There are two big issues with dark matter - what is it and why is it dark. If dark matter is made of WIMPs (and there is very good evidence that it is), the question becomes what are these particles. We know they don't have electric charge like protons or electrons. We know they don't bind together like the nuclei of atoms. The one thing that we know for sure is that they have mass, but how much mass per particle is unknown. If we know how much mass an individual WIMP has, we can use theoretical tools to tell us what it is.
The other big issue with dark matter is why is it dark, or in other words why doesn't it interact with normal matter the way we're used to. As I mentioned, we know that dark matter doesn't interact via electromagnetism or the strong nuclear force and that it does interact through gravity. The only fundamental force we're not sure about is the weak nuclear force. So the question then becomes if dark matter does interact with normal matter via the weak nuclear force, how much does it interact? That is quantified as the interaction cross-section.
That leads us to the big result from the CDMS preprint:
This plot shows two things. The filled areas are theoretical predictions based on a popular dark matter candidate, supersymmetric particles. If that theory is right, the dark matter particles should be found in those shaded areas. The lines show the area above which various experiments would have found the WIMPs if they were there. That means that the only places the WIMPs can be hiding is below those lines. Essentially, all this hubbub is about those little bumps in the CDMS upper limits on the interaction cross sections at about 40 and 70 GeV/c^2. Hopefully as detectors improve and they have more time to take data, those bumps will turn into filled regions where dark matter has actually been detected.
Like always, really great analysis Nick.
ReplyDeleteThanks Joe. Glad to hear I didn't mess it up too badly.
ReplyDeleteSo if you had to bet on who will find WIMPs first (assuming they are there to be found) between passive detectors like CDMS and particle accelerators like the LHC, where's the smart money? I think I'd go with the passive detectors.
I think I'd go with the passive detectors as well Nick. The LHC, as awesome as it is, in many respects is incredibly complex, will take years to claim any discovery at all and is highly tuned to physics at the TeV scale.
ReplyDeleteFor good or bad, as with dark matter, I think the cheap, simple focused experiments like CDMS will be more likely to claim a "first" detection then the LHC.
I think the LHC is going to have major payoffs long term. After the thing actually works as designed, and after we literally will have spent years understanding the backgrounds will we discover anything.
But once we do all that I think the LHC will be the most thorough probe of the TeV scale we have.
Just to put the complexity in prospective: the Tevatron in some ball park sense has 1/10th the complexity of the LHC. (1/10th the energy, much less luminosity ie. less collisions per second and less people, detectors etc..)
ReplyDeleteThe Tevatron, I was told, didn't make it's first discovery until it had been running 3 years at its full destined level. Its understanding the calibration and "backgrounds" as they are called that is so difficult.
It's up in the air how long all of this will take for the LHC. Everyone thinks at least as long as the Tevatron, and most I believe think it will take a little longer.
So ballpark prediction: It would take at least 5 years before the LHC can discover WIMPS behind dark matter.
In contrast, things like CMDS are closing in very quickly.
I should further add, some think the Tevatron itself has a better chance of being the first to discover this stuff, given it still has 3-5 years to beat the LHC to the punch. Furthermore, the backgrounds of the Tevatron are very well understood... comparatively.
ReplyDeleteSorry Joe, but I've got two more questions: if dark matter isn't some sort of supersymmetric particle when will we know and what else could it be?
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