>Date: Thu, 14 Mar 2002 03:32:23 +0000
>From: Rose
>
>Hello,
>I am sorry to bother you but I read about your work involving WIMPs and I
>wondered if you could spare a few moments to help me with some questions I
>have.  I'm afraid I don't have much scientific knowledge yet and I realise
>that there may not be an answer to many of these questions but I'd be
>grateful if you could have a look over them!

Sure!

>Firstly, I was wondering, what are the masses of such particles likely to
>be?  Would they be less than the 1 a.m.u of neutrons and protons?  Would
>they be fundamental particles that don't decay into other particles?

We don't know their mass for certain yet, or even very well at all.
Our best guess, based on the particle theories which predict their
existence, is that they are about 100times heavier than the proton.
This would be, as you correctly refered, 100 a.m.u.  

>Secondly, how do such particles move - do they remain in one area or are
>they continually moving in a straight line like (I think) neutrinos move?

We believe that they behave much like the individual atoms of nitrogen
and oxygen in the air in a room.  If the door is shut, the collection
of atoms remains roughly "trapped" in the room, but they bump into
the other objects in the room.  WIMPs, if we are right, would be
"trapped" in a big cloud called a halo.  Such a halo seems to surround
each galaxy -- in fact, the halo of dark matter formed first and drew
in the gas which formed stars.  Now, the WIMPs in a galactic halo
have individual thermal motions (like the gas atoms in the air in the
room) and they remain localized to the galaxy roughly (again, like 
the gas in a room) and they bump into the other atoms which constitute
the stars and stuf fof our galaxy.  In fact, we believe that our sun,
like every such star, has its own little special collection of WIMPs
which bumped into the sun's atoms and lost energy and got pulled into
the sun's gravity.  Incidentally, we suspect that the random thermal
velocities of the WIMPs in the galaxy are about 300km/s, which is about
as fast as our sun is moving through the galaxy.

>Also, are they likely to be found in the form of individual particles when
>they are found on earth or are they expected form connections with each
>other and bond together in structures?  I would have thought this is
>unlikely because they would be more easily detectable if they came together
>in clumps of some sort? 

Our best guess is that they are weakly interacting -- with atoms and
with each other, so they are most likely solitary particles, like
neutrinos.  Some models suggest that WIMPs are self-interacting, but
that would only imply they collide more easily with each other and
not form WIMP atoms.

>Do they ever react and form bonds with 'normal' particles and if not, do
>they still create electromagnetic fields of some kind?

They have no charge, so there should be no electromagnetic interactions.
Some models of dark matter particles suggest that they interact with
the nuclei of atoms very strongly.  They might then get trapped inside
of nuclei, particularly in heavy atoms with large nuclei.  These theories
are not really taken seriously by many.  However, as I mentioned before,
when they collide with atoms in the stars (rare, but after time it does
happen enough), they do lose enegry and "fall" into the stars forming
pockets of higher WIMP density.

>Also, does the detection of WIMPs rely on the idea that they have certain
>properties or would any particle with a mass be detectable from it's
>collisions with an atom, regardless of the other interactions it is or
>isn't capable of?

Good question.  We are gambling that they interact weakly with atomic
nuclei.  In particle physics, we know of basically four types of
interactions: strong (holds protons and neutrons together and
keeps them bound in the nucleus), electromagnetic (charges and static
and binds electrons to the nucleus), weak (neutrinos, decays of neutrons),
and gravity.  By the "W" in WIMPs we mean "weak" interaction as in
the third category above.  This, as you might guess, is hard to detect.
Now, just because a particle has mass does not mean it can be detected--
except by gravity, of course.  If you say something has mass, then
it has to produce some gravitational attraction.  This is actually
how we know dark matter is out there.  We measure gravitational motions
of galaxies and stars and clusters of galaxies and realize that there must
be more mass out there to be causing the motions.  

But, in terms of particle-to-particle interactions, gravity is not
important and we are left with strong, electromagnetic or weak.
Some particles interact by more than one of the above three, and
all interact gravitationally.  But, the actual degree to which a particle
engages in interactions depends on its constitution.
It might be that they interact so rarely that we will never detect them,
but it seems to be that, according to theories, they ought to have an
interaction strength such that if we wait a year or so with a good set of
detectors, we'll record just a few bumps.

>Sorry for asking so many questions but I'd be extremely grateful if you
>could help me understand any of them a little more clearly,

My pleasure!
Scott Armel-Funkhouser
Berkeley Cosmology Group