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My understanding is that dark matter cannot be (or is at least highly unlikely to be) an exotic form of any known particle. On the other hand, articles about particle accelerators seem to say that the Higgs is the last piece missing in the Standard Model jigsaw puzzle.

If dark matter is determined to be some form of new particle, what are the certain implications? Might such a discovery "stand to the side" of the Standard Model or would it certainly change the foundations?

(Forgive my extremely lay understanding and vocabulary -- please feel free to correct my mistakes.)

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The phrase "physics beyond the standard model" appears in most justifications for doing particle physics at the energy or intensity frontier. – dmckee May 15 '11 at 20:08

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The conventional wisdom about dark matter is that it is likely to be a new kind of particle that is not part of the standard model. Basically, the reason for this is that most of the stable standard model particles interact electromagnetically (and so wouldn't be "dark").

The exception is neutrinos, and for a long time neutrino dark matter was considered to be a viable possibility, but it doesn't seem to work for several reasons. The main one is that neutrino dark matter would be "hot" (meaning that the particles would have had relativistic speeds in the not-too-distant past), whereas the way dark matter is observed to cluster gravitationally only seems to work if the dark matter is "cold."

For quite a while, people tried to come up with models in which the dark matter was ordinary matter (made of atoms), but that also doesn't seem to work for several reasons. In a Universe with a high enough density of atomic matter, the abundances of light elements produced in the early Universe would be quite different from what is seen. There's no good way to make this sort of matter "dark enough": even if you make it cold and neutral,\ it still interacts with radiation too strongly to remain hidden. Also, a Universe made of only atomic matter predicts a spectrum of fluctuations in the microwave background, and a bunch of other cosmological observables, that differ by a huge amount from what we observe.

So the leading theory is that the dark matter is a different sort of stable, neutral, weakly-interacting particle. Such a particle would necessarily be "beyond the standard model."

Probably the least exotic possibility is that it's a supersymmetric particle. If supersymmetry is right, there are a lot of new particles out there waiting to be discovered. Most of them are unstable, but the lightest one is stable and would make an excellent dark matter candidate.

If supersymmetry is right, there's a good chance the LHC will detect it, so we may actually know the answer to this in the not too distant future. Then again, we may not.

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So there is no such thing as "cold" neutrinos? Not even if they are slowed by a large gravitation field? Could you explain this to me please? It's something I've always wondered. – user1567 May 15 '11 at 21:38
At early times, neutrinos would have reached thermal equilibrium with the other matter in the Universe, meaning that they would have had quite a high temperature. Once you know the temperature, you can calculate the average kinetic energy of each neutrino. The neutrino is relativistic if the kinetic energy is comparable to the rest energy $mc^2$. – Ted Bunn May 15 '11 at 22:03
Would these neutrinos, as left over from the early universe, experience a red-shift like the CMB? And being particles with mass wouldn't this result in smaller velocities? Or is it that the rest mass is so small that even relatively small kinetic energies result in relativistic neutrinos? – user1567 May 15 '11 at 23:04
@jaskey13: the rest mass of the neutrino is something like 3 eV. To put that in context, the energy required to transition an electron from the ground state of the hydrogen atom to its first excited state is 10.2 eV. The rest mass of the electron is 5.1 $\times 10^{5}$ eV. So, yes, the rest mass of the neutrinos is very very tiny, and it doen't take much to make them ultra-relativistic. But yes, they would be expected to redshift with the CMB. They'd just be too hot. Now, if you want to talk about sterile neutrinos, they're still viable: – Jerry Schirmer May 16 '11 at 0:37
That's right -- neutrinos do redshift with the expansion, but their rest mass is low enough that they remain relativistic until late times. Not necessarily all the way up to the present, but late enough to cause problems with gravitational clustering. – Ted Bunn May 16 '11 at 12:44

Let me address this part of the question, as the physics part is covered by Ted Bunn.

If dark matter is determined to be some form of new particle, what are the certain implications? Might such a discovery "stand to the side" of the Standard Model or would it certainly change the foundations?

Up to now, progress in understanding particle physics has been happening not by completely discarding previous theories, but by assimilating them. The reason is that previous theories, as the standard model will become too in the future, were based on solid data. Being based on data means that they are just a shorthand of describing them. The old data still exists when new data appear, as the example of dark matter , so what will happen is that the standard model will be incorporated in any new theory describing matter in the microcosm.

Already string theories include the standard model within their structure so no "stand aside" will happen if they are found sufficient to describe all new data. String theories have a plethora of matter forms that could well explain/describe dark matter.

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