Saturday, December 19, 2009

Primer On WIMP Dark Matter.

First, let's state that dark matter could be all sorts of things, but there is a lot of motivation to believe they are weakly interacting massive particles. (WIMPS)  It turns out our own Jonathan Feng at UC Irvine is considered one of the world's greatest experts on WIMPS and has some accessible review articles on dark matter collider physics here and here.  (The first especially should be assessable all physicists. I took images from them.)

I Think Therefore I Am

The fact that a particle exists should tell you it must somehow interact with other particles.  All particles are created and destroyed in interactions with other particles.  It turns out, using arguments from cosmology taking the expansion on the universe into account, how abundant a particle is is directly related to it's typical cross section, or likelihood that it interacts with other particles.  (See equation above.)

The graph on the right demonstrates this.  The higher the cross-section, the lower its abundance in the universe.  If it interacts too much, it won't be abundant enough.  It it interacts too little, it will be too abundant enough.

However, if a non-relativistic particle interacts with typical weak scale cross sections, its relative abundance to rest of the matter is just right to be dark matter.  This is called the "WIMP Miracle".

How Can We Detect it?

We know the the particle has to be non-relativistic because dark matter must be cold.  The typical non-relativistic weak scale cross section is given in the equation above.  Alpha is the hyperfine structure constant, m is the mass and k is a parameterizing allowed small deviations from from the weak scale to still work.  The graph on the right shows masses that work.

In some sense that's it!  If we can find any non-relativistic particle dominated by weak interactions with the right mass, 100 GeV - 1TeV, supersymmetric or not, we can feel fairly confident we have discovered dark matter.

At this point you may ask: "Wait, we know of particles whose dominate interaction is at the weak scale, like say neutrinos.  Why aren't they the dark matter?".  Well, the relativistic nature of these particles changes enough so that they don't work anymore.  (For example they have a different cross section.)

But How Do We Know If It Is Supersymmetric?

(I think) Most non-collider surveys will have a hard time answering this question.  They can answer the more important question: "Is this a non-relativistic particle dominated by weak interactions with a mass on the order of 100 GeV -1TeV?".  If yes, we have our dark matter.

Every different dark matter candidate, such as the supersymmetric ones, have specific ways they interact with other particles.  These ways are described by Feynman Diagrams.  The plot to the right shows the interactions specifically for the neutralino.  Colliders can test these diagrams better than anything else.

So here is the oversimplified formula:

  1. Find a particle dominated by weak interactions.  (Like a neutrino except non-relativistic.)
  2. Ensure the mass is on the order 100 GeV -1TeV. (You've discovered dark matter!!!)
  3. Run an experiment at the LHC for a particle of just that mass interacting mostly through weak interactions. 
  4. Compare the findings with the proposed models of such particles.
  5. If one lines up perfectly you know what particle you are dealing with. (Ie... some like a supersymmetric neutralino)
Smarter people than me may be able to pinpoint the exact particle without a collider, but as far as I know it may take the LHC to do this.  It's initial discovery however should be able to be accomplished without the LHC however.


  1. That first preprint is great. I especially love his figure 5 flowchart, particularly the steps "Did you make a mistake?" and "Are you sure?"

    I also like the idea of probing Newton's constant through the LHC. That's amazing.

  2. Yeah, that is a good article. Glad you liked it. It turns out Dr. Feng is a great lecturer too. He's proof you can be a great researcher and professor at the same time.

  3. I think that the dark matter in a way is compatible with my system.

  4. I think that is a so cool article,but I do not like algebra and mathematics so I think that is important but for my it is so difficult and complicated.


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