviolet light and

make sure it's really really clean these took another couple days and I'm gonna

make a break here and take a break here halftime show I'm gonna talk about radioactivity and then we come back to

construction you've seen that everybody is wearing cleanroom suits and to

pairs of gloves you can only see one but the two pairs of gloves mask here night etc why are we doing that

so everything is radioactive you probably heard that if you eat a banana it's about 15 backers of the activity

because bananas have potassium potassium is good for you if you don't have enough potassium your heart will stop but also

a very very small fraction of the potassium in nature is radioactive humans are radioactive as you know we

can find how long something has been dead by measuring its carbon-14 content this still comes from the Sun Sun the

radioactivity from the Sun interacts with nitrogen in the atmosphere makes a little bit of radioactivity plants take

the radioactive carbon in sweet plants or if we eat an animal the heat pits plant we ingest the carbon-14 which then

decays case in five thousand years and that's how you know how long something has been dead but also in organic matter

is radioactive this is an example of stock photo of steel piping everything

in the earth this sun blew up before the current sun that we have now so we have

all sorts of elements including some radioactive ones and specifically uranium thorium and potassium which are

all radioactive but they live very very very long life they have they live longer than the Sun actually and there

are traces of these elements in anything that you take out from the earth so then

everything is radioactive I've said earlier that the interactions from that matter are extremely rare and we must

must control the radioactivity of the detectors because everything that's the way they were built is going to be radioactive I'm gonna make two examples

the average human is a hundred times more radioactive than a Dark Matter detector per pound if you buy steel the

commercial steel that comes from the earth is gonna be 200 times more radioactive than the titanium they would

chose to build the back at the contains the pocket I'm gonna show you the titanium in a second we had to work we

actually with the company to develop a special type of the titanium there will be distilled before actually making the

ingot which is kind of crazy to make it pure in so problem here is that we don't get to

choose the rock in the Homestake mine mine is already there we went and

battered it without detector so we didn't get to choose the material so everything that we can choose a

super-clean and for the mind we do this this is the detector I showed you

earlier the white one one which is about five feet tall and then we put it in

concentric see in concentric cylinders of detectors so there is one out here

which is drawn in green this is we call a scintillator this scintillator contains gadolinium and gadolinium is an

element that is very good at trapping neutrons so if we have neutrons coming from outside from the rock they will be

trapped by this guy and then there is a large water tank that is basically a

buffer and absorbs radioactivity from the rock did I say something wrong guys

I'm looking at my colleagues with like mild concern so then I said there is a

titanium so first thing the bucket is contained another other bucket which is

made of titanium as I just said and we can go back to the assembly video have another video that's the last one of

putting the thing here the big detector inside this titanium bucket these

require cranes I think we did it in three days everybody was wearing harnesses so that they would

accidentally fall into a hole because safety first then we had guides obviously people taking pictures and

alignment alignment insertion and it - it was a very tight fit because if you

have extra space out here it decreases your this sensitivity of your detector

then it goes stuck here as you can see there was a lot of swearing and cursing and then rebuilding coming back the next

day and then if finally it was inserted so there we go it's in its bucket then

we put the lid on we seal it and we are very happy and say thumbs up so this is

the detector in its inner titanium what we call Christ the Christ that is a double wall metal object like a thermos

for your coffee and then we put if we bring it to the mind all the assembly will stand on the

surface in South Dakota but on the surface because it's kind of hard to work in the mind but then we have to

bring it down so we roll it to the side and packet we chose a snow day because

of course and then we put it on for to lift and drive it drive it to the head

frame this is what the half frame looks like and we suspended the whole the

patent dictator on on a rope that is you know few miles long and then lower it

very slowly two words of mine in terminology a vertical corridor is called the shaft and horizontal corridor

is called the drift so we sent it down the shaft when you send humans down here you put them in in a literal cage it's

called the cage and it takes about ten minutes when you put something so delicate it takes two hours because you

don't want to bump the object if you bump the humans no nobody cares just kidding safety first so then it gets

down it goes back on its side and it gets like transfer to the drifts and it

drifts are progressively cleaner where you get to the lab and it's a very tight fit so I told you earlier how many

people how many photosensors okay this many okay stop here how tall okay stop

here why did how did we know where to stop basically we build the biggest object that could go through the drifts and

into the shaft that's kind of how we design the detector and that's it this

is it on top of the water tank of the big shielding object everybody very happy

and it will go down through this flange and this is the inside of the water tank

with the postdoc waiting for the detector to be lowered those transparent

boxes are where the scintillator will go so once the inner detector is

installed we rotate these things we put them around the inner detector and then fill everything with other scintillator

robot or water so almost there I said a

few times that matter is extremely elusive there are about three term other particles per liter on earth depending

on the like give me order magnitude so show-and-tell this object contains three

their matter particles at all time these includes your bodies by the way so part

three particles per leader no matter what leader they're streaming through the earth at 150 miles per second

approximately including your head so I hope this is not upsetting they will not interact with the inside of your head

there are almost 10,000 particles inside the Z my detector that I showed you at

any given time and if they will go through the volume in something like 10 milliseconds 10 millionth of a second

so one billion particles go through the detector every second of those 1 billion particles we're hoping to see maybe one

per year if we're lucky and Nature cooperates so that's what I mean by extremely elusive so then the question

will be how does that compare to background because everything is radioactive so I said we get a 200

million reduction by going underground and then we build everything with as clean as we can so we our detector is

between a hundred times and 1 million times cleaner than regular stuff the

residual expected particles after you take these steps are going to be 5 billion 1 billion a year or 50 per

second of those that matter is one of the 1 billion so it's like an extreme

leader in a haystack problem and you know this is if you're lucky if Nature cooperates that we have 1 in 1 billion

so how are we going to distinguish those hopefully not this not this way all

right so this is how the way we designed the detector comes to our help first of all this is the detector bucket

if a particle comes from an object that emits radioactivity it will interact a

bunch of times in the Z no it will interact here and here and here and we'll have a sketch a jagged path is

that a word in English my making this up if the Dark Matter particle it interacts it will not interact but if it died

it will interact only once so the probability of disease sorry powers of

10 I shouldn't say it is 10 to the minus 17 the birthday probability of this is 10 to the minus 34 it's a number way to

say pretty much impossible so if I throw out from my 5 billion events everything

that interacts more than once I have a 95% rejection of stuff that I know comes

from radioactivity some down to 250 million already then what do I do I look

at how everything deposits energy in my detector this plot is very crowded we

I'm saying I don't expect like the public to understand it I don't really understand either although immediate and

this is the not the the deposited energy in the detector and all of these comes

from detector components from the rock from contamination radioactive

contamination of the xenon itself so we're gonna look in this region here

which is a bit less crowded and also not only does does that matter interact very

rarely but when it does it leaves very little energy so with zoom this region

and we look at the energy deposits and again there is a quiet region down here

so we only look down here and this is point four percent of the events that

end up in this in this region and this is this is you know nothing above here

will be that matter particle and this is more quiet sorry this was confusing then

we want to see some charge from those events to make sure that it's not an electric emission from those electrodes

because we're bringing a hundred kilo volt inside the liquid in the cold so you may have like electric discharge

emitted so you want to know that this is a particle event and not just an electrical phenomenon and these throws

out ninety percent more events then I told you earlier that the xenon is

extremely dense specifically it has 54 electrons per atom so everything that

has electromagnetic interaction when it comes to the to the detector will interact with those electrons because

there is a lot of an extremely dense leak xenon is like three kilograms per liter which doesn't mean anything but aluminum

floats on it so it's very very dense so everything will stop here it's basically a war made of made of its material so if

I throw out the external 20% of the volume by figuring out with the where

the interaction happened I get rid of 95% of whatever is there of of the

radiation I know the data and I get like a quiet region in the center so I can go

in here and find a quiet region then I told you the one of my outer detectors

one of my external buckets is made of scintillator that traps neutrons interestingly enough so when a neutron

hits here it will probably hit another time so then if I turn on this detector

and throughout all the events they had more than one interaction including the neutron detector what happens is this so

that this region will be inside the shaded line is very very quiet so I'm going to look for that matter there

distros out another 20% all of these are order of magnitude and if I change the

order the fractions are going to be different so don't quote me space don't don't make your own dirt metal paper

with those numbers it's like orders of magnitude then I remind you that they

let the particle that comes in has two lights one that is directly lighting one

that is from the extracted electrons so it's charge is based basically from the

electrons so the reason why we use liquid xenon or liquid argon is that

either matter particle will have an amount any a 50-50 distribution of light

that comes out first and light that comes out second for everything else most of the energy is deposited in the

form of electrons so the second flash of light will be will have a lot more light than the first flash of light so we

decide that you know for an electron this will be a hundred times bigger to a thousand times bigger for a dark matter

particle it will be the same size so I throw this out and this is 99.5% good

the rejection rejecting known that matter and so we end up with five events

out of five billion ah this is you know I said the recipe it's kind of like labor

intensive and resource intensive we do we write a lot of data one petabyte a

per ear and of that like one kilobyte is that matter and then we need a lot of

computing to analyze that this is the curie supercomputer nurse which is what we use for for these data analysis and

obviously you don't only need computers you need people this is a photo of the

members of the LC collaboration in South Dakota outside the mine we have about

250 between engineers and students and scientists some of them are here about

15 of them are here we'll introduce them at the very end so that's how you really do it is not just a computer so this is

a map of all the underground labs in the world there are about thirty to forty detectors looking for that matter in

this with techniques similar to the one I described at any given time and

spoiler alert we haven't found it yet but this is a worldwide effort and a lot

of people are working on it this is a photographs of the Milky Way galaxy from

New Zealand New Zealand is because we cannot see the galactic center from here

from the Northern Hemisphere and it's around here the galactic center it has there is gas in front of it so you don't

really see but most of what you see in this picture it is dark matter but we don't really see it and to me that's

kind of annoying little it is hiding most of the universe most matter in the

universe is dark and it is hiding in plain sight at all times like I said we

haven't found it yet it's been been working on it for thirty years if somebody some young person in the

audience wants to figure out what to do as adults we might still be looking for this in 20 years

hopefully not if not maybe we find it with the with this detector that I presented and then we have a big party

thank you for listening [Applause]

okay we have time for questions um raise

your hand be recognized all of you have microphones in front of you so before

you ask you a question just turn on the microphone the little button in front of you and then you can be heard and also

you'll be heard on the recording so who would like to ask a question ma'am can

you list the theorized dark matter particles I'm glad you asked this is a

selection of the theorized dark matter particles this is about seven years old

so there are probably more than that and just for context I don't even know which

one we're looking what is the WIMP here I don't see it

top-left webson applause I think you're

right it's somewhere around here so we're looking for one of these particles

and there is a lot lot more this is an image yes there is this is a partial

list of that matter particles as of 2013

but only one person can be right

so we can't we can't detect it because whether we have a difficult time detecting it because it doesn't interact

with matter but we have detected it because it has gravity and we have these

dark holes black holes sitting in the middle of all

these galaxies and no Einstein rings no interactional I mean are there is there

a way the people have explained that we can't see it by those kinds of

mechanisms thank you for asking these so it's very hard to see a black hole if we

had a lot of black holes in the galaxy we would see them because we would observe stuff that is falling onto them

so one of the big iPods is in the 80s and 90s was the so-called matches

massive halo comfort objects and we did a survey and figured out that in the

galaxy in our galaxy region around our galaxy we didn't have enough of those to

explain the missing factor of 10 in the galaxy then the black hole e potus's

hypothesis came back again a few years ago after the detection of gravity discover the gravitational waves because

we had intermediate scale black holes 30 solar masses falling onto each other and we started were wondering how many are

there really the beginning is we sing like we had a lot of them we don't really know the actual number because

we've seen a hand hand followed this event so far I think it got popular and went away again

so we dip the consensus again consensus are not certainty is that for what we

know about black holes there aren't enough to explain all the effects that we see and all the effects that we see

at the different scales so they have to be able to explain what they see in the galaxy what they sing galaxy clusters

what they what we see in the whole universe so we we don't think that there are there are there is enough of them am I

saying this right people will tell me if I'm not a couple of experiments and Italy have

claimed to detect some particle they thought was dark matter what do you think it is what do I think it is okay

first of all it's you know I'm Italian I don't want to get killed with this is

being recorded right first of all different experiments so these

hypotheses this happened like at when the density of the universe was made measures same here

one experiment claimed they saw something that kept the any fact that could be provoked caused by the Dark

Matter particle then like Turkey other experiments look for the same effect and didn't see anything you can find the

model that accommodates exclusion from those experiment and evidence from the first one the only way to rule it out

forever would be to remake it with the same material in the same lab with the same technique and there are efforts

around the world to do that there have been hundreds of papers to explain what

they think that the damn experiments is the important bit here is that um what I

showed in this drawing which is the ability to distinguish dark matter from

radioactivity that is not possible in the damn experiment so this can literally be anything if you want me if

you want me to say my favorite crackpot theory of what is this is the water in

the gran sasso mountain is very porous and the water level comes up and down and it's kind of synchronized with the

annual modulation that we see with that matter water is a neutron moderator can it do

that that or not I haven't done the calculations I should there are like I

said there are hundreds of papers and the Dhamma people who say that what what I just said is wrong and it's been

proven to be wrong and I'm not that your is my way of checking this is to build another detector and see if I see the

same thing I don't know if this is it answers your question it helps

oh sorry okay so I was wondering so

there are so many theories about to that matter right so how do people decide which one do I want to follow and spend

you know millions of dollars to build a detector so two ways one is we you know

the comic of like the drunk person looking for his skills under the the streetlight and his friend says is this

where you lost your keys and the answer is I don't know but that's where light is so we're looking for dark matter

interactions using the weak interaction why do we do that a because we know how to do it B because

if you rewind the clock of the Big Bang and see when that matter in the present

density was formed you get to a temperature which is roughly or was a few years ago roughly the same scale as

the wick interaction that's why I got exciting so there are those those two reasons because we can and because it

might be a possibility if you're very creative and have a there are other

experiments looking in other place of this and I think with soldering going the wrong way

and I think we're expanding the search in other what we call other parts of the parameter space if you're very creative

and can tell me how to find absolutely particles have ideas I don't know no no

I saw it but I'm like I'm looking for something that we don't know cue balls if you have an idea on how to look for

cue balls that will be great and but you know there is a process starting here

for the next you know all over us collecting ideas for the next ten years

on how to look for some of those other models

so normal matter is organized in galaxies and black holes is this also

from gravitational pool you can see is this also true for dark matter or is it

very uniformly distributed we think that that matter follows kind of it's the

other way around matter started to condense in regions of the universe they were seeded by dark

matter density so basically - baby picture of the universe I showed you is

very uniform is uniform in like except for one part in 100,000 with that level

of uniformity it's very hard to make a large-scale structure to see it what will become a cloud of gas that then

will become a star or a galaxy so we think that because that matter doesn't

work does it doesn't interact with radiation in the same way as you know the star is on the picture did it

started making clumps way earlier than anything else so he started making clumps you know years after the Big Bang

not not hundreds hundreds of thousands of years so then he started making plans

so while everything else was it was expanding the regular matter started falling on top of the pre-existing that

matter clumps basically so dark matter started making the aggregation and everything else started falling on it

what role does Dark Matter play in the formation of galaxies and does it interact with other dark matter

particles so yes the answer to the first one is what I just said so clumps of

dark matter form very very very early in the in the history of the universe and regular matter followed so basically

that matter seeded the formation of stars and the formation of galaxies and the other one is with taken up matter

interacts with other that matter one of the models in this thing although it may not be labeled that way is called the

self interacting dark matter so the dark matter has the weak interaction it will interact with itself

obviously besides regular matter and if not we still have models of the dark matter interacting with itself but not

with regular matter and you know take so-called self interact in Dark Matter

explains some of the things we see outside in the galaxy that may or may

not be explained by by like the standard dark matter model which is what I

thought mostly about tonight so probably we think so

what happens when you finally discover dark matter I'll be very happy I will

have a big party maybe somebody in this room will win the Nobel Prize not me but

maybe that person will bring all their colleagues to stock on what happens in

terms of regular life I don't know maybe 100 years from now 500 years from now we

will not take the power from like nuclear physics we will not like switch on the light using you know

electromagnetism we will switch it on using that matter that I do not know it's like well it's decades or centuries

away the universe will be a lot less annoying to me it's personally it's very

irritating that we have a very very comprehensive and detailed model of

elementary particle physics and all their interactions and that explains less than 5% of the total thing but

later can I take this one off yeah I'm a particle theorist so I have an interest

in this problem I it it's not I'm one of those people who doesn't feel like

annoyed by the universe that we don't know what dark matter is but the dark

matter particle as Maria Elena explained would be something completely outside of our current theory of particle physics

so it's like you know about electrons but you didn't know about protons and neutrons well now you know that there's

a proton and probably there's something else out there that contains that which would be a whole new set of interactions

in physics and a whole piece of the if you like grand unified picture of what

the fundamental forces are so it went up in the door and we could walk through

that door and who knows what's on the other side

more questions um

I've given that we understand that Dark Matter interacts with gravity and given that we know that black holes have lots

of gravity and we know that there's 10x more dark matter than regular matter we

consume the dark black holes are consuming a lot of dark matter is there

some detection mechanisms either at the event horizon or at the massive amount

of dark matter being consumed allow us to better detect and understand dark matter at these interfaces this is

extremely interesting I never thought about it but we see we know that black

holes exist because we see stuff falling onto it and the way we see stuff falling onto it is with electromagnetic

interaction so I don't know I I everything that we've done wouldn't work

for that matter so far but please if you have an idea that's that that is that that would be excellent that would be a

great thing to do

the third matter exists in every galaxy or Havana galaxy without dark matter

being discovered there are no galaxies without that matter there are multiple

galaxies with no regular matter or very little regular matter so there are galaxies with more than 99% that matter

and less than 5% the total but no there will be a massive massive discovery if

we found galaxies without that matter and we haven't found it that would that would prove that we are not

understanding how this works

[Applause] [Music]


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