Tuesday, October 31, 2017

Dark Matter Day

I have been informed that today is Dark Matter Day, and that I am to circulate these videos:

We search for Dark Matter in IceCube on the assumption that it is made of particles that interact with neutrinos.

It doesn't interact with photons (dark matter particles have no charge: that's what makes them dark) nor with baryonic matter (nuclei), and all we seem to get are limits on its interaction with W and Z (weak interaction--related to radioactive nuclear decay). Since it seems much too odd that we should get two completely non-interacting types of matter in a Big Bang, that just leaves neutrinos for them to interact with. And maybe dark matter interacts with W and Z after all, which would give particles that would decay in turn, which should also give some neutrinos.

Anyhow, if the dark matter particle is unstable, it can decay into a couple of neutrinos. If there are several species of dark matter particle, heavier ones might decay into a couple of neutrinos and a lighter type of dark matter particle. Or they could fuse to make neutrinos. Anyhow, look for neutrinos. Which, happily enough, is what IceCube is designed to do.

So if you see an extra number of high energy neutrinos coming from places where you expect dark matter to accumulate--inside stars, or at the center of galaxies--you might be seeing neutrinos from dark matter particles decaying/fusing. That would be very interesting, and could give you some estimate of just how massive these dark matter particles might be.

Of course if the dark matter particles are very light, they won't accumulate in stars the way you expect(*), plus the number of low energy neutrinos from cosmic ray showers in Earth's atmosphere will probably drown out your signal. (Low mass dark matter would decay to low energy neutrinos.) So the assumptions matter.

Also have a look at the search for sterile neutrinos video. Neutrinos are hard to study...

(*) The cloud of dark matter particles would be much more spread out. It's kind of intuitive that "heavy stuff sinks to the bottom," but it's a little more complicated because the dark matter particles don't bump into things very much--they don't interact and slow down in ways you're used to.

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