Direct Detection of Dark Matter?

Discussion in 'Physics & Math' started by BenTheMan, Dec 8, 2009.

  1. BenTheMan Dr. of Physics, Prof. of Love Valued Senior Member

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    http://resonaances.blogspot.com/2009/12/dark-matter-discovered.html

    The CDMS experiment has supposedly submitted a paper to the journal Nature, and has ordered a film crew for the release of their data in January.

    Could this be detection of dark matter?

    PS---In this thread, I will eventually explain how CDMS works, or someone else can, if they understand the mechanics.
     
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  3. Beer w/Straw Transcendental Ignorance! Valued Senior Member

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    I'm interested but not getting my hopes up from the first sentence.

    "The essence of blogging is of course spreading wild rumors."
     
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  5. BenTheMan Dr. of Physics, Prof. of Love Valued Senior Member

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    In this case, the statement seems to be correct

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    The rumors have been tempered a bit, and it's probably not the big news that some people thought.

    But I will still try to explain the direct detection experiments----it's really not too hard to understand what the experiments are measuring.
     
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  7. Acitnoids Registered Senior Member

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    http://www.newscientist.com/blogs/shortsharpscience/2009/12/rumours-that-first-dark-matter.html
    .
    According to this link, Nature's senior physical science editor Leslie Sage has squashed the rumours that a paper is about to appear in the journal.
    .
    Whenever you decide to explain the process, I'm sure you will have an audiance. From what I can tell, they are looking for a specific apparition of dark matter refered to as weakly interacting massive particles or WIMPs. These particles set off a nuclear recoil inside a detection medium as oppose to the electron recoil triggered by most of the background radiation. Why they think dark matter exists in this form is beyond me but, we'll never know unless we look. I'm interested in knowing how they can tell if they have indeed observed a WIMP as oppose to some other exotic type of particle.
     
  8. CptBork Valued Senior Member

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    The project I'm working with has been looking for dark matter, as well as Higgs particles and all sorts of other exotic theoretical stuff. It's all done by stastical inference in our case- we look for various particles to interact and decay in certain ways, and from the precise details of those interactions and decays, we can get a sniff of physics at the cutting edge of the Standard Model and beyond. The problem with indirect observations like this is, you can never be 100% sure about what you actually saw- there are many ways for conventional particles to produce the same kinds of signals in our detector as what we'd expect to see from certain Higgs models or dark matter. We have to try very hard to look at various types of particle interactions from as many viewpoints as possible and to watch these interactions as closely as possible, so we can get strong results and make statements along the lines of "if the Higgs particle didn't exist, we'd have only a 0.001% chance of getting results like what we found after 10 years of measurements, whereas according to such and such a Higgs model, we expect to get results like this 99.9% of the time".

    So for the case of Higgs searches and dark matter searches in the project I'm working with, it consists of a relatively small number of individuals carefully poring over mountains of old data, looking for small effects that haven't already been noticed or explained. Obviously none of them have found anything truly mindblowing up to this point, or we'd all know about it, but these are all considered longshot efforts anyhow, and probably indicate that some of these guys have just a bit too much time on their hands (not to say these searches aren't worthwhile, but there are other priorities neglected in the process). I haven't looked too deeply into how our guys are searching for dark matter, aside from attending a few presentations on occasion, but from what little I know, I have no idea how they plan to isolate whatever effects dark matter might have from the things ordinary neutrinos do; after all, since dark matter is still a vague concept and there's plenty of disagreement on what it actually is and does, no one's even sure what to look for in the first place.
     
  9. DRZion Theoretical Experimentalist Valued Senior Member

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    What do you think about this one -
    http://www.nature.com/nature/journal/v461/n7264/pdf/nature08437.pdf

    So dark matter has the same density in this halo, and coincidentally gravitational acceleration due to luminous matter is also constant in this halo. Did I understand that correctly?
     
  10. BenTheMan Dr. of Physics, Prof. of Love Valued Senior Member

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    Indeed

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    It does seem that some of us were too quick to interpret rumors.

    Anyway, I promised an explanation of the experiment, so here goes.

    When you look at how dark matter is distributed throughout the universe, you find that it exists in large spheres around galaxies, called halos. There is indirect evidence of this---the behavior of the outer arms of the Milky Way require a more or less constant mass distribution at least to the edge of the galaxy, and computer simulations of galaxy formation give us an idea about how dark matter should be distributed. But there is also direct observational evidence in the form of gravitational lensing. Einstein's equations predict how much light should bend in a gravitational field, and the observations can only be explained if there exists some approximately spherical dark matter distribution.

    There is one possibility, that General Relativity is somehow wrong at long distances. This was disproved, though, by observation of the Bullet Cluster. Basically, we see two galaxies which have collided. The luminous matter of the galaxies stopped, but the dark matter halos kept moving through each other. So if we try to understand dark matter by changing general relativity, we still need some sort of new particle explanation to fit the data. In other words, you have to reintroduce dark matter. I will ignore this possibility in what follows, but some people still try to make these models work. (Most people in the theoretical community, though, disregard this possibility.)

    Now, we have dark matter halo which is basically at rest with respect to the galaxy. Our solar system exists in one of the spiral arms, and we are rotating about the center of the Milky Way. So our solar system is moving through this cloud of dark matter---we feel a dark matter wind. This is like riding your bike down the street---the air is more or less at rest with respect to the street, but you're moving relative to the street, which means you feel a wind.

    The earth is also moving with respect to the solar system. This means that sometimes our velocity is in the same direction as the solar system's through the galaxy, and sometimes our velocity is in the opposite direction. That means that locally, on earth, sometimes we measure a change in the speed of the dark matter wind. And this is what the so-called ``direct detection'' experiments are measuring.

    If the dark matter is a weakly interacting particle, then it should interact with nuclei. In this instance, ``weak'' means ``via the weak force''. The weak force is a short range force, so that means a dark matter particle has to get close enough to a nucleus to interact. So the goal is to get a bunch of nuclei together and look for dark matter interactions. If the dark matter is a single particle, the interactions will be primarily elastic scattering. The classical analogue is hard sphere scattering. When the dark matter comes through the lattice of nuclei, it can bump into one of them. This makes the whole lattice vibrate, and this is what we measure. Because of the signal that we expect, we expect more collisions when the earth is moving parallel to the velocity of our solar system about the center of the galaxy, and less collisions when we're moving anti-parallel.

    The rate at which a dark matter particle bumps into a nucleus is given by

    \(\Gamma = n \sigma v\)

    n is the number of particles, and \(\sigma\) is the cross section---this tells us how likely a particle is to interact with another particle. In this case, the experiments are looking for weakly interacting particles, which means their cross section for interaction with nuclei is set by the weak interaction. As you can see, as v changes, so does the rate \(\Gamma\). Over time, if you plot the rate \(\Gamma\) as a function of time over the course of the year, you should see one period of a sine wave.

    Of course, the dark matter could be something different, like gravitini. (In supersymmetric models, the supersymmetric partner of the graviton is the gravitino. In certain cases, the gravitino can be very light, and makes a good dark matter candidate.) These dark matter candidates interact only through gravity, and the gravitational interaction is so weak that we could never hope to measure such a particle in an experiment like the one I've described.

    Anyway, I'm sure I've screwed something up, or left some questions unanswered. Please, ask and correct away!
     
    Last edited: Dec 12, 2009
  11. Acitnoids Registered Senior Member

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    I'm trying to improve my understanding of particle physics. Please correct me if I am wrong but I thought that the only way to find a particles cross section is by knowing what that particles compton wavelength is. If I understand the compton wavelength correctly, it is just another unit of mass. I know that you can rearrange the equation for finding the Thomson cross section of an electron (alpha sq * (8pi/3) * compton wavelength sq / 4 * pi sq = Thomson cross section) in such a way that you can find the rest mass of an electron (the square root of (alpha sq * (8pi/3) * inverse-meter to kilogram relationship sq / Thomson cross section) = rest mass * 2pi). My questions are, how did they deturmine the compton wavelength of a dark matter WIMP? Is it just a statistical approximation of a galaxies rotation curve? Is the cross section you speak of derived from the interaction between the WIMP and nucleus?
    .
    (I'm sorry if the above equations are arranged improperly.)
     
    Last edited: Dec 12, 2009
  12. BenTheMan Dr. of Physics, Prof. of Love Valued Senior Member

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    In general, the cross section for a given interaction is determined by the coupling constants involved, as well as the masses of the particles. For Thompson scattering, you have to know the coupling constant of EandM (1/137) and the mass of the electron.

    In general, we don't know the mass of the dark matter particle, and we don't know how it couples to matter, therefore we have absolutely no way of knowing what the cross section is.

    But we can guess.

    What we know is that precision experiments, like WMAP, measure that the universe is something like 25% dark matter. This number is known very precisely, and I'm not really doing the experiment any justice. The typical argument goes like this:

    IF dark matter is made of a single type of particle, and
    IF that particle interacts with our matter with a typical weak-scale cross section (~pico-barnes, if those units mean anything),
    THEN it just so happens that the relic abundance of dark matter exactly matches what WMAP measures.

    There's actually a lot of IFs above, but given these IFs, we can ask, what ELSE could we measure. These direct detection experiments are one possibility, but as I mentioned before, if one of the IFs above is wrong, then they will never see anything.

    Let me write down a formula, tell you what it means, and tell you why it makes us think the IFs above are reasonable.

    \(\Omega_X h^2 \approx \frac{0.1 \text{pb} }{\langle \sigma v\rangle }(\approx 0.25)\)

    The number that experiments, like WMAP, measure is \(\Omega_X h^2\). Theoretically, it is a function of the freeze out temperature. In other words, you imagine the universe as a hot soup (read: chemical equilibrium) of fundamental particles at early times. The universe expands and cools, and the particle species in the soup fall out of equilibrium. The point at which it falls out of equilibrium is determined by its cross section, and is called the freeze out temperature. Once a particle species freezes out, then it either decays or we're stuck with it. Dark matter doesn't decay (or, if it does, it decays very slowly), so this is what's called a ``relic abundance'' calculation, because we're left with it today.

    Anyway, this gives us a way to connect the early universe with observation. For us, WMAP can measure \(\Omega_Xh^2\), and if dark matter is a single particle species (which is what was assumed in deriving the equation), then this gives us a way to measure its cross section. We still don't know anything about masses and coupling constants, but we can measure its cross section.

    Now the magic---if you guess that \(\sigma v \approx 0.1 \text{pb}\), where pb is ``picobarns'', you get \(\omega_X h^2 \approx 0.1\), which is exactly what is measured. The reason this is magic is that 0.1 pb just so happens to be exactly the number you'd expect if the dark matter interacted via the weak force. This is why we THINK that the IFs above are pretty reasonable...coincidences like this don't happen often.

    I realize now that I may have rambled for too long, but hopefully the answers to your questions are in there somewhere

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  13. Acitnoids Registered Senior Member

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    You rambled on just enough :thumbsup: . I understood just about everything you said. Thank you.
    Coupling constant. That's the word I forgot. In the case of Thomson scattering, this would be the fine structure constant (alpha). Is there a difference between Thomson scattering and the cross section of an electron?
    I had never used this unit before so I looked it up. It sounded like one of those physicist jokes. One barn or "b" is equal to 1.0x10-28 square meters (picobarn or "pb" is equal to 1.0x10-40 square meters) and is a unit of area. As it turned out, it was an attempt at humor. It was a reference to the large cross section of Uranium (broad side of a barn).
    In the past I have used words like phase transition or universal strata to discribe this process to myself. "Freeze out temperature" sounds more accurate because it describes at least one of the functions behind the "relic abundance" of particle species.
    Here, here.
     
  14. BenTheMan Dr. of Physics, Prof. of Love Valued Senior Member

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    Well, you have to tell me a cross section between an electron and what. The electron interacts with many things

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    Thompson scattering describes a specific process.

    Indeed. I thought the barn was defined in terms of a proton-proton scattering, but oh well.
     
  15. Acitnoids Registered Senior Member

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    So then I can assume that a Thomson cross section of 0.6652458558x10-28 meters squared is describing the process of an electron interacting with another electron?
     
  16. Acitnoids Registered Senior Member

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    Well, I can see now how wrong this statement is. The Thompson cross section (elastic scattering of electromagnetic radiation by a charged particle) is a different measurement than say the differential cross section (probability to observe a scattered particle in a given quantum state per solid angle unit) of a particle. I mentioned the Thompson cross section above because it is the only one listed on my "complete list of fundamental physical constants" and it is the only one that I have ever calculated. Like I said before, my understanding of particle physics is elementary at best. I may come off as being aloof but I can assure you that I am smarter than the average bear. If I were given the opportunity to replicate one or two numerical examples of each type of cross section then I know that I could grasp the concepts better. I can understand if you don't want to get into the logistics. Hell, I don't even know if you are interested in this kind of stuff but I figured, when else could I ask such questions.
     
  17. Orleander OH JOY!!!! Valued Senior Member

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    Update: Nature contacted the source of the initial rumor, the blog Resonaances [see the "Important update" at the bottom], flatly denying any upcoming paper published in their journal by the CDMS team. Which certainly deflates the claim of a major, peer-reviewed finding and makes an incremental development on pre-existing data far more likely, if anything.

    can anyone translate that into english for me??
     
  18. Acitnoids Registered Senior Member

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    It sounds to me like the journal Nature contacted the blog Resonaance and flat out told them that there was no CDMS paper being published in their journal.
     
  19. Orleander OH JOY!!!! Valued Senior Member

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  20. Pete It's not rocket surgery Registered Senior Member

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    Here's the cast of characters:

    The Blogger = Resonaances
    The Science Gods = Nature (the journal that publishes some of the best science research)
    The Science Squad = Fermilab, home of CDMS, an experiment that is looking for dark matter.

    And here's the translation:

    The Blogger said that The Science Squad had presented an offering to the Science Gods. The Blogger thought that maybe this meant that they had actually found some Dark Matter.
    Then the Science Gods told the Blogger that he was wrong. The Science Squad have not presented any offering. So now the Blogger says that whatever the Science Squad has, it's not that they've found Dark Matter. It's probably another list of places* where they've looked but not managed to record a good sighting.


    *Not really 'places', but something more sciencey that I don't understand.
     
  21. CptBork Valued Senior Member

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    Found a couple of new articles on CDMS from UK newspapers, here's one of them. The claim is that 2 particles have been observed with the expected characteristics of dark matter. Not a great deal of statistical confidence behind a small result like this of course, but at least it's a sniff. I'm sick and tired of people not finding anything and then settling for new upper bounds.
     

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