Date: October 17, 2009
Title: The Search for WIMPs
Podcaster: Jimmy Erickson
Organization: Smart Ass Science http://www.smartassscience.com
Description: Dark Matter makes up one quarter of our Universe, and we still don’t even know what it is. Many scientists are searching for the answer. There are several different teams around the world searching for the particles that could make up Dark Matter, called WIMPs (Weakly Interacting Massive Particles.) In this podcast, I will provide an overview of Dark Matter, WIMPs, and a couple of the projects currently being run searching for Dark Matter.
Bio: I am an undergraduate senior in physics at the University of Minnesota. I have worked for both the CDMS and EDELWEISS projects, both of which are searching for WIMPs using cryogenic underground detectors. It is my goal to one day become an Astrophysicist.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Cindy Koppelman (http://www.arthouseprint.com).
Transcript:
Jimmy: Hello, I am Jimmy, and I am here with Professor Rupak Mahapatra from Texas A&M, and we’re going to talk a little bit about Dark Matter and the Cryogenic Dark Matter Search.
So, Professor Mahapatra, the big question that we should probably address is: Does Dark Matter exist?
Prof. Mahapatra: Yes, and we have seen the evidence for the last 70 years. Only in the last decade are we getting enough experimental evidence and theoretical understanding to have a pretty solid picture of where Dark Matter exists, and what faction of the Universe is made up of Dark Matter.
Jimmy: What evidence to we have for Dark Matter?
Prof. Mahapatra: The first evidence that anybody had ever seen and come up with was Zwicky at Cal Tech about 70 years ago. He found that the speed at which the stars are moving in a galaxy are actually much higher than you would expect from the mass available from the luminous matter. That was the first evidence.
Then Vera Rubin did that same study for many different galaxies and confirmed that indeed the rotation curve of the galaxies where you look at the stars that are emitting light and apply Newton’s law, for example, to understand what the rotation speeds should be and what they actually are. It’s surprisingly very different and it’s not a small effect. We find that the speeds of the stars as a function of the distance from the center of the galaxy, stays pretty flat. It does not change, it does not go down as you go up in radius. Where as if you look at the velocities of planets around the sun, the velocity goes down as one over the square root of R, as you’d expect from Newton’s law. So clearly there is a huge mass in the galaxies that you can’t see, but they are affecting the stars’ velocity through gravitational effects. So that’s the first motivation for the existence of Dark Matter, and it’s universal to all galaxies.
Jimmy: And have we found anything since then? Is there other…?
Prof. Mahapatra: So there have been other evidences in the last decade or so where precision cosmological and astrophysical measurements such as cosmic microwave background and large scale structure formation, and more rotation curves of galaxies have confirmed that the picture is a pretty unified picture. In fact, what we know now with very good precision is about one quarter of the Universe is made up of what’s called Dark Matter, and the rest of it is dominated by what is called Dark Energy, that makes up three quarters of the Universe. It’s really a puzzle and very surprising that we understand less than 1% of the Universe in terms of things that we know such as electrons, protons, and neutrons, things that make us up.
Jimmy: So we have us, Dark Matter, and Dark Energy. Now just to clarify, is there any relationship between Dark Matter and Dark Energy?
Prof. Mahapatra: We don’t know yet. There is no obvious connection, but the theoretical understanding is being developed, but we are really very far away from even thinking about the connection between Dark Matter and Dark Energy. So at this point I would say we don’t know.
Jimmy: So they’re both big mysteries.
Prof. Mahapatra: Right. Although the Dark Energy is more mysterious than Dark Matter, because energy is something the pervades all space and you can’t actually localize it, you can’t see it, where as Dark Matter you see the effect in the rotation curves of galaxies or, even more dramatically, in a more recent experiment called the Bullet Cluster.
It’s a very dramatic evidence for the presence of Dark Matter because you see two big galaxies go through each other in a collision, and when that happens the Dark Matter is very weakly interacting, and so where the two galaxies go through each other, the Dark Matter components in the galaxy they just pass through each other without much interaction at all. Whereas the normal matter in the galaxies have a lot of drag, because they have a lot of interactions with each other, so they fall behind. So, when you look at these two galaxies in x-rays, you see that there’s a big difference in x-rays, and through lensing experiments you see there’s a big separation of the masses.
Why? Because you can’t see the Dark Matter, but you can feel it gravitationally through lensing experiments, whereas you can see the normal matter through the optical spectrum because they emit x-rays. So, in x-rays, you can see that there’s one blob that represents normal matter, in lensing you see another bulk matter that represents Dark Matter, and they are clearly separated for each of those two galaxies. And we call it the Bullet Cluster because it looks like a bullet going through another object, and that’s the clearest evidence for the presence of Dark Matter.
Jimmy: So we see Dark Matter present there, do we know actually what this Dark Matter is?
Prof. Mahapatra: We don’t know yet. There are many candidates for Dark Matter. We believe it’s of particle nature because that would solve a lot of mysteries in both particle physics and astrophysics. In fact, with a basic assumption of standard model particle physics, which is very well known, if you combine that with what we’ve seen and measured, that one quarter of the Universe is made up of Dark Matter, a pretty straight forward calculation show that it has to be a particle with interaction cross section with that of the weak interaction. Weak interaction means it’s many orders of magnitude weaker than the electromagnetic force we all feel that binds the electrons and protons in the hydrogen atom.
Jimmy: What are some of the candidates then for Dark Matter?
Prof. Mahapatra: So the particle nature makes two viable candidates, although one candidate is much more believable than the other candidates. The most favored candidate is a heavy particle called the Weakly Interacting Massive Particle, and when you say massive it means about 100 times the mass of the hydrogen atom. So not too much different from a lead atom or an iron atom. This particle is the most favored because it fits in the puzzles very well. You know puzzles exist in particle physics and astrophysics, and beyond standard model particle physics called super symmetry. So this heavy mass particle with weak interaction cross section is the most favored candidate, but there are other candidates for Dark Matter on the other side of the mass spectrum. They are very light, they are billions of times lighter than the assumed WIMP. They are called Axions. They come in from a different source in particle physics. The Axion is a much harder particle to detect because again it has very little interaction, but the mass is very very low. There are a couple of experiments in the world looking for Axions, and there are about 30 experiments in the world looking for the WIMP.
Jimmy: So it has to be one of these, and it can’t just be gas and dust out there in space that we can’t see, it has to be something new?
Prof. Mahapatra: It can be, the way we understand now is that the Dark Matter is not just one type, it’s a combination of particle types such as WIMPs. Even for the WIMPs, it’s not just one particle, there could be a combination of four different particles, so it could be a combination of WIMPS and many other candidates, it could be Gravitons and Axions as well, and some component of it could be Neutrinos, although it used to be thought that Neutrinos could be Dark Matter, they’re not, though certainly they could be a component. So Dark Matter ideally is a big bag of many different things, the dominate of which we believe are the WIMPs.
Jimmy: We’re out there, we’re looking for WIMPs, lets talk about some of the projects that are looking for WIMPs. You work on the Cryogenic Dark Matter Search, also referred to as CDMS. Can you explain a little bit about that and how they are attempting to detect this Dark Matter?
Prof. Mahapatra: There are many different experiments using many different technologies in the world. The Cryogenic Dark Matter search is a collaboration of 15 US universities, one university from Canada, one from Europe. The project uses a very fancy technology for the detector. The reason we call it the Cryogenic Dark Matter Search is the detectors are at cryogenic temperatures.
Jimmy: How cold?
Prof. Mahapatra: They operate at about 50 millikelvin temperature, which is essentially absolute zero temperature. Very close to zero temperature. The way we do that is using a dilution refrigerator which can bring down the temperature very low. Our experiment uses semiconductor germanium detectors. You cool them down to very low temperatures so there are no thermal vibrations, to distract you from measuring this very weak signal from WIMPs because WIMPs are a very weakly interacting particles. So you want very quiet detectors, and we use a technology called transitional sensors so it’s a superconducting detector where very little deposition of energy can give rise to a big signal and that how we look for WIMPs. We use many different shielding technologies, both passive and active, so that we are not fooled by the presence of other particles.
Jimmy: Because any particle could come along, hit these detectors, and it would look like a WIMP to the detectors.
Prof. Mahapatra: A majority of the particles will not look like WIMPs because our detectors are designed explicitly, such as gammas you know radioactive photons, they will not look like WIMPs because our detectors are designed specifically to differentiate between electromagnetic interactions and the nuclear interactions from WIMPs. So wimps undergo a nuclear reaction. They have no charge so they cannot interact with electrons, so they interact only with neutrons and protons.
So we have a semi-conductor detector, cooled down to very low temperatures, almost absolute zero temperature, just waiting there for an interaction from a WIMP with very active, very sophisticated shielding mechanism.
Jimmy: And so the biggest shield that you’re using then is the planet Earth right? How far deep down is this experiment?
Prof. Mahapatra: Right, we can’t do this experiment very well at the surface because of the cosmic radiation, so much of the radiation such as neutrons would look like WIMPs. They are indistinguishable. So we go as deep as we can. Our CDMS experiment, currently is in a mine half a mile underground in northern Minnesota. It used to be an iron mine, now it’s a site for two different experiments, a Neutrino experiment and a Dark Matter experiment.
Jimmy: So has the CDMS project made any discoveries so far?
Prof. Mahapatra: No. No experiment in the world has successfully discovered WIMPs. There were a couple of false claims, but none of the experiments have successfully seen it. CDMS’ claim to fame is the most sensitive experiment in the world. We have the best limit in the world in the search for the WIMPs.
Jimmy: So if we’re able to use these detectors half a mile underground, that means there must be WIMPs then, ideally we’re expecting them half a mile underground, does that mean they are everywhere all the time?
Prof. Mahapatra: Right, so the WIMPs are everywhere in the Universe, they are passing through us right now, there are millions of WIMPs passing through us. A good number to think about it is, there is one WIMP in every bottle of coke, you know, so about a liter of bottle will have one WIMP at any point in time, that’s how the number density works out. So they are passing through us every moment and they can pass right through the Earth, just like the Neutrinos because of the weak interaction. So even if they pass a million times through the earth, they are unlikely to give you even a single hit. So it’s just a numbers game. So we have lots of WIMPs… for a given WIMP we are unlikely to get a hit ever, but when you add them up, all these WIMPs, we expect to get some signals, and they have no problem going down a half mile underground.
Jimmy: So the idea then is to build the biggest detector you can possible build.
Prof. Mahapatra: Exactly, so the bigger the detectors you have, the higher the chance, and that’s exactly the point of direction of CDMS is the next phases of CDMS are called Super CDMS, and the idea is to make bigger and bigger detectors, or more and more of the same detectors, so right now the mass is five kilograms total detector mass, we expect to go to 100 kilograms in the next three or four years, and about a ton in about five years.
Jimmy: Excellent, very exciting! So hopefully in the next five years, we’ll be able to talk again, and you’ll be able to tell me all about the WIMPs that CDMS has discovered.
Prof. Mahapatra: Hopefully.
Jimmy: Thank you very much Professor Mahapatra for speaking with me today about WIMPS and the CDMS search for WIMPs.
Prof. Mahapatra: Thank you Jimmy it has been a pleasure.
End of podcast:
365 Days of Astronomy
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