From another email to friends back then.
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hey all- the astronomer postdoc at Caltech i work with asked me a coupla
q's (since i have more of a particle physics background then he does), so
i wrote him back the answers below, thought it might interest you guys
too.
I find it really amazing that despite *all* our efforts, we simply may
*never* see DM directly in our lives on the Earth -- say it ain't so, Mother
Nature!! I hope very much not..
-M
---------- Forwarded message ----------
Date: Sun, 6 May 2007
From: Mandeep Gill
To: Richard M
Subject: Re: dark matter
R- just saw this email, and though i'm not a total expert in this, i'll say what i know..
On Fri, 4 May 2007, Richard M. wrote:
> Quick question - if dark matter were an LSP, would it be self-interacting?
Generally, yes (it's normally some version of a neutralino in these
scenarios). If it interacts with normal matter through some extra weak
Susy force, then it will interact with itself as well. But of course, we
have no way of knowing what the cross-section would be, and it may be so
low that it will *not* interact with itself nor normal matter much, or at
*all* through Susy forces -- in which case LHC will *not* make it.
That is, if DM interacts *only* gravitationally (e.g. gravitino DM) --
which is entirely theoretically allowed (and in fact i'm told, rather
generic in string theory models, which have plenty of particles that don't
interact through the Standard Model forces, that is, don't carry those
quantum numbers, but charges of other forces we haven't seen yet) -- *all*
the DM could've been made just after the Big Bang when gravity was strong
and temps high, and then pretty much just have never interacted again
after that.
Now for particle physicists (and maybe some cosmologists too) this is an
incredibly depressing potential scenario -- way more than the idea of
the Universe being open and ever-expanding to a cold death, which i know
personally brings you down, because while that's billions of years in the
future (and who the heck knows, maybe we'll find we can tunnel to
alternate Universes in the meantime..! ;-) ) if we can't *see* DM in a
collider or otherwise on Earth, then that is something that is going to
impact *our* very scientific lives!
So there's nothing that rules out this
logical possibility, *but* there are plenty of scenarios that it's *not* the
case, and i know people are hoping badly to see DM sometime soon.
> Does it depend on which is the lightest particle?
Well, by definition LSP *is* the "lightest Susy particle", right, so it's
the last particle in a decay chain, and it has nothing it can decay to
(and this is only in Susy models with externally imposed R-parity
conservation -- which was a symmetry first cooked up to prevent proton
decay, and turned out to result in a very nice DM candidate later). So i
guess the answer is no, in that sense.
> And if so, would we be able to see the carrier particle?
Generally not, because it's going to decay so quickly (like the W+- or Z0)
that it will only be a virtual particle in Feynman diagrams, and never
actually detectable. e.g. the W or Z never leave tracks in a detector, we
just know that they were the force carriers from the daughter particles in
the decay.
Hope that helps a bit..?!
If you have some more specific followup q's, or models in mind, feel free
to ask.
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