Direct Detection of Dark Matter With Liquid Argon
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Transcript: English(auto-generated)
00:14
Thank you. So, the last time I was here in 2016, I spoke about the work we did, which
00:23
won the Nobel Prize, which was the detection of flavour change for electron neutrinos produced in the sun. Today I'm going to tell you about some of the present and future research that we're doing. We've moved on, in addition to studying further neutrino properties that
00:46
I'll mention briefly, we've moved on to studying dark matter. In this case, what I'm involved in predominantly involves liquid argon as a means of detecting dark matter particles.
01:01
We really know a lot more about our universe than we knew 30 years ago, and a lot of that is because of the remarkable measurements that were made by physicists for whom we had Nobel Prizes awarded, and some of them are here, the cosmic microwave background,
01:23
the studies of distant supernova that made us realise that the universe is actually accelerating in its expansion, and which gives us the feeling that there is a quantity called dark energy. But in addition, the overall understanding of basically the theory
01:50
of how the universe has evolved since the Big Bang gives us a model in which we are really quite a small part of the total. It was mentioned on the first day that we're
02:04
only about 4% of the total mass energy in the universe. We think that there is the dark energy that I mentioned, but also dark matter of a totally different nature from
02:22
the matter we're familiar with. When we started studying neutrinos back in the 70s and 80s, the thought was that perhaps neutrinos were the dark matter. They had properties that would perhaps allow that, but in fact we now know that the limits on their mass mean
02:44
that their properties are inadequate to explain all of the data that we observe, and in fact they're probably only about less than 1% of the total. So the question is what is that remaining 26%? There are many models for it, the principle among them
03:05
I think are weakly interacting massive particles or WIMPs, as we physicists like to inject a little bit of whimsy into what we're doing, and that's what I'll talk about predominantly today. There are also other models, there are axions and there are things that will
03:25
be discussed in the session I'm rather looking forward to this afternoon on the dark side of the universe. But for the WIMP approach, we have really two approaches that are being taken. One is to attempt to observe directly the weakly interacting massive particles
03:46
that potentially exist in our galaxy. Certainly the structure of our galaxy is such that a form of dark matter is required, so we look for the ones that have been created in the
04:03
original Big Bang when conceivably there was enough energy to produce them, and there's major efforts at CERN to try and create dark matter particles for the first time on Earth since the Big Bang by having hopefully enough energy that they can be created.
04:21
So let's talk about the Big Bang theory, not that Big Bang theory, but rather this Big Bang theory, but let's go back to this Big Bang theory because it's probably important in this audience to recognize that that series has just come to a conclusion after a wonderful period of time. Your generation for the young scientists is going
04:46
to have the wonderful advantage that as we go forward, all of your non-physicist friends are not going to be looking at you trying to decide if you're more like Sheldon or Leonard or Amy. However, I think we have a legacy from that program, and David Salzberg,
05:07
who was the technical advisor to the Big Bang theory, he happened to have worked in our group in the summer at Princeton back in the 80s. He invited me to come to a taping as Geek of the Week. I think there's a number of the other laureates that have
05:21
had similar experience, and in fact some of them have actually appeared on the program. The idea of Geek of the Week is simply to allow a real physicist to interact with the writers and the actors, and it was quite fun. It turns out actually that they filmed
05:40
this in front of a live audience, and if they don't get a laugh, the writers change it on the spot in order to try to get something that the audience does react to. It was an interesting bit of culture associated with us as physicists. But the real Big Bang theory, I'm sure you have seen this many times, but let me just remind you that the thought is that we start with a Big Bang followed by a period
06:05
of inflation that leaves us with a universe with individual components, and as of right now, the ones that we don't know how to subdivide any further are quarks, electrons, well really the ones that Takaki showed you earlier on. After about a microsecond,
06:25
we're coming to the point where quarks condense into protons and neutrons, and after about three minutes, those become nuclei, and then at about 300,000 years, we have a situation
06:44
where full atoms, neutral atoms are formed, and at that point, you start to see light. Following that, the variations, fluctuations in that become galaxies and stars, and that
07:06
develops to the point where 13.8 billion years later, we have our present situation. Now, what's made it possible for us to understand things in the way that I described
07:20
to you in terms of the composition in the first transparency, first slide, is the very remarkable measurements that have been made. Here's an example of some of the latest data, and what you see in this plot here is data dealing with the variations in the
07:41
light observed from the original Big Bang and the cosmic microwave background, and this is a polynomial expansion of it, and what's remarkable about it is that the line that goes through those points is actually a theory with six parameters, and the fit in that
08:03
case is, to my mind, absolutely remarkable in terms of an understanding of where we are in the universe as a whole, and that's what leads to the conclusions I mentioned earlier of 26% dark matter. So that's where we are with respect to the Big Bang theory,
08:23
but there's still a number of open questions. One of them certainly was one that Professor Kajita mentioned a few moments ago, and that is where did all the antimatter go, and we hope that with neutrino-less double beta decay, we will be able to understand
08:41
one parameter of the neutrinos that we don't yet know, and that is, are neutrinos their own anti-particle, are they Majorana particles as stated. Second question is whether there is CP violation in the neutrino systems, and the hope is that by observing CP violation
09:00
perhaps with the measurements that Professor Kajita mentioned for the light neutrinos that we have access to, we may be able to infer the theory of what's happening at the models that actually have extremely massive neutrinos influencing the process called leptogenesis,
09:23
where the antimatter decays away, leaving us with a matter-dominated universe. If we do have neutrinos that have, that are their own anti-particle, then it is also possible to measure the absolute neutrino mass, because as of right now from the oscillation
09:41
and flavor change measurements, we only know differences in masses between the three masses that underlie the three flavor types, and that question of the absolute neutrino mass has cosmological implications. It's small enough that it doesn't have as big an impact on how structure forms in the universe as dark matter does, but it is significant,
10:04
and another objective of those neutrinos double beta decay experiments. But a big question is what is the dark matter, and that's what I'll spend most of my time talking about at this point, focusing on one model for what dark matter may be, and that is weakly
10:22
interacting massive particles. So there's more microscopic information about why you might think there is dark matter out there. If you look out on a starry night, there's more matter about five times as much mass, in fact, in between the stars, more matter
10:45
I said, but I should have said dark matter in between the stars as there is in the stars themselves, and if you actually measure in a galaxy like ours, a spiral galaxy, the velocity of stars as a function of radius, then what you get is the white curve, and
11:06
if you then try to infer what the velocities would be if it were simply the glowing matter, then you get the red curve, and there's clearly a discrepancy there that is what leads to that conclusion. There's also a lot of evidence relating to the formation
11:25
of the large-scale structure of the universe and other information from gravitational lensing that suggests that there is that 26% of dark matter out there. As I mentioned earlier, there's an attempt to observe dark matter particles at CERN.
11:45
There's a strong motivation for weakly interacting massive particles by the supersymmetry proposal for one of the theories that goes beyond the standard model, and the lightest supersymmetric particle is a candidate for something that could live long enough to be the dark matter.
12:05
In terms of weakly interacting massive particles, the way we're approaching it is to attempt to observe the interaction of those particles with nuclei in an experiment where there is a good way of discriminating dark matter from other interactions in your detector.
12:24
I'll describe one of the laboratories in the process of making these measurements, and that's the one we have in Canada called SNOLAB. It's two kilometres underground, and the entire laboratory is ultra-clean, better than class 2000. On the right here is where
12:42
the snow experiment was performed, the snow cavern. It is now changed to snow plus that I'll mention in a minute, but we've had many experiments from around the world that are now situated or about to be situated in this laboratory, and many of them are
13:04
dark matter. I'll talk in particular about the deep experiment with liquid argon, and there's also the PICO experiment, which uses the formation of bubbles, which radioactivity does not form in this particular configuration they have for their bubble chambers, and
13:25
that can be very sensitive to spin-dependent interactions of dark matter, and the super CDMS experiment is scheduled to start there next year, and that uses bolometry as well as ionisation in their germanium and silicon detectors to try to discriminate against
13:42
background radiation. Very sensitive in both cases. This is what the lab looks like, at least one section of it, and on the right is Stephen Hawking in his most recent visit. He visited our lab twice. This was in 2012, and everyone who met him was simply
14:06
amazed at not only the intellectual capacity but also the resilience of a person who still wanted to go two kilometres underground in a mine cage in order to observe where
14:21
measurements are being made. You can notice it's rather clean. My mother visited at one point and she couldn't believe that I had anything to do with anything that was as clean as that is, but anyway, it was very important to us to maintain that cleanliness because we're looking for extreme low radioactivity in all the materials but also keeping mine
14:45
dust out of the experiments as we perform them, so that's been the way in which we run the lab. You'll notice on the right that everyone is in lint-free clothing. Everyone takes a shower before they come into the laboratory, and that's a great advantage
15:04
when it comes to creating even lower dust situations than the remainders, well, inside the experiments themselves. So I mentioned neutrino-less double beta decay, and I'll just touch on this because I mainly want to talk about dark matter, but the SNO experiment,
15:23
the original Sudbury Neutrino Observatory has been reconfigured. We wanted to study neutrino-less double beta decay, and we realized that if we replace the heavy water in the original experiment with liquid scintillator, we could get about 100 times the light output, and then if you dissolve tellurium, one of the best candidates for
15:46
neutrino-less double beta decay in that liquid scintillator, you can get a large amount of the material and still maintain a reasonable resolution, and in fact you do get very good sensitivity with that. We have recommissioned the detector. We had to now
16:04
hold down the central sphere. It's no longer heavier than water in the middle. It's now lighter, and so we had to, under ultra-clean conditions, drill holes in the floor and so on, but we've been running for about a year stably. We're putting the liquid scintillator in, and we'll have tellurium in next year. The main thing that I want to talk about
16:25
is the use of liquid argon for the detection of dark matter, and this shows you the deep 3600 detector, which has a little over three tons of liquid argon in a detector
16:40
that has a central volume of that amount of liquid argon in an acrylic sphere at 80 7 Kelvin surrounded by a sphere of photomultiplier tubes looking in to detect faint bursts of light which are actually running at room temperature, and the gradient is taken across
17:03
light guides and other material that thermalize neutrons that are produced in any residual radioactivity in the material, but it has been a tour de force in attempting to get low radioactivity. For example, the final process of finishing the central sphere was
17:22
to put down a device which goes down that long neck, and then opens out into a pair of arms, each of which has a rotating sanding disc on the end of it, and then it rotates enough to take a half a millimeter of surface off the inside of the detector
17:44
to get rid of the daughters of radon that have deposited there, and then you at the same time are pulling the residual filings, if you like, out of it, and so we were able to obtain very good low radioactivity, and we also used the property of liquid
18:03
argon, which is a really excellent property, and that is that there are two de-excitation processes for excited states of argon. One of them, and their parameters are shown here
18:20
for the singlet and triplet states, you have six nanoseconds and 1.5 microseconds for the two cases where if you have a nuclear recoil, which is what a dark matter particle will induce in your detector, the light comes out with a time constant on the order
18:40
of six nanoseconds, whereas if you have radioactivity, then it comes out over a period with a time constant around 1.5 microseconds. It's much more favorable than xenon for this purpose. Xenon has done excellent measurements. They use a different process I'll mention
19:03
later for doing it, but in our case with the deep experiment, we simply took the short light, first 60 nanoseconds, and compared it with the total light. That's plotted on the axis on this slide on the right here, and you can see if you induce nuclear recoils,
19:21
which you can do using neutrons, you get a number for this prompt light, which shows up here. If you have gamma rays, then you see it here, and that's the way we separated such things. This is data from 230 days with the deep experiment. We're not finished yet,
19:42
but this is our preliminary reporting. No background in 230 days. Unfortunately, no dark matter particles either, but at this point, we're adding to the evidence that the interactions with matter are substantially weaker even than the weak interaction. You
20:05
can see here on the left, the spin-independent interaction with nuclei, and the red line is this result. We'll continue to count and improve our analysis techniques, and right now, the best results come from xenon. We formed a major international collaboration
20:25
with over 400 researchers in the collaboration already. Basically, everyone in the world that's been working on liquid argon for detection process came together in this global argon dark matter collaboration, and we're working on a sequence of experiments. We're running
20:43
the deep experiment, which has about a ton of fiducial volume in the center. We're building right now a 20-ton fiducial detector in the Gran Sasso Laboratory, and the ultimate objective is a 300-ton detector at SNOLAB that is to reach what's referred to as the neutrino
21:03
floor. This is the point at which neutrinos produce nuclear recoils in your detector and neutrinos are background, which for Takaki and me are, well, you described how neutrinos were the original background for super cameo candy. For me, as background
21:26
it's unusual, but nevertheless, that's our objective, and I'll say a little bit more about that later. There's another approach that's used and was used in a 50-kilogram detector to observe not only the scintillation light I described, but also accelerate in
21:47
a time projection chamber the ionization and produce electroluminescence in the gas region above the argon, and that gives you a little bit of advantage in discrimination against electron recoils and also gives you good position resolution. With the discrimination
22:07
on light alone, in fact, you can get on the order of 10 to the eighth to 10 to the ninth discrimination against electron recoils, which is going to be very important when you try to deal with background in the future detectors. Here is what the 20-ton fiducial
22:27
detector will look like. It's in the process of final design. CERN is very much involved in this, and in fact, the design that you see here is based on a very successful design
22:40
that CERN has developed over the last three or four years as the prototype for the DUNE experiment for neutrino oscillation that in fact, Professor Kajita mentioned. The idea is that you use external thermal shielding and then inside that have in the central
23:07
region here underground argon. It turns out that argon from the atmosphere has argon 39, which is an interfering radioactivity, but by extracting it from an underground source
23:21
you can reduce that by a factor of 1400. In the central volume you have the active region of underground argon. You surround that with silicon photomultipliers, a new type of light sensor, and this is all inside a cryostat similar to the protodome
23:44
technology developed, well at least made to operate at CERN. It's a very nice technology that is based on what's done to transport liquid natural gas. You can go to a company
24:00
and buy modules off the shelf for all of the external cryostat. They've built and operated 500 tons at CERN already and there's an inner membrane here that's welded in place after you bring everything underground. The other innovative part of this is the
24:22
use of silicon photomultipliers, which at 87 Kelvin work extremely well because they have essentially no dark current. They also have absolutely remarkable, this is one, two, three, four photoelectrons emission observed with this detector. If you remember
24:43
photomultipliers this is just a, the resolution is at least the width between the peaks there and a reasonably good timing resolution and low radioactivity and so they can be placed quite close to the detector. The underground argon was measured at the
25:03
original 50 kilogram detector at Gran Sasso and you can see here that argon 39 shows a evidence here of a beta decay that is a black line. When you run it and
25:22
use underground argon, the black line which comes from using argon from the atmosphere is reduced by a factor of at least 1400 and that we are going to use as a way of filling our detectors with underground argon. We have a tender out for a facility that is
25:43
to be mounted in Colorado in a location where carbon dioxide is being removed from underground and sent to Texas for pressurizing oil wells but it has a large amount of argon in the flow and we expect to be able to extract about 70 tons per year starting
26:04
next year which gives us the opportunity not only to use it in the experiment in Italy but also to stock pile it for the 400 tons we need eventually for the larger experiment to be done at SNOLAB. There is a purification facility that is in the process
26:26
of first commissioning for the first part of it in Sardinia which is a very long cryogenic distillation column, 350 meters mounted in an abandoned coal mine which has the
26:43
capability first of all for chemical purification at a rate of about a ton per day which is for the big detectors but also at a much lower rate has the capability of further enrichment removal of the argon 39 which can be valuable in another experiment that I'll mention in a moment. We have cooperation from three major laboratories around the
27:07
world and the process that we expect, we're now at about this level in terms of cross section for interaction with nuclei versus mass. We actually have sensitivity
27:21
that goes above the region which is covered by the large hadron collider. The next generation of 20 ton will be here and the eventual 300 ton fiducial will be here at dipping into what is known as the neutrino floor. That floor as I mentioned is calculated
27:45
by for the coherent scattering of atmospheric neutrinos on the nuclei in our detector, the same neutrinos that Professor Kajita was talking about and recently the coherent scattering has been observed and the cross section for that will be well known to five to 10%
28:05
by the time we come to operating our experiment. We have an advantage over xenon because there is another background which is at about this level here which is the PP neutrinos from
28:22
the sun producing electron recoils. With argon we can discriminate by factors of 10 to the eighth or greater against such electron recoil events from neutrinos. It's harder to do that in xenon so far. Factors of about 300 have been achieved but that means
28:41
a little bit higher background if you do it with xenon. For the lower mass region below about 1 GeV mass for the dark matter particle it's possible to turn off the light part and just use the electroluminescence in which case you have a lower threshold
29:00
and the red curves here show you what has been observed already with 50 kilograms and we expect in the future to have another detector very carefully designed to have low radioactivity even more so than what we have done so far because in this case you
29:21
no longer have that discrimination ability against electron recoils and so there the reduction further of argon 39 in the area facility in Sardinia could be of an advantage in reducing your overall radioactive background as well. The final summary slide in terms of our sensitivity shows you that this future detector Argo is dipping into the
29:48
limitation from neutrinos. If you want to go beyond that you have to incorporate directionality which is much more difficult to do in your detectors. It is also possible and this is a sort of an extreme projection to in the low mass region get as far as that
30:06
dotted line shows the background here is defined by boron 8 neutrinos interacting and so that actually is an electron recoil pardon me a nuclear recoil process from
30:20
coherent scattering of boron 8 neutrinos. The xenon projects will compete and so in the future 8 to 10 years from now something for my granddaughters to work on I have 8 granddaughters will have these experiments that perhaps will enable us to understand
30:44
even more our universe and how it is composed and so please stay tuned for the future. Thank you.